13.1The 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 70deg 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 70deg. 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.
13.2The assumptions are:
- athat 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;
- bthat 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;
- cthat Residential and Assembly and Recreation Purpose Groups represent a greater life risk than other uses;
- dthat 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
- ethat the amount of radiation that passes through any part of the external wall that has fire resistance may be discounted.
13.3Where a reduced separation distance is desired (or an increased amount of unprotected area) it may be advantageous to construct compartments of a smaller size.
13.4The 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 or 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 80deg or less (see Diagram 41).
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 41.
13.5The 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 41 and 42).
13.6Generally separation distance between buildings on the same site is discounted. In some circumstances the distances to other buildings on the same site 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.
A notional boundary is assumed to exist where:
- aeither or both of the buildings concerned are in the Residential or Assembly and Recreation Purpose Groups; or
- bmore than one building is constructed on the same site but is to be operated/managed by different organisations.
The appropriate rules are given in Diagram 42.
Unprotected areas and fire resistance
13.7Any 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.
External walls of protected shafts forming stairways
13.8Any part of an external wall of a stairway in a protected shaft is excluded from the assessment of unprotected area.
Notes:There are provisions in the guidance to B1 (Diagram 24) and B5 (paragraph 17.11) which refers to Section 2 of BS 5588-5:2004 about the relationship of external walls for protected stairways to the unprotected areas of other parts of the building.
Status of combustible surface materials as unprotected area
13.9If 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 43. (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).
Notes:When a classification includes “s3, d2”, this means that there is no limit set for smoke production and/or flaming droplets/particles.
Small unprotected areas
13.10Small unprotected areas in an otherwise protected area of wall are considered to pose a negligible risk of fire spread and may be disregarded. Diagram 44 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.
13.11Some 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 Works). 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 45).
In the case of a free-standing canopy structure above a limited risk or controlled hazard (for example over petrol pumps), in view of the high degree of ventilation and heat dissipation achieved by the open sided construction and provided the canopy is 1000mm or more from the relevant boundary, the provisions for space separation could reasonably be disregarded.
Large uncompartmented buildings
13.12Parts of the external wall of an uncompartmented building which are more than 30m above mean ground level, may be disregarded in the assessment of unprotected area.
External walls within 1000mm of the relevant boundary
13.13A wall situated within 1000mm from any point on the relevant boundary and 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 44 or referred to in paragraph 13.12; and
- bthe rest of the wall is fire-resisting from both sides.
External walls 1000mm or more from the relevant boundary
13.14A wall situated at least 1000mm from any point on the relevant boundary will meet the provisions for space separation if:
- athe extent of unprotected area does not exceed that given by one of the methods referred to in paragraph 13.15; and
- bthe rest of the wall (if any) is fire-resisting from the inside of the building.
Methods for calculating acceptable unprotected area
13.15Two 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 13.13 above.)
Method 1 may be used for small residential buildings which do not belong to Purpose Group 2a (Institutional type premises) and is set out in paragraph 13.19.
Method 2 may be used for most buildings or compartments for which Method 1 is not appropriate and is set out in paragraph 13.20.
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
13.16The basis of Methods 1 and 2 is set out in Fire Research Technical Paper No 5, 1963. This has been reprinted as part of the BRE report referred to in paragraph 13.15. 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:
- a84 kw/m2, if the building is in the Residential, Office or Assembly and Recreation Purpose Groups, or is an open-sided multi-storey car park in Purpose Group 7(b); and
- b168 kw/m2, if the building is in the Shop and Commercial, Industrial, Storage or Other non-residential Purpose Groups.
13.17If 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 be in accordance with paragraph 0.16. 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 1m. 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 13.15.
13.18If a building contains one or more atria, the recommendations of clause 28.2 in BS 5588-7:1997 should be followed.
13.19This method applies only to a building intended to be used for block of flats or other residential purposes (not Institutional), which is 1000mm or more from any point on the relevant boundary.
The following rules for determining the maximum unprotected area should be read with Diagram 46.
- aThe building should not exceed 3 storeys in height (basements not counted) or be more than 24m in length:
- bEach side of the building will meet the provisions for space separation if:
- ithe distance of the side of the building from the relevant boundary; and
- iithe extent of the unprotected area, are within the limits given in Diagram 46.
Note: In calculating the maximum unprotected area, any areas falling within the limits shown in Diagram 44 and referred to in paragraph 13.10, can be disregarded.
- cAny parts of the side of the building in excess of the maximum unprotected area should be fire-resisting.
13.20This method applies to a building or compartment intended for any use and which is not less than 1000mm from any point on the relevant boundary.
The following rules for determining the maximum unprotected area should be read with Table 15.
- aThe building or compartment should not exceed 10m in height except for an open- sided car park in Purpose Group 7(b) (see paragraph 11.3).
Note: 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.
- bEach side of the building will meet the provisions for space separation if either:
- ithe distance of the side of the building from the relevant boundary; and
- iithe extent of unprotected area, are within the appropriate limits given in Table 15.
Note: In calculating the maximum unprotected area, any areas shown in Diagram 44 and referred to in paragraph 13.10, can be disregarded.
- cany parts of the side of the building in excess of the maximum unprotected area should be fire-resisting.