State
England & Wales
Northern Ireland Scotland
Regulation
B3
E4 12
Source of published statutory guidance affecting structural design
Approved Document B.
Sections 8, 9, 10, 11
Technical Booklet E, Sections 3 and 6 Technical Standards D
Table A5 Internal fire spread (structure)
England & Wales
Structure that supports only a roof does not need to be fire resisting provided that the roof does not perform the function of a floor, eg for the support of plant or is used for means of escape etc.
Typically, the structural frame for a medium-rise timber frame building would require a fire resistance of 60 minutes.
Based on a floor/floor height of nominally 2.6 m, an eight-storey medium-rise timber frame building would possess a height to the top storey in excess of 18 m. Statutory guidance would then dictate a fire resistance for the structural frame of 90 minutes.
If a basement storey deeper than 10 m is used, the requirement also becomes 90 minutes for elements of structure within the basement.
Northern Ireland
Structure that supports only a roof does not need to be fire resisting provided that the roof does not perform the function of a floor, eg for the support of plant or is used for means of escape etc.
Typically, the structural frame for a medium-rise timber frame building would require a fire resistance of 60 minutes.
The guidance in respect of fire resistance steps from a 60-minute to 90-minute requirement at a floor height of more than 20 m. Hence, in N.
Ireland, the eight-storey medium rise timber frame building described above would possess a height to the top storey less than 20 m and would be expected to have a loadbearing frame with 60 minutes fire resistance.
If a basement storey deeper than 10 m is used then the requirement also becomes 90 minutes for elements of structure within the basement.
Scotland
Structure that supports only a roof does not need to be fire resisting. It is then, however, a requirement that the roof does not support plant or is used for means of escape or parking etc.
Typically, the structural frame for a medium rise timber frame building would require a fire resistance of 60 minutes.
The actual period of fire resistance required depends on the height to the floor of the top storey above ground level:
up to 7.5m = Short,
>7.5m and up to 18m = Medium
>18m = Long.
For all elements of structure in a basement, the requirement is Medium.
The Technical Standard recommends the following fire resistance periods:
Low = 30 minutes Medium = 60 minutes Long = 120 minutes
and refers to both British and European fire test standards.
Table A6 Loadbearing elements of structure
Notes
Fire resistance in the context of loadbearing elements of structure means that the structural elements must continue to support the design load under the conditions of the fire resistance test (BS 476: Parts 20/21) for the designated period. Failure criteria for loadbearing capacity relate to collapse or unacceptably large deflections.
The latest design details for timber frame panels and connection details are contained in Timber Frame
Construction[10]. Chapter 6 of the document deals with intermediate floors (ie those located wholly within a single occupancy and that therefore are not also compartment floors) and Chapter 7 of the document deals with floors that also have to provide compartmentation. The guide shows typical structural solutions for up to and including 60 minutes fire resistance. Where higher periods of fire resistance are required then the British Gypsum Whitebook[32]provides a useful point of reference for wall and ceiling lining combinations that will provide the necessary performance. Where alternative solutions are adopted, some changes to joist/studwork and fixing specifications may be necessary.
It must be noted that any floor that is not also a compartment floor will still need to be fire resisting since it is loadbearing. The same applies to loadbearing walls.
England & Wales
The following are recommended as being constructed as compartmenting elements:
❐all floors between individual dwellings;
❐all walls separating individual dwellings from the remainder of the building;
❐all walls enclosing refuse storage chambers.
Northern Ireland
The following are recommended as being constructed as
compartmenting elements:
❐all floors between individual dwellings;
❐all walls separating individual dwellings from the remainder of the building;
❐all walls enclosing a communal waste container;
❐all walls common to two or more buildings;
❐all walls provided to divide a building into separated parts;
❐all walls or floors provided to separate different purpose groups.
Table A7 Compartmentation
Notes
The fire resistance ratings required for compartmenting elements of structure are derived as for loadbearing elements unless a specific minimum value is stipulated depending on the nature of the compartmentation. Compartmenting elements of structure are required to provide fire resistance in terms of integrity and insulation when exposed to the conditions of the fire resistance test BS 476: Parts 20/21.
The use of combustible materials in separating walls is allowed in the Scottish guidance up to 18 m (as a result of the proposed 6th amendment).
As stated in Table A2, timber will not pass the non-combustibility test however treated, and the limitation of 11 m for separating floors comprising of timber frame construction would preclude the construction of multi-storey timber frame buildings in Scotland above five storeys.
The latest design details for timber frame compartment walls and compartment floors are contained in Chapter 5 an 6 respectively of Timber Frame Construction[10]. The guide shows typical solutions for up to and including 60 minutes fire resistance. Where higher periods of fire resistance are required, the British Gypsum Whitebook[32]provides a useful point of reference for wall and ceiling lining combinations that will provide the necessary performance. Where alternative solutions are adopted, some changes to joist/studwork and fixing specifications may be necessary.
Scotland
The Scottish statutory guidance retains special forms of compartmentation known as separating walls and separating floors.
These separating walls and floor are required to have a Medium duration of fire resistance (see Table A6) and are required as follows:
❐all floors between individual dwellings;
❐all walls between individual dwellings.
For buildings with a height to the floor of the top storey not in excess of 18 m separating walls are not required to be constructed from non-combustible materials. This is on the required proviso that insulation and surfaces within the wall cavity have at least a Low Risk (see Table A4) surface spread of flame performance or are non-combustible and contains no pipes, wires or other services. The structural frame may therefore be timber but care should be taken in respect to the specification of sheathing boards that may be desired for racking resistance.
The guidance requires by implication dealing with non-combustibility
requirements for separating floors that the floor over solid waste storage rooms should be constructed as a separating floor of non-combustible construction irrespective of the overall height of the building.
England & Wales
The guidance recommends that compartment walls are carried through the roof void to the underside of the roof membrane.
Up to a building height of 15 m, combustible materials used as a substrate to the roof covering may be continued over the compartment wall provided that they are embedded in mortar or other cementitious non-combustible material over the width of the wall.
For buildings in excess of 15 m, fire rated roof coverings (classification AA, AB or AC) on a deck of a material of limited combustibility are required to a distance of 1500 mm to either side of the wall.
Northern Ireland
The guidance recommends that compartment walls are carried through the roof void to the underside of the roof membrane.
Up to a building height of 15 m, combustible materials used as a substrate to the roof covering may be continued over the compartment wall provided that they are embedded in mortar or other cementitious non-combustible material over the width of the wall.
For buildings in excess of 15 m, fire rated roof coverings (classification AA, AB or AC) on a deck of a material of limited combustibility are required to a distance of 1500 mm to either side of the wall.
Scotland
The guidance requires that separating walls are continued through the roof void to the underside of the roof membrane.
The guidance recommends the separating or compartment wall is continued through the roof by a distance of at least 375 mm except:
❐ where the roof substrate is non-combustible;
❐ the junction is fire-stopped and the roof covering is low vulnerability for a distance of at least 1.7 m to each side of the centre line of the wall;
❐in a pitched roof covered by slates that are nailed directly to sarking and underlay where the junction between the sarking and the wall head is fire stopped;
❐in a pitched roof covered by slates or tiles fixed to tiling battens and any counter battens, where only the tiling battens and underlay are carried over the wall and are fully bedded in mortar (or similar) at the wall head.
Table A8 Roof voids
Notes
Typical solutions for roof void compartmentation details provided in Timber Frame Construction[10]. Classifications for roof coverings are derived from the British Standard fire test BS 476: Part 3.
An alternative approach may be to consider designing the ceiling of the uppermost storey as fully fire resisting, ie as a non-loadbearing compartment floor with a rating of 60 minutes (for a medium rise building). This could let an unoccupied roof void be treated as a separate void compartment that may then only require lightweight cavity barriers (if at all).
A precedent for this type of approach has already been set by instances where the regulatory authorities have allowed compartmentation between the units in shared residential accommodation (eg student halls of residence) to be
terminated at the soffit of the uppermost storey. The compensatory measure has been the specification of a 30-minute fire rated ceiling lining to the uppermost storey. The reason for this alternative approach has been to simplify the amount of work necessary in the roof void.
England & Wales
The guidance recommends a fire resistance for enclosing walls of 30 minutes.
Northern Ireland
The guidance recommends a fire resistance for enclosing walls of 30 minutes.
Scotland
The guidance requires the following fire resistance levels for the enclosing structure of:
Protected Zone = Medium (60) Protected Lobby = Short (30) Protected Enclosure = Short (30) Table A9 Enclosures to protected corridors, lobbies and stairs
that are not also compartment walls separating dwellings from these zones
Note
Such walls need not necessarily be constructed to the specification for compartment walls given in Chapter 5 of
Timber Frame Construction[10]but instead may be more akin to the specification given in Chapter 4 of the document for internal walls. Where the specification in Chapter 4 of the document is used, care must be taken to establish that the quoted fire resistance rating of 30 minutes relates to integrity and insulation performance and not just loadbearing capacity.
England & Wales
The guidance recommends that a protected service shaft that conveys piped flammable gas should be provided with ventilation openings to outside air at the top and bottom of the shaft. The fire resistance of the protected shaft should be as derived for compartmenting elements of structure.
The guidance allows for suitably firestopped pipes and fire resisting doors to penetrate compartment walls separating occupancies. Therefore, by implication, it is recommended that other services such as ventilation ducts and cables do not penetrate these compartment walls.
Northern Ireland
The guidance recommends that a protected service shaft that conveys piped flammable gas should be provided with ventilation openings to outside air at the top and bottom of the shaft. The fire resistance of the protected shaft should be as derived for compartmenting elements of structure.
The guidance allows for suitably firestopped pipes and fire resisting doors to penetrate compartment walls separating different occupancies.
Therefore, by implication, it is
recommended that other services such as ventilation ducts and cables do not penetrate these compartment walls.
Scotland
The guidance makes no reference to protected shafts and therefore places no special requirements on
construction and makes no recommendations with respect to deemed to satisfy solutions.
The approach is simply then to treat service risers etc as creating holes in separating floors that need to be addressed in order to maintain the compartmentation offered by the separating floor.
Table A10 Protected services and shafts
Notes
A protected escape stair is a form of protected shaft (see Table A2). A firefighting shaft (if required) is also a form of protected shaft. Service risers do not necessarily have to be designed as protected shafts but there may be benefits to be gained in reducing the amount of firestopping if a large number of services penetrate compartment floors with a few of these services branching out at each floor level. Maintenance and the addition of new services can also be facilitated by employing service risers that are designed as protected shafts.
Should the concept of a protected service shaft be designed for a project in Scotland, the approach for designing the enclosing walls of the shaft should be approached as for separating walls, ie the enclosing walls would be required to be of non-combustible construction if the floor height of the top storey is greater than 18 m.
Chapter 10 of Timber Frame Construction[10]shows solutions for maintaining the fire resistance rating of
compartment/separating walls where service risers pass through compartment floors. The guide also shows a detail for maintaining the fire resistance of compartment/separating walls at the location of socket outlets.
TRADA Technology Report RR1/2001 [33]describes the results of fire resistance testing that demonstrated that
electrical power socket outlets may not compromise the fire resistance of timber frame compartment/separating walls and this data could potentially be used on a case by case basis to demonstrate an alternative approach.
England & Wales
The guidance recommends by
implication in Clause 9.35c that refuse chutes should be of non-combustible construction.
Northern Ireland
The technical provisions allow for waste chute systems constructed of non-combustible materials to penetrate compartment floors.
Regulation J3 requires waste chute systems to be constructed of non-combustible materials.
Scotland
Makes no specific requirements or recommendations
Table A11 Waste chutes
Notes
Waste chutes are effectively treated as a special form of protected shaft.
BS 5906 gives specific guidance with respect to the design of on-site storage/treatment of solid waste from buildings.
The provisions of this standard for waste chute systems are considered as deemed to satisfy in the Northern Ireland Regulations.
A non-combustible refuse chute can be provided in the form of an insulated steel duct. Where the chute passes through compartmentation boundaries, it would be required to be fire resisting in terms of integrity and insulation performance.
England & Wales
Cavity barriers are recommended to be provided:
❐ at the junction between the external cavity wall and every compartment wall and compartment floor;
❐ at the junction between a cavity wall and every compartment floor,
compartment wall, or other wall or door assembly that forms a fire resisting barrier;
❐ in a protected escape route, above and below any fire resisting
construction which is not carried full storey height, or (for the top storey) to the underside of the roof covering (this is relaxed if the cavity is fully enclosed throughout the compartment on its lower side by a ceiling with a fire resistance of 30 minutes);
in the void behind the external face of rainscreen cladding at every floor level, and on the line of compartment walls abutting the external wall of buildings that have a floor 18 m or more above ground level;
❐ at the edges of cavities (including around openings): guidance allows for window and door frames to form cavity barriers in this regard (a PVC
continued ...
Northern Ireland
Cavity barriers are recommended to be provided:
❐ at the top of an external cavity wall;
❐ at the junction between the external cavity wall and every compartment wall and compartment floor;
❐ at the junction between a cavity wall and every compartment floor,
compartment wall, or other wall or door assembly that forms a fire resisting barrier;
❐ in a protected escape route, above and below any fire resisting
construction which is not carried full storey height, or (for the top storey) to the underside of the roof covering (this is relaxed if the cavity is fully enclosed throughout the compartment on its lower side by a ceiling with a fire resistance of 30 minutes);
❐ in the void behind the external face of rainscreen cladding at every floor level, and on the line of compartment walls abutting the external wall, of buildings that have a floor 20 m or more above ground level.
Cavity barriers are recommended to have a fire resistance of 30 minutes
continued ...
Scotland
Cavity barriers are required to be provided:
❐ around the edges of the cavity where a wall, floor or other part of the building which is required to be fire resisting, other than a wall which is required to be fire resisting only because it is loadbearing, abuts a structure containing a cavity: it must be installed so as to extend the line of the fire resisting structure;
❐ where the cavity is above a ceiling and continues over a wall between the accommodation and an escape route, or above a fire door across an escape route: the barrier must be installed in the same plane as the wall and door;
❐ where required to maintain the distance between cavity barriers to a maximum of 10 m: where the surfaces in the cavity are Low Risk (see Table A4) or non-combustible, the distance can be extended to 20 m;
❐ within the ventilated void between a rainscreen panel and an external wall at every floor level, and on the line of compartment walls abutting the external wall, of buildings that have a floor 11 m or more above ground
continued ...
Table A12 Cavity barriers
window or door frame should not be considered as performing as a cavity barrier).
Cavity barriers are recommended to have a fire resistance of 30 minutes integrity and 15 minutes insulation.
Deemed to satisfy cavity barriers for stud walls or partitions include:
❐ steel sheet with a minimum thickness of 0.5 mm;
❐ timber with a minimum thickness of 38 mm;
❐ polyethythene sleeved glass wool or glass wool slab placed under compression when installed;
❐ calcium silicate, cement or gypsum based boards at least 12 mm thick.
The guidance also recommends that cavity insulation material in external walls should be of limited
combustibility if any storey has a floor height in excess of 18 m above ground level (this recommendation is made in Section 13 of the statutory guidance dealing with the control of external fire spread).
integrity and 15 minutes insulation.
However, cavity barriers for stud walls or partitions may include:
❐ steel sheet with a minimum thickness of 0.5mm;
❐ timber with a minimum thickness of 38 mm;
❐ polyethythene sleeved glass wool or glass wool slab placed under
compression when installed;
❐ calcium silicate, cement or gypsum based boards at least 12.5mm thick.
The guidance also recommends that cavity insulation material in external walls and any materials used to support cladding should be of limited combustibility if the building has a storey with a floor height in excess of 20 m above ground (this guidance is given in Section 4 of Technical Booklet E).
level. If a rainscreen is formed by overcladding with Low Risk (see Table A4) surfaces attached to a masonry
level. If a rainscreen is formed by overcladding with Low Risk (see Table A4) surfaces attached to a masonry