This Approved Document deals with the following Requirement from Part B of Schedule 1 to the Building Regulations 2010.
External fire spread
- 1The external walls of the building shall adequately resist the spread of fire over the walls and from one building to another, having regard to the height, use and position of the building.
- 2The roof of the building shall adequately resist the spread of fire over the roof and from one building to another, having regard to the use and position of the building.
In the Secretary of State’s view the Requirements of B4 will be met:
- a.if the external walls are constructed so that the risk of ignition from an external source, and the spread of fire over their surfaces, is restricted by making provision for them to have low rates of heat release;
- b.if the amount of unprotected area in the side of the building is restricted so as to limit the amount of thermal radiation that can pass through the wall, taking the distance between the wall and the boundary into account; and
- c.if the roof is constructed so that the risk of spread of flame and/or fire penetration from an external fire source is restricted.
In each case so as to limit the risk of a fire spreading from the building to a building beyond the boundary, or vice versa.
The extent to which this is necessary is dependent on the use of the building, its distance from the boundary and, in some cases, its height.
B4.iThe construction of external walls and the separation between buildings to prevent external fire spread are closely related.
The chances of fire spreading across an open space between buildings, and the consequences if it does, depend on:
a. the size and intensity of the fire in the building concerned;
b. the distance between the buildings;
c. the fire protection given by their facing sides; and
d. the risk presented to people in the other building(s).
B4.ii Provisions are made in Section 8 for the fire resistance of external walls and to limit the susceptibility of the external surface of walls to ignition and to fire spread.
B4.iii Provisions are made in Section 9 to limit the extent of openings and other unprotected areas in external walls in order to reduce the risk of fire spread by radiation.
B4.iv Provisions are made in Section 10 for reducing the risk of fire spread between roofs and over the surfaces of roofs.
B4 Section 8
Construction of external walls
8.1 Provisions are made in this section for the external walls of the building to have sufficient fire resistance to prevent fire spread across the relevant boundary. The provisions are closely linked with those for space separation in Section 9 which sets out limits on the amount of unprotected area of wall. As the limits depend on the distance of the wall from the relevant boundary, it is possible for some or all of the walls to have no fire resistance, except for any parts which are loadbearing (see paragraph B3.iii).
External walls are elements of structure and the relevant period of fire resistance (specified in Appendix A) depends on the use, height and size of the building concerned. If the wall is 1000mm or more from the relevant boundary, a reduced standard of fire resistance is accepted in most cases and the wall only needs fire resistance from the inside.
8.2Provisions are also made to restrict the combustibility of external walls of buildings that are less than 1000mm from the relevant boundary. This is in order to reduce the surface’s susceptibility to ignition from an external source.
In the guidance to Requirement B3, provisions are made in Section 4 for internal and external loadbearing walls to maintain their loadbearing function in the event of fire.
Fire resistance standard
8.3 The external walls of the building should have the appropriate fire resistance given in Appendix A, Table A1, unless they form an unprotected area under the provisions of Section 9.
8.4The external surfaces of walls within 1000mm of the relevant boundary should meet Class 0 (National Class) or Class B-s3,d2 or better (European class). The total amount of combustible material on walls more than 1000mm from the relevant boundary may be limited in practice by the provisions for space separation in Section 9 (see paragraphs 9.7 to 9.17.).
B4 Section 9
9.1The provisions in this Section are based on a number of assumptions and, whilst some of these may differ from the circumstances of a particular case, together they enable a reasonable standard of space separation to be specified. The provisions limit the extent of unprotected areas in the sides of a building (such as openings and areas with a combustible surface) which will not give adequate protection against the external spread of fire from one building to another.
A roof is not subject to the provisions in this Section unless it is pitched at an angle greater than 70° to the horizontal (see definition for ‘external wall’ in Appendix E). Similarly, vertical parts of a pitched roof such as dormer windows (which taken in isolation might be regarded as a wall), would not need to meet the following provisions unless the slope of the roof exceeds 70°. It is a matter of judgement whether a continuous run of dormer windows occupying most of a steeply pitched roof should be treated as a wall rather than a roof.
9.2The assumptions are:
- a.that the size of a fire will depend on the compartmentation of the building, so that a fire may involve a complete compartment, but will not spread to other compartments;
- b.that the intensity of the fire is related to the use of the building (i.e. purpose group), but that it can be moderated by a sprinkler system;
- c.that Residential (1 and 2) and Assembly and Recreation (5) Purpose Groups represent a greater life risk than other uses;
- d.that there is a building on the far side of the boundary that has a similar elevation to the one in question and that it is at the same distance from the common boundary; and
- e.that the amount of radiation that passes through any part of the external wall that has fire resistance may be discounted.
9.3Where a reduced separation distance is desired (or an increased amount of unprotected area) it may be advantageous to introduce additional compartment walls and/or floors.
9.4 The use of the distance to a boundary, rather than to another building, in measuring the separation distance, makes it possible to calculate the allowable proportion of unprotected areas, regardless of whether there is a building on an adjoining site and regardless of the site of that building and the extent of any unprotected areas that it might have.
A wall is treated as facing a boundary if it makes an angle with it of 80° or less (see Diagram 17).
Usually only the distance to the actual boundary of the site needs to be considered. But, in some circumstances, when the site boundary adjoins a space where further development is unlikely, such as a road, then part of the adjoining space may be included as falling within the relevant boundary for the purposes of this section. The meaning of the term boundary is explained in Diagram 17.
9.5 The boundary which a wall faces, whether it is the actual boundary of the site or a notional boundary, is called the relevant boundary (see Diagrams 17 and 18).
Diagram 17 – Relevant boundary
9.6 The distances to other buildings on the same site also need to be considered. This is done by assuming that there is a boundary between those buildings. This assumed boundary is called a notional boundary. The appropriate rules are given in Diagram 18.
Unprotected areas and fire resistance
9.7Any part of an external wall which has less fire resistance than the appropriate amount given in Appendix A, Table A2, is considered to be an unprotected area.
Status of combustible surface materials as unprotected area
9.8 If an external wall has the appropriate fire resistance, but has combustible material more than 1mm thick as its external surface, then that wall is counted as an unprotected area amounting to half the actual area of the combustible material, see Diagram 19. (For the purposes of this provision, a material with a Class 0 rating (National class) or Class B-s3, d2 rating (European class) (see Appendix A, paragraphs 7 and 13) need not be counted as unprotected area.)
Small unprotected areas
9.9 Small unprotected areas in an otherwise protected area of wall are considered to pose a negligible risk of fire spread and may be disregarded. Diagram 20 shows the constraints that apply to the placing of such areas in relation to each other and to lines of compartmentation inside the building. These constraints vary according to the size of each unprotected area.
9.10 Some canopy structures would be exempt from the application of the Building Regulations by falling within Class 6 or Class 7 of Schedule 2 to the Regulations (Exempt Buildings and Work). Many others may not meet the exemption criteria and, in such cases, the provisions in this section about limits of unprotected areas could be onerous.
In the case of a canopy attached to the side of a building, provided that the edges of the canopy are at least 2m from the relevant boundary, separation distance may be determined from the wall rather than the edge of the canopy (see Diagram 21).
External walls within 1000mm of the relevant boundary
9.11 A wall situated within 1000mm from any point on the relevant boundary, including a wall coincident with the boundary, will meet the provisions for space separation if:
a. the only unprotected areas are those shown in Diagram 20; and
b. the rest of the wall is fire-resisting from both sides.
External walls 1000mm or more from the relevant boundary
9.12 A wall situated at least 1000mm from any point on the relevant boundary will meet the provisions for space separation if:
a. the extent of unprotected area does not exceed that given by one of the methods referred to in paragraph 9.13; and
b. the rest of the wall (if any) is fire-resisting.
Notes:When a classification includes ‘s3, d2’, this means that there is no limit set for smoke production and/or flaming droplets/particles.
Methods for calculating acceptable unprotected area
9.13 Two simple methods are given in this Approved Document for calculating the acceptable amount of unprotected area in an external wall that is at least 1000mm from any point on the relevant boundary. (For walls within 1000mm of the boundary see paragraph 9.11 above.)
Method 1 may be used for small residential buildings and is set out in paragraph 9.16.
Method 2 may be used for most buildings or compartments for which Method 1 is not appropriate, and is set out in paragraph 9.17.
There are other more precise methods, described in a BRE report External fire spread: Building separation and boundary distances (BR 187, BRE 1991), which may be used instead of Methods 1 and 2. The ‘Enclosing Rectangle’ and ‘Aggregate Notional Area’ methods are included in the BRE report.
Basis for calculating acceptable unprotected area
9.14 The basis of Methods 1 and 2 was originally set out in Fire Research Technical Paper No 5, 1963. This has been reprinted as part of the BRE report referred to in paragraph 9.13. The aim is to ensure that the building is separated from the boundary by at least half the distance at which the total thermal radiation intensity received from all unprotected areas in the wall would be 12.6 kw/m2 (in still air), assuming the radiation intensity at each unprotected area is 84 kw/m2.
9.15 If a building is fitted throughout with a sprinkler system, it is reasonable to assume that the intensity and extent of a fire will be reduced. The sprinkler system should meet the relevant recommendations of BS 9251 Sprinkler systems for residential and domestic occupancies. Code of practice. In these circumstances the boundary distance may be half that for an otherwise similar, but unsprinklered, building, subject to there being a minimum distance of 1000mm. Alternatively, the amount of unprotected area may be doubled if the boundary distance is maintained.
Notes:The presence of sprinklers may be taken into account in a similar way when using the BRE report referred to in paragraph 9.14.
9.16 This method applies only to a building, which is 1000mm or more from any point on the relevant boundary and meets the following rules for determining the maximum unprotected area, which should be read with Diagram 22:
a.The building should not exceed 3 storeys in height (basements are not counted) or be more than 24m in length; and
b.Each side of the building will meet the provisions for space separation if:
i the distance of the side of the building from the relevant boundary; and
ii the extent of the unprotected area, are within the limits given in Diagram 22; and
Note: In calculating the maximum unprotected area, any areas falling within the limits shown in Diagram 20, and referred to in paragraph 9.9, can be disregarded.
c. Any parts of the side of the building in excess of the maximum unprotected area should be fire-resisting.
Diagram 21 – The effect of a canopy on separation distance
Diagram 22 – Permitted unprotected areas for Method 1
9.17 This method applies to a dwellinghouse which is more than 1000mm from any point on the relevant boundary. The following rules for determining the maximum unprotected area should be read with Table 4.
a. The building or compartment should not exceed 10m in height.
Notes: For any building or compartment more than 10m in height, the methods set out in the BRE report External fire spread: Building separation and boundary distances can be applied.
b. Each side of the building will meet the provisions for space separation if either:
i the distance of the side of the building from the relevant boundary; or
ii the extent of unprotected area, are within the appropriate limits given in Table 4.
Notes: In calculating the maximum unprotected area, any areas shown in Diagram 20, and referred to in paragraph 9.9, can be disregarded.
c. Any parts of the side of the building in excess of the maximum unprotected area should be fire-resisting.
B4 Section 10
10.1 The provisions in this section limit the use, near a boundary, of roof coverings which will not give adequate protection against the spread of fire over them. The term roof covering is used to describe constructions which may consist of one or more layers of material, but does not refer to the roof structure as a whole. The provisions in this Section are principally concerned with the performance of roofs when exposed to fire from the outside.
10.2 The circumstances when a roof is subject to the provisions in Section 9 for space separation are explained in paragraph 9.1.
Other controls on roofs
10.3 There are provisions concerning the fire properties of roofs in other Sections of this document. In the guidance to B1 (paragraph 2.10) there are provisions for roofs that are part of a means of escape. In the guidance to B2 there are provisions for the internal surfaces of rooflights as part of the internal lining of a room or circulation space. In the guidance to B3 there are provisions in Section 4 for roofs which are used as a floor and in Section 6 for roofs that pass over the top of a compartment wall.
Classification of performance
10.4 The performance of roof coverings is designated by reference to the test methods specified in BS 476-3:2004 Fire tests on building materials and structures. Classification and method of test for external fire exposure to roofs or determined in accordance with BS EN 13501-5:2005 Fire classification of construction products and building elements. Classification using data from external fire exposure to roof tests, as described in Appendix A. The notional performance of some common roof coverings is given in Table A5 of Appendix A.
Rooflights are controlled on a similar basis, and plastic rooflights described in paragraphs 10.6 and 10.7 may also be used.
10.5 The separation distance is the minimum distance from the roof (or part of the roof) to the relevant boundary, which may be a notional boundary.
Table 5 sets out separation distances according to the type of roof covering and the size and use of the building. There are no restrictions on the use of roof coverings designated AA, AB or AC (National class) or BROOF(t4) (European class) classification. In addition, roof covering products (and/or materials) as defined in Commission Decision 2000/553/EC of 6 September 2000 implementing Council Directive 89/106/EEC as regards the external fire performance of roof coverings can be considered to fulfil all of the requirements for performance characteristic ‘external fire performance’ without the need for testing provided that any national provisions on the design and execution of works are fulfilled.
Notes: The boundary formed by the wall separating a pair of semi-detached houses may be disregarded for the purposes of this Section (but see Section 5, Diagram 11(b), which deals with roofs passing over the top of a compartment wall).
10.6 Table 6 sets out the limitations on the use of plastic rooflights which have at least a Class 3 (National class) or Class D-s3, d2 (European class) lower surface, and Table 7 sets out the limitations on the use of thermoplastic materials with a TP(a) rigid or TP(b) classification (see also Diagram 23). The method of classifying thermoplastic materials is given in Appendix A.
10.7 When used in rooflights, a rigid thermoplastic sheet product made from polycarbonate or from unplasticised PVC, which achieves a Class 1 (National class) rating for surface spread of flame when tested to BS 476-7:1997 (or 1987 or 1971), or Class C-s3,d2 (European class) can be regarded as having an AA (National class) designation or BROOF(t4) (European class) classification, other than for the purposes of Diagram 11.
Unwired glass in rooflights
10.8 When used in rooflights, unwired glass at least 4mm thick can be regarded as having an AA designation (National class) or BROOF(t4) (European class) classification.
Thatch and wood shingles
10.9 Thatch and wood shingles should be regarded as having an AD/BD/CD designation or EROOF(t4) (European class) classification in Table 5 if performance under BS 476-3:2004 (or 1958) or BS EN 1187:xxxx cannot be established.
Notes:Consideration can be given to thatched roofs being closer to the boundary than shown in Table 5 if, for example, the following precautions (based on Thatched buildings. New properties and extensions [the ‘Dorset Model’]) are incorporated in the design:
a. the rafters are overdrawn with construction having not less than 30 minutes fire resistance;
b. the guidance given in Approved Document J Combustion appliances and fuel storage is followed; and
c. the smoke alarm installation (see Section 1) extends to the roof space.
Table 7 – TP (a) and TP (b) plastic rooflights – limitations on use and boundary distance