Requirements: Internal fire spread (structure)
B3. (1) The building shall be designed and constructed so that, in the event of fire, its stability will be maintained for a reasonable period.
(2) A wall common to two or more buildings shall be designed and constructed so that it adequately resists the spread of fire between those buildings. For the purposes of this sub-paragraph a house in a terrace and a semi-detached house are each to be treated as a separate
(3) Where reasonably necessary to inhibit the spread of fire within the building, measures shall be taken, to an extent appropriate to the size and intended use of the building, comprising either or both of the following:
a. sub-division of the building with fire-resisting construction;
b. installation of suitable automatic fire suppression
(4) The building shall be designed and constructed so that the unseen spread of fire and smoke within concealed spaces in its structure and fabric is inhibited.
Limits on application
Requirement B3(3) does not apply to material alterations to any prison provided under Section 33 of the Prison Act 1952.
The RequirementThis Approved Document deals with the following Requirement from Part B of Schedule 1 to the Building Regulations 2010.
Internal fire spread (structure)
The building shall be designed and constructed so that, in the event of fire, its stability will be maintained for a reasonable period.
A wall common to two or more buildings shall be designed and constructed so that it adequately resists the spread of fire between those buildings. For the purposes of this sub-paragraph a house in a terrace and a semi-detached house are each to be treated as a separate building.
Where reasonably necessary to inhibit the spread of fire within the building, measures shall be taken, to an extent appropriate to the size and intended use of the building, comprising either or both of the following:
- asub-division of the building with fire-resisting construction;
- binstallation of suitable automatic fire suppression systems.
The building shall be designed and constructed so that the unseen spread of fire and smoke within concealed spaces in its structure and fabric is inhibited.
In the Secretary of State’s view the Requirements of B3 will be met
A. if the loadbearing elements of structure of the building are capable of withstanding the effects of fire for an appropriate period without loss of stability;
B. if the building is sub-divided by elements of fire-resisting construction into compartments;
C. if any openings in fire-separating elements (see Appendix E) are suitably protected in order to maintain the integrity of the element (i.e. the continuity of the fire separation); and
D. if any hidden voids in the construction are sealed and sub-divided to inhibit the unseen spread of fire and products of combustion, in order to reduce the risk of structural failure and the spread of fire, in so far as they pose a threat to the safety of people in and around the building.
The extent to which any of these measures are necessary is dependent on the use of the building and, in some cases, its size, and on the location of the element of construction.
B3.i Guidance on loadbearing elements of structure is given in Section 4. Section 5 is concerned with the sub-division of a building into compartments, and Section 6 makes provisions about concealed spaces (or cavities). Section 7 gives information on the protection of openings and on fire-stopping which relates to compartmentation and to fire spread in concealed spaces. Common to all these sections and to other provisions of Part B, is the property of fire resistance.
B3.ii The fire resistance of an element of construction is a measure of its ability to withstand the effects of fire in one or more ways, as follows:
A. resistance to collapse, i.e. the ability to maintain loadbearing capacity (which applies to loadbearing elements only);
B. resistance to fire penetration, i.e. an ability to maintain the integrity of the element; and
C. resistance to the transfer of excessive heat, i.e. an ability to provide insulation from high temperatures.
B3.iii ‘Elements of structure’ is the term applied to the main structural loadbearing elements, such as structural frames, floors and loadbearing walls. Compartment walls are treated as elements of structure although they are not necessarily loadbearing. Roofs, unless they serve the function of a floor, are not treated as elements of structure. External walls, such as curtain walls or other forms of cladding which transmit only self weight and wind loads and do not transmit floor load, are not regarded as loadbearing for the purposes of B3.ii(a), although they may need fire resistance to satisfy requirement B4 (see Sections 8 to 9).
Loadbearing elements may or may not have a fire-separating function. Similarly, fire-separating elements may or may not be loadbearing.
Guidance elsewhere in the Approved Document concerning fire resistance
B3.iv There is guidance in Section 2 concerning the use of fire-resisting construction to protect means of escape. There is guidance in Section 9 about fire resistance of external walls to restrict the spread of fire between buildings. Appendix A gives information on methods of test and performance for elements of construction. Appendix B gives information on fire doors. Appendix C gives information on methods of measurement. Appendix D gives information on purpose group classification. Appendix E gives definitions.
B3 Section 4
Loadbearing elements of structure
4.1 Premature failure of the structure can be prevented by provisions for loadbearing elements of structure to have a minimum standard of fire resistance, in terms of resistance to collapse or failure of loadbearing capacity. The purpose in providing the structure with fire resistance is threefold, namely:
- A. To minimise the risk to the occupants, some of whom may be unable to make their own escape if they have become trapped or injured;
- B.To reduce the risk to firefighters, who may be engaged in search or rescue operations; and
- C. To reduce the danger to people in the vicinity of the building, who might be hurt by falling debris or as a result of the impact of the collapsing structure on other buildings.
Fire resistance standard
4.2 Elements of structure such as structural frames, beams, columns, loadbearing walls (internal and external), floor structures and gallery structures should have at least the fire resistance given in Appendix A, Table A1.
Application of the fire resistance standards for loadbearing elements
4.3 The measures set out in Appendix A include provisions to ensure that where one element of structure supports or gives stability to another element of structure, the supporting element has no less fire resistance than the other element (see notes to Table A2). The measures also provide for elements of structure that are common to more than one building or compartment, to be constructed to the standard of the greater of the relevant provisions. Special provisions about fire resistance of elements of structure in single storey buildings are also given and there are concessions in respect of fire resistance of elements of structure in basements where at least one side of the basement is open at ground level.
Exclusions from the provisions for elements of structure
4.4 The following are excluded from the definition of element of structure for the purposes of these provisions:
A. Structure that only supports a roof, unless:
i. the roof performs the function of a floor, such as a roof terrace, or as a means of escape (see Section 2), or
ii. the structure is essential for the stability of an external wall which needs to have fire resistance; and
B. The lowest floor of the building.
4.5 Guidance in other sections of this Approved Document may also apply if a loadbearing wall is:
A. a compartment wall (this includes a wall common to two buildings), (see Section 5);
B. a wall between a dwellinghouse and an integral garage, (see Section 5, paragraph 5.4);
C. protecting a means of escape, (see Section 2); or
D. an external wall, (see Sections 8 to 9).
4.6 If a floor is also a compartment floor, see Section 5.
Floors in loft conversions
4.7 In altering an existing two-storey single family dwellinghouse to provide additional storeys, the provisions in this Approved Document are for the floor(s), both old and new, to have the full 30 minute standard of fire resistance shown in Appendix A, Table A1. However, provided that the following conditions are satisfied, namely:
A. Only one storey is being added;
B. The new storey contains no more than 2 habitable rooms; and
C. The total area of the new storey does not amount to more than 50m2;
then the existing first floor construction may be accepted if it has at least a modified 30 minute standard of fire resistance, in those places where the floor separates only rooms (and not circulation spaces).
1. The ‘modified 30 minute’ standard satisfies the test criteria for the full 30 minutes in respect of loadbearing capacity, but allows reduced performances for integrity and insulation (see Appendix A, Table A1, item 3(a)).
2. A floor which forms part of the enclosure to the circulation space between the loft conversion and the final exit needs a full 30 minute standard.
Conversion to flats
4.8 Where an existing dwellinghouse or other building is converted into flats the guidance in Volume 2 should be followed.
B3 Section 5
5.1 The spread of fire within a building can be restricted by sub-dividing it into compartments separated from one another by walls and/or floors of fire-resisting construction. The object is twofold:
- A.to prevent rapid fire spread which could trap occupants of the building; and
- B.to reduce the chance of fires becoming large, on the basis that large fires are more dangerous, not only to occupants and fire and rescue service personnel, but also to people in the vicinity of the building. Compartmentation is complementary to provisions made in Section 2 for the protection of escape routes, and to provisions made in Sections 8 to 10 against the spread of fire between buildings.
Provision of compartmentation
5.2 Compartment walls and compartment floors should be provided in the circumstances described below, with the proviso that the lowest floor in a building does not need to be constructed as a compartment floor. Provisions for the protection of openings in compartment walls and compartment floors are given in paragraph 5.13 and Section 7.
5.3 Every wall separating semi-detached houses, or houses in terraces, should be constructed as a compartment wall and the houses should be considered as separate buildings.
5.4 If a domestic garage is attached to (or forms an integral part of) a dwellinghouse, the garage should be separated from the rest of the dwellinghouse, as shown in Diagram 10.
5.5 Where a door is provided between a dwellinghouse and the garage, the floor of the garage should be laid to fall to allow fuel spills to flow away from the door to the outside. Alternatively, the door opening should be positioned at least 100mm above garage floor level.
Construction of compartment walls and compartment floors
5.6 Every compartment wall and compartment floor should:
A. Form a complete barrier to fire between the compartments they separate; and
B. Have the appropriate fire resistance as indicated in Appendix A, Tables A1 and A2.
Note: Timber beams, joists, purlins and rafters may be built into or carried through a masonry or concrete compartment wall if the openings for them are kept as small as practicable and then fire-stopped. If trussed rafters bridge the wall, they should be designed so that failure of any part of the truss due to a fire in one compartment will not cause failure of any part of the truss in another compartment.
Diagram 10 – Separation between garage and dwellinghouse
Compartment walls between buildings
5.7 Compartment walls that are common to two or more buildings should run the full height of the building in a continuous vertical plane. Thus adjoining buildings should only be separated by walls, not floors.
5.8 Compartment walls in a top storey beneath a roof should be continued through the roof space (see definition of compartment in Appendix E).
Junction of compartment wall or compartment floor with other walls
5.9 Where a compartment wall or compartment floor meets another compartment wall, or an external wall, the junction should maintain the fire resistance of the compartmentation. Fire-stopping should meet the provisions of paragraphs 7.12 to 7.14.
5.10 At the junction of a compartment floor with an external wall that has no fire resistance (such as a curtain wall) the external wall should be restrained at floor level to reduce the movement of the wall away from the floor when exposed to fire.
Junction of compartment wall with roof
5.11 A compartment wall should be taken up to meet the underside of the roof covering or deck, with fire-stopping where necessary at the wall/ roof junction to maintain the continuity of fire resistance. The compartment wall should also be continued across any eaves.
5.12 If a fire penetrates a roof near a compartment wall there is a risk that it will spread over the roof to the adjoining compartment. To reduce this risk either:
A. The wall should be extended up through the roof for a height of at least 375mm above the top surface of the adjoining roof covering (see Diagram 11a). Where there is a height difference of at least 375mm between two roofs or where the roof coverings on either side of the wall are AA, AB or AC this height may be reduced to 200mm; or
B. A zone of the roof 1500mm wide on either side of the wall should have a covering of designation AA, AB or AC. Any combustible boarding used as a substrate to the roof covering, wood wool slabs, or timber tiling battens that are carried over the compartment wall should be fully bedded in mortar or other suitable material over the width of the wall (see Diagram 11b).
Note:Double-skinned insulated roof sheeting with a thermoplastic core should incorporate a band of material of limited combustibility at least 300mm wide centred over the wall.
Openings in compartmentation
Openings in compartment walls separating buildings or occupancies
5.13 Any openings in a compartment wall which is common to two or more buildings should be limited to those for:
A. A door which is needed to provide a means of escape in case of fire and which has the same fire resistance as that required for the wall (see Appendix B, Table B1) and is fitted in accordance with the provisions of Appendix B; and
B. the passage of a pipe which meets the provisions in Section 7.
5.14 Information on fire doors may be found in Appendix B.
Diagram 11 – Junction of compartment wall with roof
B3 Section 6
Concealed spaces (cavities)
6.1 Concealed spaces or cavities in the construction of a building provide a ready route for smoke and flame spread e.g. in walls, floors, ceilings and roofs. As any spread is concealed, it presents a greater danger than would a more obvious weakness in the fabric of the building.
Provision of cavity barriers
6.2 Provisions are given below for cavity barriers in specified locations. The provisions necessary to restrict the spread of smoke and flames through cavities are broadly for the purpose of sub-dividing cavities, which could otherwise form a pathway around a fire separating element, and closing the edges of cavities; therefore reducing the potential for unseen fire spread. See also paragraph 2.14.
Note: These should not be confused with fire stopping details, see Sections 5 and
7. Consideration should also be given to the construction and fixing of cavity barriers provided for these purposes and the extent to which openings in them should be protected. For guidance on these issues, see paragraphs 6.6 to 6.9 respectively.
Diagram 12 – Interrupting concealed spaces (cavities)
6.3 Cavity barriers should be provided at the edges of cavities, including around openings (such as window and door openings). Additionally, cavity barriers should be provided at the junction between an external cavity wall and a compartment wall that separates buildings, see Diagram 12; and at the top of such an external cavity wall, except where the cavity wall complies with Diagram 13.
It is important to continue any compartment wall up through a ceiling or roof cavity to maintain the standard of fire resistance – therefore compartment walls should be carried up to the roof, see paragraph 5.11. It is not appropriate to complete a line of compartment walls by fitting cavity barriers above them.
Double-skinned insulated roof sheeting
6.4 Cavity barriers need not be provided between double-skinned corrugated or profiled insulated roof sheeting, if the sheeting is a material of limited combustibility; and both surfaces of the insulating layer have a surface spread of flame of at least Class 0 or 1 (National class) or Class C-s3, d2 or better (European class) (see Appendix A); and make contact with the inner and outer skins of cladding.
Notes:When a classification includes “s3, d2”, this means that there is no limit set for smoke production and/or flaming droplets/particles.
Diagram 13 – Cavity walls excluded from provisions for cavity barriers
Construction and fixings for cavity barriers
6.5 Every cavity barrier should be constructed to provide at least 30 minutes fire resistance and may be formed by any construction provided for another purpose if it meets the provisions for cavity barriers (see Appendix A, Table A1, item 10). Every cavity barrier should be constructed to provide at least 30 minutes fire resistance and may be formed by any construction provided for another purpose if it meets the provisions for cavity barriers (see Appendix A, Table A1, item 10). However, cavity barriers in a stud wall or partition, or provided around openings may be formed of:
a. Steel at least 0.5mm thick; or
b. Ttimber at least 38mm thick; or
c. Polythene-sleeved mineral wool, or mineral wool slab, in either case under compression when installed in the cavity; or
d.Calcium silicate, cement-based or gypsum- based boards at least 12mm thick.
Notes: Cavity barriers provided around openings may be formed by the window or door frame if the frame is constructed of steel or timber of the minimum thickness in (a) or (b) above as appropriate.
6.6 A cavity barrier should, wherever possible, be tightly fitted to a rigid construction and mechanically fixed in position. Where this is not possible (for example, in the case of a junction with slates, tiles, corrugated sheeting or similar materials) the junction should be fire-stopped. Provisions for fire-stopping are set out in Section 7.
6.7 Cavity barriers should also be fixed so that their performance is unlikely to be made ineffective by:
A. Movement of the building due to subsidence, shrinkage or temperature change; and movement of the external envelope due to wind; and
B. Collapse in a fire of any services penetrating them; and
C. Failure in a fire of their fixings (but see note below); and
D. Failure in a fire of any material or construction which they abut. (For example, if a suspended ceiling is continued over the top of a fire- resisting wall or partition, and direct connection is made between the ceiling and the cavity barrier above the line of the wall or partition, premature failure of the cavity barrier can occur when the ceiling collapses. However, this may not arise if the ceiling is designed to provide fire resistance of 30 minutes or more.)
Notes:Where cavity barriers are provided in roof spaces, the roof members to which they are fitted are not expected to have any fire resistance (for the purpose of supporting the cavity barrier(s)).
Openings in cavity barriers
6.8 Any openings in a cavity barrier should be limited to those for:
A. Doors which have at least 30 minutes fire resistance (see Appendix B, Table B1, item 1(a)) and are fitted in accordance with the provisions of Appendix B;
B. The passage of pipes which meet the provisions in Section 7;
C. The passage of cables or conduits containing one or more cables;
D. Openings fitted with a suitably mounted automatic fire damper; and
E. Ducts which are fire-resisting or are fitted with a suitably mounted automatic fire damper where they pass through the cavity barrier.
B3 Section 7
Protection of openings and fire-stopping
7.1 Sections 7 and 8 make provisions for fire-separating elements and set out the circumstances in which there may be openings in them. This section deals with the protection of openings in such elements.
7.2 If a fire-separating element is to be effective, then every joint, or imperfection of fit, or opening to allow services to pass through the element, should be adequately protected by sealing or fire-stopping so that the fire resistance of the element is not impaired.
7.3 The measures in this section are intended to delay the passage of fire. They generally have the additional benefit of retarding smoke spread but the test specified in Appendix A for integrity does not directly stipulate criteria for the passage of smoke.
7.4 Consideration should also be given to the effect of services that may be built into the construction that could adversely affect its fire resistance. For instance, where downlighters, loudspeakers and other electrical accessories are installed, additional protection may be required to maintain the integrity of a wall or floor.
7.5 Detailed guidance on door openings and fire doors is given in Appendix B.
Openings for pipes
7.6 Pipes which pass through fire-separating elements (unless the pipe is in a protected shaft), should meet the appropriate provisions in alternatives A, B or C below.
Alternative A: Proprietary seals (any pipe diameter)
7.7 Provide a proprietary sealing system which has been shown by test to maintain the fire resistance of the wall, floor or cavity barrier.
Alternative B: Pipes with a restricted diameter
7.8 Where a proprietary sealing system is not used, fire-stopping may be used around the pipe, keeping the opening as small as possible. The nominal internal diameter of the pipe should not be more than the relevant dimension given in Table 3.
Alternative C: sleeving
7.9 A pipe of lead, aluminium, aluminium alloy, fibre-cement or uPVC, with a maximum nominal internal diameter of 160mm, may be used with a sleeving of non-combustible pipe as shown in Diagram 14. The specification for non-combustible and uPVC pipes is given in the notes to Table 3.
Table 3 – Maximum nominal internal diameter of pipes passing through a fire separating element (see paragraph 7.6 to 7.9)
Diagram 14 – Pipes penetrating structure
Diagram 15 – Enclosure for drainage or water supply pipes
Ventilation ducts, flues etc.
7.10 Air circulation systems which circulate air within an individual dwellinghouse with a floor more than 4.5m above ground level should meet the guidance given in paragraph 2.16. Where ventilation ducts pass through compartment walls into another building then the guidance given in Approved Document B Volume 2 should be followed.
Diagram 16 – Flues penetrating compartment walls or floors (note that there is guidance in Approved Document J concerning hearths adjacent to compartment walls)
7.11 If a flue or duct containing flues or appliance ventilation duct(s), passes through a compartment wall or compartment floor, or is built into a compartment wall, each wall of the flue or duct should have a fire resistance of at least half that of the wall or floor in order to prevent the by-passing of the compartmentation (see Diagram 16).
7.12 In addition to any other provisions in this document for fire-stopping:
A. Joints between fire-separating elements should be fire-stopped; and
B. All openings for pipes, ducts, conduits or cables to pass through any part of a fire-separating element should be:
i. Kept as few in number as possible; and
ii. Kept as small as practicable; and
iii. Fire-stopped (which in the case of a pipe or duct should allow thermal movement).
7.13 To prevent displacement, materials used for fire-stopping should be reinforced with (or supported by) materials of limited combustibility in the following circumstances:
A. in all cases where the unsupported span is greater than 100mm; and
B. in any other case where non-rigid materials are used (unless they have been shown to be satisfactory by test).
7.14 Proprietary fire-stopping and sealing systems, (including those designed for service penetrations) which have been shown by test to maintain the fire resistance of the wall or other element, are available and may be used.
Other fire-stopping materials include:
- cement mortar;
- gypsum-based plaster;
- cement-based or gypsum-based vermiculite/ perlite mixes;
- glass fibre, crushed rock, blast furnace slag or ceramic-based products (with or without resin binders); and
- intumescent mastics.
These may be used in situations appropriate to the particular material. Not all of them will be suitable in every situation.
Guidance on the process of design, installation and maintenance of passive fire protection is available in Ensuring Best Practice for Passive Fire Protection in Buildings (ISBN: 1 87040 919 1) produced by the Association for Specialist Fire Protection (ASFP).
Further information on the generic types of systems available, information about their suitability for different applications and guidance on test methods is given in the ASFP Red Book: Fire Stopping and Penetration Seals for the Construction Industry – the ‘Red Book’ published by the Association for Specialist Fire Protection and freely available from the ASFP website at www.asfp.org.uk.