The common rafters for a gable roof are all the same length, have only two bevels and may be set out from one pattern rafter. The length of this pattern rafter may set out by calculation, as previously mentioned in calculating roof pitch, or be set out using a steel roofing square, which
will also have the two bevels required for the rafter.
Proportions of common rafters
Some of these proportions were dealt with earlier in this unit. The critical elements are shown below:
Fig 83 Common rafter proportions C e n t r e
l i n e s e t - o
u t 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
True length of reduction for ½ thickness of ridge
Plumb bevel
½ Thickness of ridge on plan
PLUMB BEVEL LEVEL BEVEL
Birdsmouth notch
Allowance for plumb cut
Level bevel
Plumb and Level bevels
The simplest method used to establish the plumb and level bevels is the ‘ pitch board method’ . Once the rise per metre run is known it is simply a matter of reducing the full measurements to a scaled size, say by dividing both measurements by 10.
This means that a roof with a pitch of 577 mm to 1000 mm, or 30°, divided by 10 would be:
These smaller measurements are then set out to look like half the roof on a piece of timber or board material. A sliding bevel is then laid against the edge and adjusted to suit the angle:
Fig. 84 Pitch board method
Fig. 85 Setting sliding bevels to the plumb and level bevels
Note: These bevels may be transferred to the rafter or any other roof member, which requires a plumb or level bevel.
577
10 =
57.7mmand
1000
10 =
100mmSTEP 1 Set up the pitch board by drawing the half roof shape, using the scaled measurements.
STEP 2 Lay a sliding bevel against each edge and adjust the blade to suit the angles formed.
1 0 0
5 7
. 7
Level bevel Plumb
bevel
Sliding bevel set to the plumb bevel angle
Sliding bevel set to the level bevel angle
THE STEEL SQUARE
A common method used to set out the length of a rafter is to use the steel roofing square.
This is a very versatile tool as it may be set up with the plumb and level bevels within the 90°
triangle formed by the square and the adjustable fence or buttons. Again, the pitch of the roof is set up on the square using scaled measurements, which in this case are usually half the full size proportions.
Example 1:
If the pitch ratio is 1 : 1.732 or 30° it is firstly changed to a rise per metre run, which equals 577 to 1000 mm, then these measurements are halved to become 288.5 to 500 mm.
Setting up the square
Fig. 86 The steel square
Fig. 87 Adjustable timber fence
STEP 1 The proportion for the rise, i.e. 288.5 mm, is placed on the tongue of the square and the run or half span, i.e. 500 mm, is placed on the blade of the square.
STEP 2 A timber fence may be used to link the measurements and form the right-angled triangle.
The timber fence sits on the top edge of the rafter to be set out. This
allows it to slide along the rafter edge travelling in increments of 500 mm, until the desired distance is reached.
STEP 3 An alternative to the timber fence is the use of patent type steel buttons or clips. They are attached to the tongue and blade measurements and then the square is used the same as for the timber fence.
Note: When the square is turned upside down it may be used to set out risers and treads for stairs.
40
Patent buttons, clips or guides
There are a number of brand types available for use with the steel square.
One common type is the steel or brass button type produced by ‘Paulcall’ and manufactured in Australia.
Another common type is the steel clip type produced by ‘Starrett’ and manufactured in the
USA.
Fig. 89 Common buttons and clips
Graduations
Steel squares are marked off around the inside and outside edges of the tongue and blade in 2 mm graduations.
Also, there are 10 mm and 100 mm graduations to allow for larger dimensions to be identified.
Fig. 90 Graduations on the steel square 'Paulcall' type
'Starrett' type
Using the steel square to set up a pattern rafter
Once the pitch of the roof has been determined, set the scaled dimensions on the square, select a straight length of timber to use as a pattern rafter and step along the required number of times until the run or half span distance has been travelled.
Example 1:
Set up a steel square with a pitch ratio of 1 : 1.732 or 30° and set out a pattern rafter, which has a run or half span of 2700 mm and an eaves width of 400 mm.
Fig. 91 Steel square ready for use
STEP 1 Set the fence or buttons on the steel square to suit the scaled proportions, which will provide the pitch and bevels for the roof, i.e:
•
Pitch =1 : 1.732
•
Rise per metre run =1.000 1.732
=
0.577m (Therefore 577 mm : 1000 mm)
•
Divide both measurements by2
=
577 =
288.5 mm, and 1000 =
500 m2 2
STEP 2 Place the square over the rafter with the fence on the top edge.
Mark a plumb cut line across the face to represent the centre line of the ridge.
Measure back half the thickness of the ridge and mark a firm plumb cut line on the face. This will be the cutting line for the top of the pattern rafter.
Now move the steel square back to the original centre line ready to start stepping along the rafter.
Fig. 92 Detail of the reduction for the ridge
Fig. 93 Setting out the length plus overhang
STEP 3 Divide the measurement on the blade, i.e. 500 mm, into the run or half span:
=
2700 500
= 5.4
∴
The square will be moved along 5 times plus an additional 200 mm to equal the run or half span of 2700 mm.Note:The overhang is marked by placing the edge of the tongue on the birdsmouth plumb cut and marking off 400 mm from the blade
Plumb out True length of
reduction for ½ Plumb cut set back ½
thickness of ridge Ridge
position
Run or half span of rafter plus eaves width = 3100 Run or half span of rafter = 2700
400
l e n g t h o f c o m m
o n r a f t e r = 3 5 8 0 C e n t r e
l i n e
s e t o u t
l e n g t h o f c o m m
o n r a f t e r =
Forming bevels on the square
Once the scaled measurements are set on the square, with the fence or buttons fixed in place, the plumb and level bevels will automatically be formed in the complimentary angles of the square.
When the square is laid over the edge of a rafter these bevels may be easily transferred as the square slides along the rafter.
Fig. 94 Rafter bevels on the square
Setting out with a pattern rafter
A straight length of timber is selected for the pattern rafter, it is set out by measurement or using the steel square. It is then cut to form a finished rafter with a plumb cut at the top, plumb cut at the bottom and the birdsmouth checked out. A short length of batten is then nailed
directly above the plumb cut at the top and the birdsmouth at the bottom. The pattern rafter is now ready for use.
Lay the rafters to be cut on top of a pair of saw stools with the spring uppermost. The pattern rafter is then laid over each rafter, making sure the top edges are hard up under the short batten, and then the plumb cuts and birdsmouth positions are transferred by marking with a pencil.
Note: The purpose of positioning each rafter hard up against the short battens is to ensure all the top edges will be in-line to maintain a straight roof surface.
Although the plumb cut at the bottom of the rafter may be marked and cut at this time, it is usually better to leave the ends and cut them to a string line once the roof frame is complete.
After the first pair of rafters is marked and cut, they should be tried in place to ensure the length and bevels are correct before proceeding with the remainder.
Plumb bevel
Level bevel
Rafter
Stop batten Rafter to be marked out
Stop batten
Pattern must be straightest rafter possible Keep round edge of rafter to top always
Fig. 96 Transferring pattern set out to the rafters
Alternative pattern method
A modified version of the pattern rafter is preferred by some tradespersons to mark out the common rafters. It is called a ‘rafter boat’ , which is a short template having a plumb cut at both ends, a birdsmouth and a cleat on top to act as a guide when the boat slides along the top edge of the rafter.
All the rafters are laid across saw stools and placed together on edge so the length and position of the birdsmouth can be marked. These positions are squared across the top edge of all the rafters. The boat is moved along the top edge of each rafter to align with these marks and then the plumb cuts and birdsmouth shapes are traced onto the rafter, ready to be cut.
Note: When metal fascias are used the ends of the rafters don’t have to be exactly in-line, therefore they may be pre-cut, as the fascia brackets are adjusted to a string line before they are fixed.
Fig. 97 Using the rafter boat to set out rafters
Position 1 Position 2
Fascia cut (Pattern only)
Bird's mouth Stop batten
Rafter Rafter
Stop batten Plumb cut
Plumb cut for fascia
E a v e s o v e r h a n g
Birds mouth Plumb cut for ridge
Timber guide
Centre line length of rafter Eaves overhang
Mark plumb cut for ridge Mark birdsmouth Mark plumb cut for fascia