Section 11: Resisting fire spread from one building to another

Introduction

11.1 The following assumptions enable a reasonable standard of resistance to the spread of fire to be specified.

a. The size of a fire depends on the compartmentation within the building. A fire may involve a complete compartment, but will not spread to other compartments.

b. The intensity of a fire is related to the building use, but can be moderated by a sprinkler system.

c. Fires in ‘residential’ and ‘assembly and recreation’ buildings (purpose groups 1, 2 and 5) represent a greater risk to life.

d. A building on the far side of the relevant boundary meets both of the following conditions.

i. Has a similar elevation to the one in question.

ii. Is the same distance as the one in question from the common boundary.

e. The radiated heat passing through any part of the fire resisting external wall may be discounted.

11.2 Where regulation 7(2) applies, that regulation prevails over the provisions within this section.

11.3 If a reduced separation distance between buildings, or increased amount of unprotected area, is required, smaller compartments should be considered.

Boundaries

11.4 The fire resistance of a wall depends on its distance from the relevant boundary (see Diagram 11.1). Separation distances are measured to boundaries to ensure that the location and design of buildings on adjoining sites have no influence on the building under consideration.

11.5 The boundary that a wall faces is the relevant boundary (Diagram 11.2). It may be one of the following.

a. The site boundary.

b. The centre line of a space where further development is unlikely, such as a road, railway, canal or river.

c. An assumed notional boundary between two buildings on the same site (Diagram 11.3) where either of the following conditions is met.

i. One or both of the buildings are in the ‘residential’ or ‘assembly and recreation’ purpose groups (purpose group 1 or 5).

ii. The buildings will be operated/managed by different organisations.

Diagram 11.1 Principles of space separation

Diagram 11.2 Relevant boundary

Diagram 11.3 Notional boundary

Unprotected areas and fire resistance

11.6 Parts of an external wall with less fire resistance than the appropriate amount given in Appendix B, Table B4, are called unprotected areas.

11.7 Where a fire resisting external wall has a surface material that is worse than class B-s3, d2 and is more than 1mm thick, that part of the wall should be classified as an unprotected area equating to half its area (Diagram 11.4).

External walls on, and within 1000mm of, the relevant boundary

11.8 Unprotected areas should meet the conditions in Diagram 11.5, and the rest of the wall should be fire resisting from both sides.
External surface materials facing the boundary should be class B-s3, d2 or better.

External walls 1000mm or more from the relevant boundary

11.9 Unprotected areas should not exceed the result given by one of the methods in paragraph 11.16, and the rest of the wall (if any) should be fire resisting but only from the inside of the building.

External walls of protected stairways

11.10 Exclude external walls of protected stairways when assessing unprotected areas (see Diagram 3.10).

Diagram 11.4 Status of materials achieving class B-s3, d2 or worse as unprotected area

Small unprotected areas

11.11 In an otherwise protected wall, small unprotected areas may be ignored where they meet the conditions in Diagram 11.5.

Diagram 11.5 Small unprotected areas that may be disregarded in assessing the separation distance from the boundary

Canopies

11.12 Where both of the following apply, separation distances may be determined from the wall rather than from the edge of the canopy (Diagram 11.6).

a. The canopy is attached to the side of a building.

b. The edges of the canopy are a minimum of 2m from the relevant boundary.

Canopies that fall within class 6 or class 7 of Schedule 2 to the regulations (Exempt Buildings and Work) are exempt from the Building Regulations.

11.13 Space separation may be disregarded if a canopy is all of the following.

a. Free-standing.

b. Above a limited risk or controlled hazard.

c. A minimum of 1000mm from the relevant boundary.

Diagram 11.6 The effect of a canopy on separation distance

Roofs

11.14 Roofs with a pitch of more than 70 degrees to the horizontal should be assessed in accordance with this section. Vertical parts of a pitched roof, such as dormer windows, should be included only if the slope of the roof exceeds 70 degrees.

It is a matter of judgement whether a continuous run of dormer windows that occupies most of a steeply pitched roof should be treated as a wall rather than a roof.

Portal frames

11.15 Portal frames are often used in single storey industrial and commercial buildings where there may be no need for fire resistance of the structure (requirement B3). However, where a portal framed building is near a relevant boundary, the external wall near the boundary may need fire resistance to restrict the spread of fire between buildings. It is generally accepted that a portal frame acts as a single structural element because of the moment-resisting connections used, especially at the column/rafter joints. Thus, in cases where the external wall of the building cannot be wholly unprotected, the rafter members of the frame, as well as the column members, may need to be fire protected. The design method for this is set out in SCI Publication P313.

NOTE: The recommendations in the SCI publication for designing the foundation to resist overturning do not need to be followed if the building is fitted with a sprinkler system in accordance with Appendix E.

NOTE: Normally, portal frames of reinforced concrete can support external walls requiring a similar degree of fire resistance without specific provision at the base to resist overturning.

NOTE: Existing buildings may have been designed to comply with all of the following guidance, which is also acceptable.

a. The column members are fixed rigidly to a base of sufficient size and depth to resist overturning.

b. There is brick, block or concrete protection to the columns up to a protected ring beam providing lateral support.

c. There is some form of roof venting to give early heat release. (The roof venting could be, for example, PVC rooflights covering some 10% of the floor area and evenly spaced over the floor area.)

Methods for calculating acceptable unprotected area

11.16 Two simple methods are given for calculating the acceptable amount of unprotected area in an external wall that is a minimum of 1000mm from any point on the relevant boundary. More precise methods are described in BRE report BR 187 and may be used instead.

Method 1

11.17 This method applies to small buildings intended to be used for blocks of flats or dwellinghouses.

11.18 The building should not exceed three storeys in height (excluding basements) or 24m in length. Each side of the building should meet the limits stated in Diagram 11.7. Any small unprotected areas falling within the limits shown in Diagram 11.5 can be ignored.

Diagram 11.7 Permitted unprotected areas in small residential buildings

Method 2

11.19 This method may be used for buildings or compartments for which method 1 is not appropriate.

11.20 The building should not exceed 10m in height. Each side of the building should meet the limits stated in Table 11.1. Any areas falling within the limits shown in Diagram 11.5 can be ignored.

Table 11.1 Permitted unprotected areas in small buildings or compartments

Sprinkler systems

11.21 If a building is fitted throughout with a sprinkler system in accordance with Appendix E, either of the following is permitted.

a. The boundary distance can be halved, to a minimum distance of 1m.

b. The amount of unprotected area can be doubled.