Basic Roof and
Basic Roof and
Ceiling Framing
Ceiling Framing
CARP11
CARP11
Carpentry - Housing
Carpentry - Housing
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These notes were prepared by These notes were prepared by Teachers of Carpentry Teachers of Carpentry TAFE NSW TAFE NSW 2003 Edition 2003 Edition
NSW TAFE Commission / DETNSW TAFE Commission / DET
CONSTRUCTION & TRANSPORT DIVISION CONSTRUCTION & TRANSPORT DIVISION WESTERN SYDNEY INSTITUTE OF TAFE WESTERN SYDNEY INSTITUTE OF TAFE
For Construction and Transport For Construction and Transport Division TAFE NSW
Division TAFE NSW Victoria Road Victoria Road Castle
Castle Hill Hill NSW 2NSW 2154154 Ph. (02) 9204 4600 Ph. (02) 9204 4600 First Published 1999 First Published 1999 Second Edition 2003 Second Edition 2003 ISBN 0 7348 1007 5 ISBN 0 7348 1007 5
Construction and Transport Division TAFE Construction and Transport Division TAFE NSW, 1999NSW, 1999
Copyright of this material is reserved to Construction and Transport Division TAFE NSW Copyright of this material is reserved to Construction and Transport Division TAFE NSW
Reproduction or transmittal in whole or part, other than for the purposes and subject to the provision of the Copyright Act, is Reproduction or transmittal in whole or part, other than for the purposes and subject to the provision of the Copyright Act, is prohibited without the written authority of Construction and Transport Division, TAFE NSW
prohibited without the written authority of Construction and Transport Division, TAFE NSW Published by
Published by
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Printed and Distributed by
Resource Distribution - TAFE Manufacturing and Engineering Division Resource Distribution - TAFE Manufacturing and Engineering Division
Publishing details: Publishing details:
These notes were prepared by These notes were prepared by Teachers of Carpentry Teachers of Carpentry TAFE NSW TAFE NSW 2003 Edition 2003 Edition
NSW TAFE Commission / DETNSW TAFE Commission / DET
CONSTRUCTION & TRANSPORT DIVISION CONSTRUCTION & TRANSPORT DIVISION WESTERN SYDNEY INSTITUTE OF TAFE WESTERN SYDNEY INSTITUTE OF TAFE
For Construction and Transport For Construction and Transport Division TAFE NSW
Division TAFE NSW Victoria Road Victoria Road Castle
Castle Hill Hill NSW 2NSW 2154154 Ph. (02) 9204 4600 Ph. (02) 9204 4600 First Published 1999 First Published 1999 Second Edition 2003 Second Edition 2003 ISBN 0 7348 1007 5 ISBN 0 7348 1007 5
Construction and Transport Division TAFE Construction and Transport Division TAFE NSW, 1999NSW, 1999
Copyright of this material is reserved to Construction and Transport Division TAFE NSW Copyright of this material is reserved to Construction and Transport Division TAFE NSW
Reproduction or transmittal in whole or part, other than for the purposes and subject to the provision of the Copyright Act, is Reproduction or transmittal in whole or part, other than for the purposes and subject to the provision of the Copyright Act, is prohibited without the written authority of Construction and Transport Division, TAFE NSW
prohibited without the written authority of Construction and Transport Division, TAFE NSW Published by
Published by
Construction and Transport Division Construction and Transport Division Printed and Distributed by
Printed and Distributed by
Resource Distribution - TAFE Manufacturing and Engineering Division Resource Distribution - TAFE Manufacturing and Engineering Division
Overview
Overview 11
Introduction to Pitched Roof Framing - definitions and roof types
Introduction to Pitched Roof Framing - definitions and roof types 22
Ceiling Framing
Ceiling Framing - Introduction to ceiling framing- Introduction to ceiling framing 1010
Ceiling trimmers
Ceiling trimmers 1212
Hanging beams
Hanging beams 1414
Set Out and Erection of Ceiling
Set Out and Erection of Ceiling FramesFrames 1515
Ceiling frame calculations
Ceiling frame calculations 1818
Alternative Ceiling Types
Alternative Ceiling Types 2323
Skillion roof construction
Skillion roof construction 2525
Lean-to roof construction
Lean-to roof construction 2626
Gable Roofs
Gable Roofs - parts, proportions and definitions- parts, proportions and definitions 2727
Structural Roof Members
Structural Roof Members 2929
Wind bracing Wind bracing 3131 Purlins Purlins 3232 Struts Struts 3333
Patent Type Strutting
Patent Type Strutting 3838
Collar ties Collar ties 3939 Gable Ends Gable Ends 4040 Calculating Drop-off Calculating Drop-off 4646 Eaves finishes Eaves finishes 4848
Erection Procedure for the Gable
Erection Procedure for the Gable Roof Roof 5050
Roof Pitch
Roof Pitch 5656
Setting Out and Cutting
Setting Out and Cutting RaftersRafters 6060
The Steel Square
The Steel Square 6262
Calculating Frame Quantities
Calculating Frame Quantities 6868
Glossary of Terms Glossary of Terms 78 78 Further Reading Further Reading PART 1: PART 1: PART 2: PART 2:
Acknowledgments: Acknowledgments:
Acknowledgment is due to the following for their permission to reproduce product materials Acknowledgment is due to the following for their permission to reproduce product materials and copyright materials or for development of this
and copyright materials or for development of this text;text;
•
• ACE Guttering Pty LACE Guttering Pty Ltd - for use of the fascia and gutter details contained in their salestd - for use of the fascia and gutter details contained in their sales
brochure brochure •
• Ivanka SusnjaraIvanka Susnjara - for desktop publishing and preparation work for printing.- for desktop publishing and preparation work for printing.
•
• Rob YoungRob Young - for preparation and editing of these notes, including development of new- for preparation and editing of these notes, including development of new graphics.
graphics.
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• Special thanks -Special thanks - to Bob Bulkeley for the many years of dedication to reto Bob Bulkeley for the many years of dedication to re search,search,
development and production of quality resources for use in the area of vocational development and production of quality resources for use in the area of vocational education.
This text introduces subject matter related to ceiling framing and basic roofing. Reference may be made to “Basic Building and Construction Skills”, produced by TAFE and Addison, Wesley, Longman Australia Pty Limited, to re-examine and reinforce these basic skills.
There are two parts to the text, PART 1 – Ceiling Framing and PART 2 – Gable
Roofing, which address the following:
Ceiling frames, roof types and terminology are explained, with special reference to
gable roofs. Structural members are detailed and their purpose defined.
Methods for determining lengths of members, setting out, cutting and erection
processes are covered, including eaves construction for various situations and the
various materials used.
The text also covers calculation of members, their lengths, quantities and costs.
Note: Only conventionally pitched roofs are dealt with in this text, as Trussed roofing will be dealt with in a separate text.
A comprehensive ‘Glossary of Terms’ is included at the end of the text, which provides
a detailed description of trade terms, technical content and some trade jargon.
BASIC ROOF
and CEILING
INTRODUCTION TO PITCHED ROOF FRAMING
Definitions:
Roof: “A roof is the weatherproofed upper covering of a building or structure, which is
designed to protect the interior from atmospheric elements such as sun, rain, hail, snow, frost and wind.”
Flat roof: Is a roof with a minimum of slope to allow water run-off, usually with a fall of 1 in 40 but a minimum of 1 in 60. These are usually confined to sheet roofs with the sheets being full length, no joins, for a slope less than 1 in 40.
Pitched roof: This is a roof with a slope, or pitch, greater than 1 in 12 or 5 degrees. A roof may have a single pitch, like a skillion, a double pitch, like a gable, or an unequal pitch, where both sides of the roof have different angles.
Coupled roof: This is where pairs of rafters are attached on opposite sides of a ridge and the feet are fixed to the wall plate. There is no tie between the feet, allowing the rafters to spread under load. It is restricted to small span gable roofs, which may be simply coupled.
Close-coupled roof: This is the same as the coupled roof except there is a tie, such as a ceiling joist, placed between the feet of the rafters. This method is used for most roof construction,
especially for gables with a wide span.
Cut roof: Also known as a conventionally pitched roof, has all of its members cut and
assembled individually. These roofs are made up of separate rafters, ridge, purlins, collar ties, struts, etc.
Free roof: This refers to any roof, which does not have enclosed walls under it. It is typically used for a freestanding carport, portico, covered walk-way, lichgate, etc.
Gablets:These are simply small versions of gables. They may be used on the ends of ridges for ventilation, over a dormer window or as an adornment to the main roof surface.
Monoslope roof: Also known as a ‘Monopitch’ roof, it is any roof with a continuous slope, which has no ridge. Skillion and lean-to roofs are monoslope roofs.
Open roof: Any roof, which is not enclosed underneath. Verandah, free roofs and garage roofs are typically not lined or framed with a ceiling and therefore classified as open roofs.
Shell roof: Made from a thin self-supporting and curved structural membrane used over long spans. Precast, prestressed concrete is commonly used for this construction.
Southlight roof:Generally refers to the vertical glass of a sawtooth roof, which faces south to allow glare-free light to enter the building.
THE ROOF
The roof, and roof covering, makes up a large part of the external fabric of the building. It may be designed and constructed to create a picturesque roofscape using a variety of materials or
they may be very plain and use only one type of material.
The design of roofs has changed dramatically from the early part of the 20th century where common features included ornate gables, gablets, turrets, spires, crested ridge capping, finials, vents, and complex chimneys decorated with terracotta pots.
These steeply pitched and highly decorative roofs have given way to the modern low pitched, plainly coloured roofs of today commonly seen in most new housing developments.
Roofing materials such as glazed and unglazed terracotta, slate and ‘ fibro’ have been generally superseded by concrete tiles and ‘Colorbond’ roof sheeting.
As with most styles in building, the older t ypes of design eventually become incorporated into contemporary design, or make a comeback, therefore methods of development and construction of roof types should not be forgotten.
TYPES OF ROOFS
There are many styles of roof, most of which are made up from variations on specific types. Some of these roof types are described below:
A-frame: This is a steeply pitched roof, which forms a shape similar to the letter ‘A’. More commonly used in snow areas to allow the snow to slide off easily, rather than have it add excessive load to the roof frame.
Bellcast: This is a roof, which changes its pitch to a lower pitch or angle near
the eaves. It is commonly used where the main roof pitch meets the lower pitch of a covered balcony or verandah.
Clerestory: This is a roof having two levels separated by a row of windows, which provide light and/or
ventilation to the rooms below. It gets its name from the upper part of a church nave, which is the main source of light.
Deck: This roof type takes the form of a truncated or cut-off top pyramid with a flat or near flat section in the middle. This may occur where a hip roof has a deck or landing on top, with a handrail around it, used for entertaining or an observation deck. A-Frame Wall of cottage Bellcast Clerestory Clerestory Windows Flat section with a handrail around perimeter
Dutch gable: This is a hip type roof with small gables or gablets at either end of the ridge. It may also be referred to as a ‘half-hipped roof’ or a
‘Gambrel’.
Gable: This is a roof with a double pitch and vertical ends. It may also be used as an add-on to a main roof in the form of gablets over entries or simply decorating the main roof surface in the form of a dummy gable.
Gambrel roof: This is similar to the Dutch gable having gablets at either end of the ridge on a hip roof. In recent times the size of the gablet has increased providing a more distinctive style
of roof surface.
Half-pitch : This refers to a roof, which has a pitch angle where the rise is equal
to the half span of the roof, i.e. forms a 45° angle. Dutch gable Gambrel Gable Gablet Gablet Half-pitch
Helm : This is a pyramidal roof, having a square base, with four gables
connected at the bottom horizontal position. The remaining roof
surfaces are diamond-shaped. This type of roof was commonly used for spires on square towers.
Hip or Hipped: This is a roof with four sloping sides on a rectangular base. The ends are triangular in shape and the sides form a trapezoidal shape.
Hip & valley: This is basically a hip roof, which is ‘T’ or ‘L’ shaped on plan. The ridge lines are the same height for the main and extended roof
sections.
(Broken) Hip & valley:
Again it is similar to the hip & valley type except the ridge(s) of the extended sections are not at the same height as the main roof. This creates a shortened or broken hip used to link the minor ridge to the major ridge.
Helm
Gables on Four sides
Hip or Hipped
Hip & Valley
Valley Hip
Broken Hip & Valley Broken hip
Hyperbolic paraboloid:
This is a form of shell roof construction, which has raised diagonally opposite corners on a square base. This creates a convex curve between the low corners and a concave between the high ones. They have been used for small architecturally designed airport terminals and swimming centre shade roofs.
Jerkin head: This is a roof, which is hipped from the end of the ridge half way down to the eaves, and gabled from half way to the eaves. It is also sometimes called a ‘Hipped gable’ or a ‘Clipped gable’.
Mansard: This is similar to a hipped roof except all four sides have a double pitch. Each side has a steeply
sloping section up from the eaves, then the top section flattens out up to the ridge. It was named after the French architect Francois Mansart, who died in 1666. It has also been referred to as a ‘Curb roof’ or a ‘French roof’.
Monitor: This is a portion of a roof, which has been raised up above the main roof, usually flat, with continuous vertical glazing around the
perimeter for natural lighting. Mainly used for industrial buildings. Hyperbolic paraboloid Jerkin Head Hip end Gable end Mansard Monitor Window area
Monoslope: Also known as a ‘Monopitch’roof, it is any roof with a continuous slope, which has no ridge. Skillion and lean-to roofs are monoslope roofs.
Pyramid: This is a roof with square or other regular polygon shaped base, with all hips being equal and
converging at a pointed apex.
Sawtooth: This is a made up of a series of connected monoslope roofs, which appear to be sawtooth-shaped when viewed from the end. The shape is a series of right-angled triangles connected at the base or trough with a common box gutter. The vertical face is usually glass to allow natural light to enter. Commonly used for commercial and industrial work.
Station: This type of roof is typically used for train and bus stations where the roof is to cantilever past supports on both sides to provide shelter. Monoslope Pyramid Sawtooth Glass along vertical faces Station
Fig. 2 Typical modern combination style hip and gambrel roof
Troughed: This is a double-pitched roof with a valley between the two surfaces. This roof is also referred to as a "Valley Roof" or a "Butterfly roof".
Tudor: This is a steeply pitched roof, usually a gable style, with dormer windows on one or both sides.
Troughed
INTRODUCTION TO CEILING FRAMING
The ceiling frame is the horizontal area between the top of walls and the roof, which is designed to enclose the room by providing a dust barrier, insulation and security.
The frame consists of ceiling joists, ceiling trimmers, hangers and hanging beams. This system is designed to tie-in with a conventionally pitched skillion, gable or hip roof.
Note:The ceiling frame of a trussed roof is made up of the bottom chords of the individual trusses and does not require additional ceiling joists, hangers or hanging beams.
Ceiling joists
These are the horizontal members with ends that rest on top of the wall plates. They carry the ceiling sheets, and provide a lateral tie between the feet of opposing rafters to form a strong, coupled frame. They may be nailed or bolted to the rafters.
They are spaced at maximum centres of 450 mm and 600 mm depending on their stress grade, section size and thickness of ceiling lining being used. They may be joined in length over a wall or under a hanger, where the join can be supported.
Refer to AS 1684 for sizes and stress grades.
PART 1 :
CEILING FRAMING
Ceiling Joist
STABILITY UNDER LOAD FAILURE UNDER LOAD
Vertical deflection
X1+X2= lateral deflection
RAFTER FIXED NEXT TO CEILING JOIST CEILINGS JOIST FIXED TO TOP
PLATE
Ceiling joist fixed next to rafter position
CEILING TRIMMERS
These are short lengths of ceiling framing material, fixed at right angles between ceiling joists, placed at the same maximum spacings as the joists.
They are designed to:
• Provide fixing for the ends of ceiling sheets and cornices; • Provide fixing for the top internal wall plates; and
• Provide continuous lateral stability for the ceiling frame once hangers are fixed.
Ceiling Joists
Trimmers
Top plate to internal wall
Patent type metal connectors may be used to provide a secure load-bearing connection between hangers and ceiling joists, which is particularly useful when work or an inspection is to be
carried out inside the roof space in the future. The connectors are fixed on alternate sides, every second joist, to assist in preventing the hanger from overturning.
‘JOIST STRAP’ ‘TRIPLE GRIP’ (Trip-L-grip) ‘CEILING DOG’
Ceiling joist Hanger
Timber batten
Top wall plate Solid blocking Solid blocking
Top wall plate Hoop iron strap
When very deep, narrow hangers are used it may be necessary to fit a timber brace or hoop iron strap to the ends to prevent the hanger from twisting or rolling over. Alternatively, if the end of the deep hanger runs past the face of a hip end rafter or gable stud it may be bolted to it.
Fig. 8 Final fixing of joists to hanger with typical patent connectors
Where hangers run across ceiling joists with the end protruding past the
line of the rafters, as would occur at the end of a hip roof, the load is transferred to the wall plate via a ceiling trimmer. The top end of the hanger is bolted to the rafter and then the hanger is strapped to the ceiling trimmer.
Note: The end of the hanger is cut to the pitch of the roof before being fixed into place. Hanger Rafter Ceiling dog Ceiling joist Trimmer Top plate Approved metal strap
Hanger bolted to rafter Ends cut to roof pitch
Trimmer support to hanger Approved metal connector Bolted joint
Hanging beams
A hanging beam, also known as a counter beam, runs at 90° to the line of hangers and supports them where their length exceeds the allowable span. The hangers are cut onto a bearing cleat on either side of the hanging beam to allow continuous support for the length of the room. The ends of the hanging beam are packed up slightly to allow for deflection. The section size and stress grade will be greater than the hanger. Refer to AS 1684 for stress grades and section sizes.
Note: The roof frame must not be supported off hangers or hanging beams unless designed and specified by a structural engineer
Hanger checked out over hanging beam Hanger
Underside of ceiling joists adjusted to a
common level
Ceiling dog or other approved fastener
Ledger
Hanging/Strutting beam packed at end support points to allow for deflection SECTION Hanger Hanging beam Ledger Packing Reinforcement blocking
SET OUT AND ERECTION OF CEILING FRAMES
The set out of the ceiling frame is based on the set out for the roof rafters. Whether the roof type is a gable or a broken hip and valley the set out of the rafter positions is carried out first so the joists may be fixed alongside.
Procedure
STEP 1 Check the top wall plates for straight and that the wall frames are square overall. Gable:
Mark the positions of the gable end rafters, at each end, then working from one end set out the positions of the common rafters, in-to-over, at the specified maximum spacing, i.e. 450 or 600 mm.
Place a mark on one side, ‘R’ or ‘X’ to represent the position of the rafter and a ‘J’ on the other side to represent the position of the joist.
Hip:
Measure in the distance equal to the half span from both ends. This represents the centre line of the centring rafters. Measure half the rafter thickness on either side of this line and place an ‘R’ between the two outside marks. Place a ‘J’ on one side of the rafter position to show the joist position.
Repeat this process for the other end of the roof, and then work from the centring rafter to the end of the walls marking rafters, in-to-over, at the maximum spacings. Mark rafter positions at both ends of the roof working from the centre to the outside. Finally, start from the centring rafter position at one end and mark rafter spacings to wards the other centring rafter, until the spacings run out. Mark these with an 'R' or 'X' and then place a 'J' beside them to show joist positions.
Joist TYPICAL DETAIL Common rafter Top plate Top wall plates
Gable end rafter
position for a flush gable
6 0 0 6 0 0 6 0 0
Note: Ends are trimmed later at 90º to the ceiling joists to provide fixing for the ceiling sheets.
STEP 2 Cut all ceiling joists to length and fix into position by double skew nailing the ends. Cut and fix ceiling trimmers to ends and above internal walls, which run parallel to the joists. TYPICAL DETAIL 6 0 0 6 0 0 6 0 0 6 0 0 6 0 0 6 0 0 6 0 0 6 0 0 6 0 0 ½ s p a n ½ s p a n
Centre line position of hip roof members
C L Joist Top plate Centring Rafter C L
End of ceiling trimmed to take ceiling sheets and cornice
Ceiling trimmers fitted over internal walls
Top wall plates Internal walls
Ceiling joists fixed beside rafter positions
End trimmers may be laid flat to allow for rafter over
Fig. 13 Setting out plates for a hip roof
STEP 3 Fit hangers to the centre of the ceiling joist length over each room as required. Hangers are joined over internal walls and should be blocked off the top wall plate equal to the depth of the ceiling joists. Where ceiling trimmers are close to the required hanger position they may be used as the means of blocking.
Note: Hangers may be run continuously over walls, but it may be more economical to treat each room separately and reduce unnecessary cost by using smaller
sectioned members where possible.
Smaller sectioned hangers over short spans
Ceiling dogs on alternate sides of hanger
End of deep hanger strapped with hoop iron and supported on a ceiling trimmer End of hanger bolted to
gable stud to prevent twisting, for a gable roof
Cut the end to suit the pitch of the roof, for a hip roof.
Fig. 15 Fix hangers into position for the ceiling of a Gable or Hip roof
Note: Where hangers are over their allowable length, hanging beams may be required to support them at mid span.
CEILING FRAME CALCULATIONS
The basic procedures are similar to that used for wall framing.
Once a system is adopted and formulae are identified it will be necessary to gather the following details for calculation purposes:
Plan - This will be required so the dimensions of rooms can be identified to allow a cutting list of material to be formed; and
Specification or Tables
-The specification or AS 1684 tables will provide section sizes and stress grades of framing members.
Joists - Where possible the joists should run the short dimension of the room, but should always be placed to tie the feet of the rafters for the length of the roof.
Calculate the ceiling joists for each room separately. Formula = (width of room) - 1 (as there is no 1stjoist)
Max. spacing
Length of joists = internal room length + (2 x wall plate width) Ceiling trimmers are calculated separately.
Ceiling Trimmers
-Allow for ceiling trimmers where ever internal or external walls run paral-lel to the ceiling joists.
Calculate each wall separately.
Formula = (internal room length of wall) - 1 (as there is no 1sttrimmer) Max. spacing
Length of trimmers = maximum spacing of joists – (2 x joist thickness)
Hangers - Allow one hanger at 2100 mm maximum centres, unless otherwise speci-fied.
Length of hangers = internal room width + (2 x wall plate width)
Hanging beam- Placed where the length of the hanger is greater than it’s maximum allow-able span. (as per tallow-ables or specification)
Length of hanging beam = between supporting walls + (2 x plate width)
Hanger / joist connectors
-Type as per specification.
Allow one per ceiling joist for each row of hangers, per room.
WORKED EXAMPLE FOR CEILING FRAME QUANTITIES and COSTS
The following worked example provides details of how the quantities are arrived at and how the individual materials are presented and costed.
Details are as per plan and specification based on AS 1684 - 1999 Part 2.
SPECIFICATION
Joists - 150 x 38 sawn Oregon F8 at 600 mm max. c/c (max. 3.6m continuous span) Joists are to be joined on a hanger where they exceed 3.6m in length.
Where joists are joined on hangers the joins should be staggered, if possible. Trimmers 100 x 38 sawn Oregon F5 at 600 mm max. c/c
Hangers - Hangers to be spaced at max. 3600 mm c/c Max. span
(mm)
Section size (mm) Stress grade Material
5400 240 x 45 F27 Seasoned hardwood
3000 190 x 35 F11 Seasoned hardwood
Hanging beams - Not required. Hanger / joist
connectors
- Allow one for each joist, per hanger, per room.
Fit ceiling dogs on alternate sides for the length of the hangers.
Material costs - Material Cost
150 x 38 F8 sawn Oregon $ 4.20/m
100 x 38 F5 sawn Oregon $ 3.10/m
240 x 45 F27 Hardwood (seasoned) $19.20/m
190 x 35 Hardwood (seasoned) $12.45 /m
Ceiling dogs $ 0.55/ each
Length = internal room length + (2 x wall thickness) + (100 mm for laps if required) Number = width of room – 1
Max. spacing
•
Max. span is 3600 mm, therefore join joists on the hanger in the centre of the room;
•Length = 6700 + 200 + 100 for join = 3.5m
2
Order - 16 / 3.6 •Length = 3400 + 200 = 3.6m
Order – 6 / 3.6 •Length = 4200 + 200 = 4.4m
Order – 4 / 4.5 •Length = 3200 + 200 = 3.4m
Order – 6 / 3.6 •Length = 2400 + 200 = 2.6m
Order – 4 / 2.7Order = 150 x 38 sawn Oregon F8 – 4/ 4.5, 28/ 3.6, 4/ 2.7 Ceiling joists Room A1 = 5250 – 1 600 = 9 – 1 = 8 Room A2 = 4200 – 1 600 = 7 – 1 = 6 Room A3 = 2650 – 1 600 = 5 – 1 = 4 Room A4 = 4200 – 1 600 = 7 – 1 = 6 plus = 2650 – 1 600 = 5 – 1 = 4
Length = max. spacing – (2 x joist thickness) Number = length of wall – 1
Max. spacing
•
Length = 600 – (2 x 38) = 524 mm
∴37 x 0.524m = 19.4m
Order = 100 x 38 sawn Oregon F5 – 1/ 6.0, 3/ 4.5
Required as per specification.
Length = span of room + (2 x wall plate width)
Order = 240 x 45 seasoned hardwood F27 – 1/ 5.7
Order = 190 x 35 seasoned hardwood F11 – 1/ 3.0
Allow one per joist, per hanger, per room
Order = 20 ceiling dogs Ceiling trimmers Hangers Connectors Room A1 = (6700 – 1) x 2 walls 600 = (12 –1) x 2 = 22 Room A3 = (4200 – 1) x 2 walls 600 = (7 – 1) x 2 = 12 Room A4 = (2400 – 1) 600 = (4 – 1) = 3 Room A1 = 5250 + ( 2 x 100) = 5450 = 1/ 5.7 Room A3 = 2650 + ( 2 x 100) = 2850 = 1/ 3.0 Room A1 = 16 Room A3 = 4
ALTERNATIVE CEILING TYPES
Flat Roof Construction
Generally, the construction for a flat roof combines the roof frame and ceiling frame to form one structure, which carries the load of the roof and the ceiling linings.
A flat roof is one which is pitched at less than 10° or has a slope of less than approximately 1in 6. To allow for this extra load the rafters/ceiling joists are increased in section size and stress grade. They normally have a single span and are lined on-the-rake, either on top or under the rafters. The roof surface is covered with full-length sheets of corrugated iron, metal tray or decking sheets, clear or coloured fibreglass sheets and/or clear or coloured polycarbonate sheets.
Fig. 17 Flat roof construction
To provide a fall for the roof covering, where the ceiling frame is to be level, different thickness battens may be used. These are referred to as ‘grading battens’. The batten at the guttering end is
the thinnest and the other battens gradually increase in thickness to the high end of the roof. This may require some battens to be laid on their flat, some on their edge and some may need to be checked-in slightly to achieve the correct height, at the nominated spacing.
To prevent sideways movement of the ceiling frame members, solid blocking is provided where the joists span more than 2100 mm. Ceiling trimmers are fixed the same as for gable and hip roofs where the internal walls run parallel to the joists.
Batten Sarking Rafter Top plate Solid blocking Trimmer Stud
FLAT ROOF CONSTRUCTION – SMALL SLOPE PROVIDED
Solid Blocking
Barge board
Trimmer
Batten Top plate
Rafter Internal wall CeilingTrimmer Sarking Fascia Metal tray decking
Metal tray decking
Fall
Fall
Fall Metal tray decking
Metal tray decking
Batten thickness graded to provide fall
Vapour barrier sarking insulation Solid blocking where span
exceeds 2100
Batten thickness graded to provide fall to roof gutter Vapour barrier, sarking, insulation laid loosely over joists
Batten thickness graded to provide fall to roof gutter Sarking lapped and taped
Sheets joined over a joist EXPOSED JOISTS CEILING:
ALTERNATE FINISH LAY-IN PANEL INFILLS TO CEILING: CONVENTIONALLY LINED CEILING:
Skillion Roof Construction
This type of freestanding roof/ceiling frame is usually constructed by having the wall frame at one end of the building higher than the other end. Internal walls running parallel to the end walls would be built at different heights, depending on their location in the building. Walls running at 90° to the end walls would taper in height to fit under the sloping rafters/joists. This type of roofing system may also have its ceiling lined on-the-rake or be fitted as a false ceiling and placed level, as shown below:
Fig. 19 Basic design of the skillion roof/ceiling system
The simplest method of marking the skillion rafters/ceiling joists is to scribe them over the supporting plates in position. Once cut they are spaced at the maximum centres, to suit the battens and roof sheets, and then fixed into position by double skew nailing to the plates.
They are also connected to the plates with patent metal connectors to prevent wind uplift forces.
Fig. 20 Practical method of marking rafters/ceiling joist RAKED CEILING
LEVEL CEILING
Rule (first position) Rafter rested on wall plates
Rule (first position)
Mark Low wall Rule (second position) Rule (second position) High wall Stud Mark Notching at top Notching at foot Eave width Eave width
Lean-to Roof Construction
Fig. 21 Various methods used to line a lean-to roof/ceiling
This type of roof system is constructed against an existing wall or other roof structure.
It is mainly used for simple extensions, carports, awnings, verandahs, etc. and provides an alternative to re-pitching the
existing main roof to cover the extra room or space.
The ceiling finish may be one of the following:
i) The rafters and the covering may be left exposed for a carport, verandah, awning, etc;
ii) The ceiling may be fixed to the underside of the rafters, making it a raking ceiling; or iii) A separate ceiling frame or false ceiling may be installed to give a level ceiling line.
Attached to wall under eaves
Top of rafters may be lined with fibrous cement sheeting to improve appearance
i) OPEN, EXPOSED RAFTER TYPE
Abutting main roof Adjoining high wall
GABLE ROOFS
The gable roof is classified as being double-pitched and one of the simplest roof forms, due to the fact that all rafters in the roof are exactly the same length and have the same bevels.
Gables are best suited for use on buildings or structures with a simple quadrilateral shape, which is typical of the freestanding car garage, most outbuildings, lichgates (or lychgate), portico’s, etc.
They may also be used in a modified form, such as a gablet, to enhance the surface of any other roof type or may also be used as a means of providing light and ventilation to a room or roof space. Many modern roof designs use dummy gables to enhance plain designs or to break up large areas of straight roof surface.
In a conventionally pitched gable roof there are many individual members, which have specific structural roles to perform. Each member is reliant on the next to form an unyielding structure and at the same time provide a framework for the roof covering.
PARTS, PROPORTIONS and DEDINITIONS
Span: This is the horizontal width of the roof, measured overall the wall plates.
Half span or Run of rafter:
This is the horizontal distance measured from the centre of the ridge to the outside of the wall plate. It is also the plan length of the rafter.
Centre line length of
rafter:
This is measured along the top edge of the rafter taken from the centre of the ridge to plumb over the outside of the wall plate. It is equal to the length of the hypotenuse of the right-angled triangle formed by the rise and half span.
Hypotenuse: This is the sloping length of a right-angled triangle.
Rise: This is the vertical distance between the ‘X-Y’ line and where the hypotenuse meets the centre of the ridge.
X-Y line: This is an imaginary horizontal line, which passes through the position where the outside of the walls is plumbed up to meet the hypotenuse or top edge of the rafter. It is used to identify the centre line positions to calculate rafter set out length and the rise of the roof.
Plumb bevel: This is the angle found at the top of the right-angled triangle, formed by the rise, half span and top of rafter edge. This bevel is used for the angled cut on the top end of the common rafters.
Level bevel: This is the angle found at the bottom of the right-angled triangle, formed by the rise, half span and top of rafter edge. This bevel is used for the angled
STRUCTURAL ROOF MEMBERS
COMMON RAFTERS
These are the main sloping members, which all have the same length, running from the wall plate to either side of the ridge. They are spaced at 450 to 600 mm centres for tiled roofs, and up to 900 mm centres for sheet roofs. They support the roof battens, which in turn support the roof covering.
The rafters may be set out using a variety of methods, which include use of the steel square, full size set out and by calculating length.
Since the rafters are all the same lengths, they are usually set out from a pattern. This pattern has the cutting length, plumb cuts and birdsmouth marked on it to allow for consistent accuracy during repetitive mark transfer.
Note:Section size, timber species and stress grades for rafters may be obtained from AS 1684.
Eaves width: This is the horizontal distance measured between the outside face of the wall frame, for a timber-framed cottage, or the outside face of the
brickwork, for a brick veneer and cavity brick cottage, to the plumb cut on the rafter end .
Eaves overhang:
This is the distance measured along the top edge of the rafter from the position plumb up from the outside of the wall frame, where the X-Y line passes through the hypotenuse, to the short edge of the plumb cut on the
end of the rafter.
Birdsmouth: This is a right-angled notch taken out of the lower edge of the rafter, where it rests on the top wall plate. The purpose of the birdsmouth is to locate the bottom of the rafter over the wall plate and to provide an equal amount
left-on so the top edges of the rafters will all be the same. This is only necessary when rough sawn timber is used. The depth of the notch should not be greater than ²/3 the width or depth of the rafter, to prevent it from
being weakened.
Height of the roof:
This is the vertical distance taken from the top of the wall plates to the top of the rafters where they butt against the ridge.
Note:this should not be confused with the ‘Rise’ of the roof .
C e n t r e l i n e l e n g t h o f r a f t e r T r u e l e n g t h o f r a f t e r T r u e l e n g t h o f o v e r h a n g Y X
Fig. 24 Bevels for the common rafter
RIDGE
Usually a deep and narrow member, it is the highest member of the roof, which runs
horizontally for the length of the roof. It must be level and parallel to wall plates, for the length of the roof with the rafters being nail-fixed onto it on opposite sides.
Gable roofs may be very long, therefore the ridge may require one or more joins to create a continuous length, as shown below:
Fig. 25 Methods of joining ridge boards
Before the roof is erected, the ridge is set out (usually on one side only) to suit the position of the rafters. The easiest way to set out the ridge is to lay the ridge on top of the completed ceiling frame, over the external wall plate, and transfer the rafter positions onto the ridge board. This ensures the rafters will be parallel and consistent with the positions marked along the wall plate.
True length of reduction for ½ thickness or ridge
Plumb bevel ½ Thickness of ridge on plan Allowance for plumb cut Level bevel Birdsmouth notch LEVEL BEVEL PLUMB LEVEL
SCARF JOINTED CLOSE BUTT JOINTED
Joint spliced with full depth timber fish plates each side, 25 mm thick Scarf jointed at abutment of rafter pair Ridge Rafters
Fig. 26 Setting out the rafter positions onto the ridge board
WIND BRACING
Effect of racking of the roof
Fig. 27 Inclined wind bracing
Wind bracing is designed to prevent any movement of the roof, or racking out of plumb. Wind forces on the gable ends
usually cause racking.
Bracing of the roof frame may be done by having two opposite 45º timber braces from the ridge onto an internal load-bearing wall, normally using 75 mm square timber. Alternatively, the roof frame may be
permanently braced using metal speed bracing over the surface of the frame.
Temporary bracing may also be inclined, as shown, or be diagonally fixed under the rafters from the ridge to the external wall plate.
Ceiling joists
Rafter positions marked onto ridge Stiffner Gable rafter Inclined brace Ridge Chock 45° Top plate
PURLINS PURLINS
Purlins, also called
Purlins, also called underpurlins,underpurlins, are fixed to the underside of the rafters parallel to the ridgeare fixed to the underside of the rafters parallel to the ridge and wall plates. They provide continuous support under the rafters, similar to bearers under and wall plates. They provide continuous support under the rafters, similar to bearers under joists in a floor frame.
joists in a floor frame.
They are normally spaced at 2100 mm centres, but this will depend on the section size and They are normally spaced at 2100 mm centres, but this will depend on the section size and stress grade, including the section size and stress grade of the rafters.
stress grade, including the section size and stress grade of the rafters.
Fig. 28
Fig. 28 Spacing of Spacing of purlinspurlins
Joining purlins Joining purlins
Fig. 29
Fig. 29 Joining purlins over Joining purlins over a supporta support
Positioning purlins Positioning purlins
Purlins are supported by struts at 2100 mm Purlins are supported by struts at 2100 mm centres, depending on the section size and centres, depending on the section size and stress grade, with an additional strut under stress grade, with an additional strut under any join.
any join.
The most common method of joining is to The most common method of joining is to half-lap and nail together. Joints may also be half-lap and nail together. Joints may also be cleated to prevent spreading by using timber cleated to prevent spreading by using timber or metal connector plates.
or metal connector plates.
Purlins are positioned by measuring up from Purlins are positioned by measuring up from the wall plate, the desired spacing, on the the wall plate, the desired spacing, on the underside of the end rafters, and marking the underside of the end rafters, and marking the top side of the purlin thickness. A string line top side of the purlin thickness. A string line or chalk line is run through and a temporary or chalk line is run through and a temporary 75 mm nail driven into the underside of 75 mm nail driven into the underside of every third rafter. The purlin is lifted into every third rafter. The purlin is lifted into position and pulled hard up against the position and pulled hard up against the
temporary nail, clamped and double skew temporary nail, clamped and double skew nailed.
nailed.
Note:
Note:Each rafter is sighted for straight Each rafter is sighted for straight
before being nailed to the purlins. before being nailed to the purlins.
2 2 1 1 0 0 0 0 m m a a x x 2 2 1 1 0 0 0 0 m m a a x x Halved scarfing Halved scarfing Purlin Purlin Strut Strut Rafter Rafter
Rafters sighted for straight prior Rafters sighted for straight prior to nailing off to nailing off Purlins cramped Purlins cramped to rafter to rafter 75mm nail 75mm nail Desired spacing Desired spacing up from wall to up from wall to centre of purlin centre of purlin
STRUTS STRUTS
These members are placed under the purlins to transfer the roof load to the internal load-bearing These members are placed under the purlins to transfer the roof load to the internal load-bearing walls, strutting beams or between other struts or supports.
walls, strutting beams or between other struts or supports.
There are several strutting systems used for roofing, as follows: There are several strutting systems used for roofing, as follows:
Inclined and Flying or Fan struts Inclined and Flying or Fan struts
These struts are cut around the purlin and run at 90º, or as near as possible, to the underside of the These struts are cut around the purlin and run at 90º, or as near as possible, to the underside of the purlin to the top of a load-bearing wall or strutting beam. Ch
purlin to the top of a load-bearing wall or strutting beam. Chocks are placed behind the foot of theocks are placed behind the foot of the strut to prevent it sliding under load. Flying or Fan struts are chocked at the top as well by fixing strut to prevent it sliding under load. Flying or Fan struts are chocked at the top as well by fixing blocks onto the face of the purlin, on the outside edge of the strut.
blocks onto the face of the purlin, on the outside edge of the strut. Alternatively, they may be bolted through the purlin or have a sprea
Alternatively, they may be bolted through the purlin or have a sprea der bolted across the fader bolted across the face toce to prevent them sliding apart under load.
prevent them sliding apart under load.
Solid timber struts are normally 75 x 75 mm, however this will depend on the stress grade, length Solid timber struts are normally 75 x 75 mm, however this will depend on the stress grade, length of strut and imposed roof load. The following table provides a guide for imposed roof loads, of strut and imposed roof load. The following table provides a guide for imposed roof loads, which will assist in the selection of strutting material:
which will assist in the selection of strutting material:
TABLE 1 TABLE 1
MASS OF ROOF MATERIALS MASS OF ROOF MATERIALS
Average mass of metal roof
Average mass of metal roof covering and battens is approx. 15 kg/m²; andcovering and battens is approx. 15 kg/m²; and Average mass of tiled roof covering and battens is approx. 60 kg/m².
Average mass of tiled roof covering and battens is approx. 60 kg/m².
Note:
Note:The size of struts should be based on AS 1684 - 1999 Part 2The size of struts should be based on AS 1684 - 1999 Part 2
Fig.
Fig. 31 31 Common Common struttistrutting ng methodsmethods TYPE
TYPE MATERIAL MATERIAL Approx. Approx. MASS MASS in in kg/m²kg/m² Steel
Steel sheet sheet 0.76 0.76 mm mm thick thick 10.010.0 0.55
0.55 mm mm thick thick 6.06.0 Aluminium she
Aluminium sheet et 1.2 mm thick 1.2 mm thick 5.05.0
Roof
Roof tiles tiles Terra-cotta Terra-cotta 58.058.0
Concrete 54.0
Concrete 54.0
Pressed
Pressed metal metal 7.57.5
Fibre
Fibre cement cement (F.C.) (F.C.) Corrugated Corrugated sheet sheet 16.016.0 Flat
Flat shingles shingles 15.015.0 Unseasoned
Unseasoned hardwood hardwood 38 38 x x 75 75 battens battens at at 900 900 mm mm c/c c/c 3.23.2 25
25 x x 50 50 battens battens at at 330 330 mm mm c/c c/c 3.83.8 Seasoned
Seasoned pine pine 35 35 x x 35 35 battens battens at at 330 330 mm mm c/c c/c 2.02.0
Chock Chock Rafters Rafters Chock Chock Alternative Alternative spreader bolted spreader bolted to struts to struts Flying Struts Flying Struts Chock Chock Load-bearing Load-bearing wall wall Rafter Rafter Purlins Purlins Inclined Strut Inclined Strut Chock Chock Load-bearing Load-bearing wall wall Chock Chock
SINGLE INCLINED STRUT
SINGLE INCLINED STRUT FLYING OR FAN STRUTSFLYING OR FAN STRUTS
ELEVATION
ELEVATION SECTIONSECTION ELEVATION
ELEVATION SECTIONSECTION
Chock Chock
Fitting struts to
Fitting struts to various anglesvarious angles
It may not be possible to fit
It may not be possible to fit the struts at exactly 90° to the the struts at exactly 90° to the underside of the purlin, therefore theunderside of the purlin, therefore the following adjustments may be made:
following adjustments may be made:
Rafter Rafter 90° 90° 'x' Small variations 'x' Small variations permitted (approx. permitted (approx. or truly or truly perpendicular) perpendicular) Struts Struts 'A' Not less than 44 mm
'A' Not less than 44 mm 'B' Not less than 25
'B' Not less than 25 mm and not over 38 mm and not over 38 mm measured atmm measured at bottom of purlin
bottom of purlin
STRUT PERPENDICULAR TO RAFTER STRUT PERPENDICULAR TO RAFTER
B B A
A A A
B B
Rafter Rafter 'Y' Small variation 'Y' Small variation permitted (approx permitted (approx or truly vertical) or truly vertical) Top of strut Top of strut must reach at must reach at least to top least to top edge of purlin edge of purlin Struts Struts C C CC D D
C' Not less than 38 mm C' Not less than 38 mm 'D' Not over 12 mm 'D' Not over 12 mm
STRUT VERTICAL OR 'PLUMB' TO RAFTER STRUT VERTICAL OR 'PLUMB' TO RAFTER
Strut Strut
2/2.5 x75 mm nails 2/2.5 x75 mm nails Not less than 5 nails Not less than 5 nails at least twice length at least twice length of chock thickness of chock thickness chock chock Stiffener Stiffener Studs Studs Low angle Low angle flat strutting flat strutting
NOTE: Long chock required to prevent slip
NOTE: Long chock required to prevent slip caused caused by greater thrust
by greater thrust
LOW ANGLE FLAT STRUT LOW ANGLE FLAT STRUT
Strut Strut Perpendicular Perpendicular or steep angle or steep angle strutting strutting
Not less than 3 nails Not less than 3 nails at least twice length at least twice length of chock thickness of chock thickness 2/2.7x75 mm nails 2/2.7x75 mm nails
Top wall plate Top wall plate NOTE
NOTE : Short chock required due to a more direct: Short chock required due to a more direct load to the wall
load to the wall
STEEP ANGLE PERPENDICULAR STRUT STEEP ANGLE PERPENDICULAR STRUT 'Z' Any angle from 0° upwards, provided that the
'Z' Any angle from 0° upwards, provided that the strut is not flatter than 1 vertical to 2 horizontal units strut is not flatter than 1 vertical to 2 horizontal units for a roof slope of 1:2, or 1 vertical to 1.5 horizontal for a roof slope of 1:2, or 1 vertical to 1.5 horizontal units for a roof slope of 5:12
units for a roof slope of 5:12
E
E EE
E' Not less than 38 mm, but not more than ½ width of E' Not less than 38 mm, but not more than ½ width of strut.
strut.
FLAT STRUTTING FLAT STRUTTING
Supporting struts over internal walls Supporting struts over internal walls
It is preferable to position struts directly over studs, however as this is not always possible the It is preferable to position struts directly over studs, however as this is not always possible the load must be distributed in an alternative way. This may be achieved by strengthening the plate load must be distributed in an alternative way. This may be achieved by strengthening the plate either between or over the studs with additional blocking.
either between or over the studs with additional blocking.
Also, where struts are placed at a low or flat angle, it will be necessary to fit a block, referred to Also, where struts are placed at a low or flat angle, it will be necessary to fit a block, referred to as a
as a chock,chock, behind the foot of the strut to prevent it from sliding under load.behind the foot of the strut to prevent it from sliding under load.
Fig. 33
Fig. 33 Methods of Methods of reinforcing top wall reinforcing top wall plates for strutsplates for struts
Chock with not Chock with not less than 3 nails at less than 3 nails at least twice length least twice length of chock thickness of chock thickness Strut
Strut
Top wall plate Top wall plate
Intermediate blocking to Intermediate blocking to stiffen top plate
stiffen top plate
STRUT BEARING BETWEEN STUDS STRUT BEARING BETWEEN STUDS
Top wall plate reinforced Top wall plate reinforced over two studs. Stiffener over two studs. Stiffener 50mm thick, full width of 50mm thick, full width of wall plate
wall plate
NOTE
NOTE : Avoid strutting between studs: Avoid strutting between studs
ALTERNATIVE STIFFENING METHOD ALTERNATIVE STIFFENING METHOD Braces, 38mm
Braces, 38mm thick, full stud thick, full stud width
width
ALTERNATIVE STIFFENING METHOD ALTERNATIVE STIFFENING METHOD Top plate locally reinforced to
Top plate locally reinforced to distribute load over two or distribute load over two or more studs
more studs
NOTE
NOTE : Avoid strutting over openings: Avoid strutting over openings
STRUTTING OVER DOOR OPENINGS STRUTTING OVER DOOR OPENINGS
Lintel deemed to be a Lintel deemed to be a strutting beam. Refer to strutting beam. Refer to AS 1684
AS 1684
Strut landing only if Strut landing only if unavoidable unavoidable Preferred strut Preferred strut landing over a landing over a stud stud
STRUTTING OVER AN OPENING STRUTTING OVER AN OPENING
Scissor struts
These struts consist of deep-sectioned timber members supported over external walls and bolted where they cross in the centre of the roof space.
They are designed to transfer the roof load to the external walls where there are no internal walls for support or the internal walls are non load-bearing.
Fig. 34 Full scissor type strutting
If there is internal support available, but would cause an inclined strut to be too flat to be effective, then a half scissor may be used.
Fig. 35 Half scissor type strutting
Note:The foot of the scissor struts must be bolted
to a rafter, and preferably a ceiling joist as well, and bolted together where they cross over in the centre of the roof. If ceiling joists are not full length they should also be bolted together where they lap over a wall, to prevent the external walls spreading under load.
Section sizes and stress grades should be taken from AS 1684 - 1999 Part 2
20 Bolt, nut and washer Spacer block
∅12 Bolt, nut and
washer or well nailed Underpurlin
Scissor struts
NOTE: Scissor struts must be
kept clear of hangers Hanger 20 Bolt, nut and washer to spliced joint of tie member Hanger
20 Bolt, nut and washer to top connection. Use∅20 for all other
connections.
Half scissor strut
Tie member required where ceiling joist can not be used as bottom chord of truss Regular purlin strutting Strut Common rafter Scissor strut 15 min 20 bolt Ceiling joist (tie member)
Strutting beams
An alternative to using scissor struts over large room spans would be the use of large timber or steel strutting beams.
They are usually placed parallel to the ceiling frame hangers, but must not rest on any part of the ceiling frame. To achieve this the ends must be packed up at least 25 mm above the ceiling joists, which allows for any deflection.
In some situations the hanger and strutting beam may be the one member, but the member should comply with AS 1684 tables or be designed by a structural engineer to cater for the additional load.
The following materials may be used for strutting beams:
• Solid timber (hardwood or softwood with the correct stress grade); • Horizontally laminated timber (similar to Glulam beams);
• Vertically laminated timber (similar to L.V.L. beams); • Boxed beams (made from plywood); and
• Steel beams (either Universal channels, U.C., or Universal beams, U.B.)
Note: Refer to AS 1684 for section sizes and stress grades of timber members. If deep solid
timber is used, it should be seasoned to reduce the risk of shrinkage. Most timber species over 175 mm deep will shrink excessively.
Fig. 37 Positions of strutting beams
An alternative position for the strutting beam is directly under the rafters, which are notched over it as if it were another wall plate. The plumb struts under it would then rest on an internal load-bearing wall. Rafter Purlin Inclined Strut Chock 12mm max Purlin 38mm min Plumb strut
Deep strutting beam 25mm clearance to allow for deflection
Ceiling joist STRUTTING BEAM RUNNING
PARALLEL WITH HANGER STRUTTING BEAM RUNNINGPARALLEL WITH HANGER
Strutting beam See adjacent detail Beam Plumb strut Birdsmouth to rafter provides secure seating to beam
PATENT TYPE STRUTTING Super Barap
This is a patent type of strutting system used where conventional strutting methods cannot be used or it will be too expensive to use them. These patent struts consist of steel saddle brackets at either end connected by a 12 mm diameter steel rod and a centrally located adjustable fulcrum. They may be used under purlins, rafters, hips or valleys to provide the required support where sagging or excessive deflection of the member has or may occur.
This system operates similar to a truss as it is made up of ties, which are in tension, and a strut, which is in compression. They may be purchased through hardware stores or direct from the manufacturer.
Note:See manufacturers brochure and specification for further details and fitting instructions.
Fig. 39 Patent type Super barap strut/brace
Cable truss
This is another very effective patent type system, similar to the Super Barap, which uses two tensile steel cables instead of a solid steel rod. Each cable is made up of 7/ 1.6 mm Ø wire strands. The truss may also be fitted with an additional adjustable fulcrum or strut for use on longer members. The ends of the cables must be bolted within 200 mm Max. of the end supports.
To calculate the length of the truss, measure the distance between the bolted ends and add 80 to 150 mm to allow for the cables to run over the single or double fulcrums.
Note:See manufacturers brochure and specification for further details and fitting instructions.
Maximum span 2 fulcrums
Saddle bracket Tie rod Strut 6.700m Adjustable fulcrums Purlin Saddle bracket Strut
Maximum span 1 fulcrum 5.500m
Purlin
Saddle bracket
Strut Adjustable fulcrum
Saddle bracket Tie rod Strut Purlin Support block
Twin wire support system Adjustable fulcrum Rafter 'Tyloc' plates and bolt
COLLAR TIES
Fig. 41 Collar tie fitted to rafter over purlin
Note: The size of collar ties depends on the stress grade and length of timber used. As a guide,
they are normally 75 x 50 or 125 x 38 F5 to F7 up to 4200 mm long, and 100 x 50 or 125 x 38 F5 to F7 over 4200 mm long. Refer to AS 1684 for specific details.
Fig. 42 Placement of collar ties in the roof frame
They are light sectioned horizontal members used for additional support, like spreaders, to prevent the rafters from sagging at the purlin position. These are fixed to alternative pairs of rafters, i.e. at 900 to 1200 mm spacings, and placed on top of the purlins running parallel to the ceiling joists. They may be half scarfed around the face and edge of the rafters and nail fixed with 2/75 mm
nails.
Alternatively, they may be run past the face of the rafters and be bolted to them at both ends using a single 10 mm Min. mild steel, cuphead bolt.
Common rafter Collar-tie half scarfed or bolted to rafter Strut Underpurlin
Placed every 2nd pair of rafters, 900 to 1200 mm apart
GABLE ENDS
There are three main methods used to finish the ends of gables: 1. Flush gable with no eaves;
2. Flush gable with raked eaves; and 3. Boxed gable.
Flush gables (no eaves)
Fig. 43 Flush gable
Fig. 44 Framed flush gable
The end of the gable is flush or in-line with the outside face of the end wall. The end of the roof has no overhanging eaves, only a barge fixed flush with the outside of the end
wall. This finish may be applied to timber framed cottages, where the walls are clad with boards or sheeting, or to brick veneer and
cavity brick cottages, where the brickwork runs to the underside of the roof covering. The triangular section formed between the top of the standard wall frame and the underside of the rafters is framed with stud material, spaced at the same centres as the wall frames, fixed on flat or on edge.
Gable studs for cladding fixing or trying to brick work.
Flush gables with raked eaves
Fig. 45 Flush gable with raked eaves
Fig. 46 Framing for eaves lined on-the-rake
The gable finish for this type is similar to that of a gable with no eaves. The main difference is the end of the roof frame is extended past the end wall to form an eaves overhang, which is lined on the rake.
The ridge and top wall plates may be extended to provide support for the gable rafters. Where the overhang is particularly wide or the length of the gable rafters is excessive, the purlins may also be extended to provide additional support.
Where the raked ends are required to adjoin level side eaves, the ends of the eaves are usually boxed to allow the raked section to terminate neatly. Wall stud Top plate Gable stud Trimmer Trimmers Rafters
Framing variations for raking eaves
The top wall plates may be extended to take the gable rafters.
It is only necessary where the ends of the side eaves are to be boxed, which will allow the raked gable eaves to terminate neatly.
Note:It is necessary to extend the ridge for
this method as well.
Fig. 47 Extended top plate
When the eaves width at the gable ends is excessive, i.e. say greater than 450 mm, or the unsupported length of the gable rafter is
excessive for the section size of rafter, then it may be necessary to extend the purlins on both sides to support the mid length of the gable rafters.
Also, with some roof design or when the eaves are lined on top of the rafters, it is desirable to expose the framing members.
(this was a typical method used for the ‘Bungalow’ style of cottage during the Federation period)
Fig. 48 Cantilevered gable framing
To provide continuous raked eaves with no framing members visible, it will be necessary to place cantilevered trimmers to support the gable rafters.
These trimmers, also known as outriggers, are either checked into the rafters on their flat or will be supported on-edge over the gable end wall frame, which has raking top plates. Short rafter trimmers are then cut between them to provide fixing for tile or roof sheet battens.
Fig. 49 Extended purlin Stiffener to support extended top plate Trimmers or outriggers Gable rafter Purlin
Boxed gable
A boxed gable occurs where it is desirable to have level eaves on both sides and ends of the roof. The face of the boxed gable may be clad with the same material as the end wall, but may also be featured by cladding with an alternative material finish.
The fascia may be returned level around the corner or the barge may extend to the outside of the gutter and have a small timber ‘bellcast’ added to the top edge.
The end of the boxed gable is framed up with gable studs, eaves trimmers and a full width bottom chord or tie, to allow for fixing of the
cladding and eaves soffit lining.
Fig. 51 Framing for a boxed gable
Soffit lining Fascia
Gable cladding Gable stud
Plates and purlins extended for support Batten for fixing Soffit bearer Top plate Ceiling joist Purlin Ridge Ceiling trimmers
Verge finishes
The verge is the section at the end of the gable roof where the roof surface meets the barge or verge board. The type of finish will depend on the roofing material used and the finish required. Tiled roofs may have a coloured mortar pointed verge, which is laid on a narrow fibre cement (F.C.) strip, it may be covered with purpose made barge cover tiles or the tiles may be cut against a pre-formed ‘barge soaker’. In recent times, the Colorbond barge soaker has become the
preferred method of finishing the verge as it does not require any maintenance.
Metal sheet roofs may also have a barge soaker or may be fitted with a covering pre-formed Colorbond barge capping.
Fig. 52 Metal barge capping profiles
Fig. 53 Pointed verge Fig. 54 Barge tiled verge
Screw Fixing Pop
rivet Clip
Clip and/or bracket fixing
Vapour barrier
Standard ridge tiles
Tiles bedded and pointed
Fibrous cement strip Reflective foil insulation/sarking Barge cover tiles
Steep angle ridge Vapour barrier Gutter bracket Barge capping Fixing clips
NOTE: Metal barge capping may also be used on the verge of tiled roofs
End cap Ridge capping
‘Colorbond ® ’ patent verge finishes
In recent years the method of finishing the verge for gable roofs has changed. New metal fascia and barge profiles provide an alternative to using primed fascia boards. They are attached directly to the ends of the rafters or outriggers, using special brackets, and the gutter or barge soaker is attached to them.
The benefits of using these Colorbond ® products is that they do not require painting, they are
not susceptible to decay caused by leaking gutter joints, they do not cup or twist, they are available in a full range of popular colours and they are relatively simple to fit.
The Colorbond ® metal barge is attached using barge rafter brackets and then the Colorbond ®
metal barge soaker is placed over the top. The benefit of using the barge soaker removes the need to bed and point the verge, which eventually cracks and becomes loose over time.
The following details were supplied courtesy of ACE GUTTERS PTY LTD.
Fig. 57 Barge, soaker, and accessories
Steel Fascia 2 6 m m 180mm 10mm 3 6 m m 97mm 70mm 28mm 95mm 16mm
Barge Rafter Bracket Straight Joiner Barge Mould L.H. & R.H.
Barge Soaker
Barge Apex Cover
CALCULATING ‘DROP-OFF’
The finished height of the brickwork is determined by the height of the timber frame and the drop-off needed to give the eaves width required. In turn the pitch of the roof and the head height of the windows influence the eaves width and drop-off.
Where eaves soffit finishes above the head of the windows, the space may be infilled with brickwork or with timber framing and cladding material.
The drop-off measurement is taken vertically from the top of the wall plate to finished height of the brickwork. The purpose of the drop-off measurement is to provide the bricklayer with a finished height, in relation to the height of the wall frame, to allow an even gauge to be set out on the storey rod .
Note:In a brick veneer cottage with a suspended timber floor, the brickwork is normally
completed to the underside of the bearer and then the brick gauge is calculated to drop-off level.
Same Roof Pitch 30°
Drop-off
Larger eaves width
LARGER DROP-OFF (LOWER WALL) SMALLER DROP-OFF
(HIGHER WALL) Smaller eaves width
Drop-off Same Roof Pitch
30°
Larger Roof Pitch 30°
Smaller Roof Pitch 22½°
Larger eaves width
SAME WALL HEIGHT SAME WALL HEIGHT
Method used to calculate drop-off
To calculate the amount of drop-off , it will be necessary to use the mathematical process known as trigonometry, which deals with the measurement of sides and angles of triangles, i.e. sine, cosine and tangent.
The method required to calculate drop-off is tangent or simply Tan, when:
•
Tan = opposite side ( Note: Use the Tan button on a scientific calculator) adjacent side•
θ= the known angle of the triangle or the pitch of the roof, e.g. 30°, 22.5°, etc.Example 1:
Find the drop-off measurement for a brick veneer cottage with a roof pitched at 30° and a required eaves width of 400 mm.
Formula = Tan
θ= opposite side
adjacent side
To find the opposite side, transpose the formula by cross multiplying to allow the unknown measurement,
i.e. opposite side, to be on its own:
Fig. 59 Detail of eaves
∴
The total drop-off as measured from the top of the wall plate to top of the brickwork will be:317 + 20
(which is the depth of the birdsmouth) =337 mm
Tan 30°
=
opposite side
400 + 110 + 40
Tan 30°
=
opposite side
550
∴
Tan 30°
1
x
opposite side
0.550
= (opposite side x 1) = ( Tan 30° x 550)
To find Tan 30°, insert 30 into the
calculator and then press the Tan button. The answer will equal 0.577350269.
Reduce this to 3 decimal places and use it for the remainder of the calculation, i.e.
0.577