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B4 External fire spread

The external walls and roof of a building shall be so designed and constructed that they afford adequate resistance to the spread of fire to and from neighbouring buildings.

Performance

The requirements of B4 may 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 spread of flame, and in some cases 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.

4.0 Introduction to Provisions

4.0.1 The 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:

  • the size and intensity of the fire in the building concerned;

  • the risk it presents to people in the other building(s);

  • the distance between the buildings; and

  • the fire protection given by their facing sides

Provisions are made in sub-section 4.1 for the fire resistance of external walls, and to limit the susceptibility of the external surface of walls to ignition and to fire spread.

Provisions are made in sub-section 4.2 to limit the extent of openings and other unprotected areas in external walls in order to reduce the risk of fire spread by radiation.

Provisions are made in sub-section 4.3 for reducing the risk of fire spread between and over roofs.

Definitions

4.0.2 The following definitions apply specifically to B4. Other terms applicable more widely throughout this Document are given in Appendix D.

Boundary The boundary of the land belonging to the building, or where the land abuts a road, railway, canal or river, the centreline of that road, railway, canal or river (see Diagram 25).

Class 0 See Appendix A, paragraph A.11.

Conservatory A single storey part of a building where the roof and walls are substantially glazed with a transparent or translucent material.

External wall (or side of a building) includes a part of a roof pitched at an angle of 70° or more to the horizontal - if that part of the roof adjoins a space within the building to which persons have access (but not access only for repair or maintenance).

Notional boundary A boundary presumed to exist between buildings on the same site (see Diagram 26).

Relevant boundary The boundary which the side of the building faces (see Diagram 25). A notional boundary can be a relevant boundary.

Rooflight Any domelight, lantern light, skylight or other element intended to admit daylight through a roof.

Thermo-plastic material See Appendix A, paragraph A15.

Unprotected area In relation to a side or external wall of a building means:

a. a window, door or other opening; and

b. any part of the external wall which has less than the relevant fire resistance set out in Section 4.1; and

c. any part of the external wall which has combustible material more than 1 mm thick attached or applied to its external face, whether for cladding or any other purpose (combustible material in this context is any material that is not included in Tables A7 or A8 in Appendix A).

Provisions meeting the Requirement

4.1 Construction of External Walls

Introduction

4.1.1 Under B3, provisions are made in sub-section 3.1 for internal and external loadbearing walls to maintain their loadbearing function in the event of fire.

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 sub-section 4.2 which sets out limits on the amount of wall area that need not be fire-resisting (termed unprotected area). 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 be permitted to have no fire resistance except for any parts which are loadbearing.

External walls are elements of structure and the relevant period of fire resistance (which is specified in Appendix A) depends on the use, height and size of the building concerned, and whether the wall is within 1 m of the relevant boundary.

Provisions are also made to restrict the amount of combustible surfaces on buildings that are very close (less than 1 m) to the relevant boundary and/or on high buildings. T his is in order to reduce the susceptibility of ignition of the surface from an external source, and to reduce the possibility of fire spread up the external face of the building.

Fire Resistance Standard

4.1.2 The external walls of the building should have the appropriate fire resistance given in Appendix A, Table A1, unless they are permitted to form an unprotected area under sub-section 4.2.

Portal Frames

4.1.3 Portal frames are often used in single storey industrial and commercial buildings where there may be no provisions under B3 for fire resistance of the structure. W here, however, the building is near a relevant boundary, then the provisions in par. 4.1.2 require the external wall to be fire-resisting.

It is generally accepted that a portal frame acts as a single element because of the moment-resisting connections used, especially the column/rafter joints. Thus the rafter members of the frame, as well as the column members may need to be fire protected in cases where the external wall of the building cannot be wholly unprotected.

Following an investigation of the behaviour of steel portal frames in fire, it was considered technically and economically feasible to design the connection of the portal frame to the foundation so that it would transmit the overturning moment caused by the collapse in a fire of unprotected rafters, purlins and some roof cladding so as to allow the external wall to continue to perform its structural function. The design method is set out in “Single storey steel framed buildings in fire boundary conditions”, published by the Steel Construction Institute, Silwood Park, Ascot, Berks, SL5 7QN, England.

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

Notes:

  • The recommendations in the SCI publication for designing the foundation to resist overturning need not be followed if the building is fitted with a sprinkler system meeting the relevant recommendations of BS 5306: Part 2 Fire extinguishing installations and equipment on premises, Specification for sprinkler systems, ie the relevant occupancy rating together with the additional requirements for life safety.

  • 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.

  • Existing buildings may have been designed to the following guidance which is also acceptable:

a. the column members area fixed rigidly to abase 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; and

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 per cent of the floor area and evenly spaced over the floor area.)

External Surfaces

4.1.4 The external surfaces of walls should meet the provisions in Table 4.1. However, the total amount of combustible material may be limited in practice by the provisions for space separation in sub-section 4.2 (see par. 4.2.6).

Where a mixed use building includes Assembly and Recreation Purpose Group accommodation, the external surfaces of walls should meet the provisions in Table 4.1 (Note 1)

Note: One alternative to meeting the provisions in Table 4.1 could be BRE Fire Note 9 Assessing the fire performance of external cladding systems: a test method (BRE, 1999).

External Wall Construction

4.1.5 The external envelope of a building should not provide a medium for fire spread. T e use of combustible materials for cladding framework, or of combustible thermal insulation as an overcladding in drained and/or ventilated cavities, may present such a risk in tall buildings, even though the provisions for external surfaces in Table 4.1 may have been satisfied.

In the case of the outer cladding of a wall with a drained and/or ventilated cavity, the surface of the outer cladding which faces the cavity should also meet the provisions of Table 4.1

In a building more than 18m high, (see Diagram 37, Appendix C) insulation material used in drained and/or ventilated cavities in the external wall construction should be of limited combustibility (see Appendix A). This restriction does not apply to masonry cavity wall construction which complies with Diagram 17.

Advice on the use of thermal insulation material is given in the BRE Report Fire performance of external thermal insulation for walls of multi-storey buildings (BR 135, 1988).

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Table 4.1 Provision for external surface of walls

4.2. Space Separation

Introduction

4.2.1 The provisions in this Section limit the extent of openings and other unprotected areas in the sides of the building (including areas with a combustible surface) which will not give adequate protection against the spread of fire.

The provisions assume:

a. that the size of the fire will depend on the compartmentation of the building, so that the fire will involve a complete compartment, but will not spread across lines of compartmentation;

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 and assembly and recreation purpose groups represent a greater life risk than other uses;

d. that the building on the adjoining site has an identical elevation to the one in question, and is at the same distance from the common boundary; and

e. that no significant radiation will pass through any parts of the external wall that have fire resistance.

It may sometimes be advantageous to construct compartments of a smaller size than indicated by B3, or to provide compartments where none would otherwise be necessary, in order to reduce the separation distance (or to increase the amount of unprotected area in the wall without increasing the separation distance).

Boundaries

4.2.2 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, even where another building does not exist but may do.

A wall should be treated as facing a boundary if it makes an angle with it of 80° or less (see Diagram 25).

Usually only the distance to the boundary of the site needs to be considered. The meaning of the term boundary is explained in Diagram 25.

Notional Boundaries

4.2.3 In some circumstances the distances to other buildings on the same site needs to be considered. This should be done by assuming a boundary called a notional boundary between those buildings.

The concept of a notional boundary between two buildings on the same site and the rules that apply are illustrated in Diagram 26.

In general, it is not necessary to consider the separation distance between buildings on the same site unless one of the buildings, whether new or existing, is of Residential (Purpose Groups 1(a), 1(b), 1(c), 2(a), 2(b)) or Assembly and Recreation (Purpose Group 5) use.

Where a number of buildings of any purpose group occupy the same site and where each building is under different ownership, tenancy or occupancy or where sub-division of the site is likely to occur, it would in these circumstances be appropriate to consider space separation between all buildings on the same site.

Where buildings of any purpose group occur on the same site and where space separation has not been considered, then it is necessary to take into account the compartmentation requirements under Section B3 (3.2) as if they were connected together as one building.

Space separation between opposing elevations of the same building (see 3.2.5.9) may also need to be considered. The principles contained in this Section are also relevant to these situations.

Relevant Boundaries

4.2.4 The boundary which a wall faces whether it is the boundary of the site or a notional boundary is called the relevant boundary (see Diagrams 25 and 26).

Unprotected Areas and Fire Resistance

4.2.5 Any part of an external wall which has less fire resistance than the appropriate amount indicated in T able A1(5) and T able A2 of Appendix A is considered to be an unprotected area.

Status of Combustible Surfaces as Unprotected Area

4.2.6 Besides the restrictions on combustible surfaces in 4.1, their extent may also be limited by the result of the calculation of unprotected area if they are more than 1 mm in thickness.

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Diagram 25 Relevant boundary

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Diagram 26 Notional boundary

a. If the combustible material is used as a surface on a wall that has the necessary fire resistance, then half of the area of combustible material should be counted as unprotected area.

b. If the combustible material is used as a surface on a wall that does not have the necessary fire resistance, then the whole of the area of combustible material is counted as unprotected area.

External Walls Within 1 m of the Relevant Boundary

4.2.7 A wall situated within 1 m from any point on the relevant boundary will meet the provisions for space separation if:

a. the only unprotected areas are those shown inDiagram 27, and

b. the rest of the wall is fire-resisting from both sides.

External Walls 1 m or more from the Relevant Boundary

4.2.8 A wall situated at least 1 m 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 4.2.8.1 below, and

b. the rest of the wall (if any) is fire-resisting.

4.2.8.1 Methods for calculating acceptable unprotected area Two methods are given in this Document for calculating the acceptable amount of unprotected area in an external wall that is at least 1 m from any point on the relevant boundary (for walls within 1 m of the boundary see 4.2.7 above).

Method 1 (see 4.2.8.4) is only suitable for small residential buildings which do not belong to Purpose Group 2(a), Residential (Institutional).

Method 2 (see 4.2.8.5) may be used for buildings or compartments for which Method 1 is not appropriate.

Other methods are described in the Building Research Establishment Report (BR 187) "External fire spread: building separation and boundary distances".

The methods outlined in the BRE report are based on the following:

  • enclosing rectangle (geometric method);

  • aggregate notional areas (protractor method); and

  • a calculation method (Part 2 of BRE Report)

The calculation method, which takes into account a number of fire parameters, including fire-load densities and ventilation conditions, may be the most appropriate in certain critical situations. Fire load densities may be determined by consideration of the intended use of the building, including any likely variations in fire load with time.

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Diagram 27 Unprotected areas which may be disregarded for space separation purposes

4.2.8.2 Basis for calculating acceptable unprotected area The 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 4.2.8.1 above. 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 emitted 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:

a. 84 kW/m2, if the building is in the residential, office or assembly or Recreation purpose groups, and

b. 168 kW/m2, if the building is in the shop, commercial, industrial, storage or other nonresidential purpose groups.

4.2.8.3 Sprinkler systems It may be assumed that the intensity of radiation from a fire in a compartment which is fitted throughout with a sprinkler system will be half the values given in 4.2.8.2 (a) and (b) above. T he sprinkler system should meet the relevant recommendations of BS 5306: Part 2: 1990, i.e. the relevant occupancy rating together with the additional requirements for life safety.

In the case of a building containing one or more atria, the recommendations of clause 28.2 in BS 5588: Part 7 “C ode of practice for incorporation of atria in buildings” should be followed.

4.2.8.4 Method 1 - small residential buildings This method applies only to a building intended to be used as a dwelling house, or for flats or other residential purposes (not Institutional), which is not less than 1 m from any point on the relevant boundary.

The following rules for determining the maximum permitted unprotected area should be read with Diagram 28 and Table 4.2:

  1. The building should not exceed 3 storeys in height (basements not counted) or be more than 24 m in length.

  2. Each side of the building will meet the provisions for space separation if:

(a) the distance of the side of the building from the relevant boundary, and

(b) the extent of unprotected area,

are within the limits given in Table 4.2.

Note: In calculating the maximum permitted unprotected area, any areas shown in Diagram 27 can be disregarded.

  1. Any parts of the side of the building in excess of the maximum permitted unprotected area should be fire-resisting.

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Diagram 28 Small residential building

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Table 4.2 Permitted unprotected areas in small residential buildings

4.2.8.5 Method 2 - Other buildings or compartments This method applies to a building or compartment intended for any use and not less than 1 m from any point on the relevant boundary. The following rules for determining the maximum permitted unprotected area should be read with Table 4.3.

a. Except for open sided car parks in Purpose Group 7(b), the building or compartment should not exceed 10 m in height.

Note: For any building or compartment more than 10 m in height, the methods set out in the BRE Report 'Building separation and boundary distances' can be applied.

b. Each side of the building will meet the provisions for space separation if:

(a) the distance of the side of the building from the relevant boundary, and

(b) the extent of unprotected area,

are within the limits of unprotected area.

Note: In calculating the maximum permitted unprotected area, any areas shown in Diagram 27 can be disregarded.

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Table 4.3 Permitted unprotected areas in small buildings or compartments

c. Any parts of the side of the building in excess of the maximum permitted unprotected area should be fire-resisting.

Material Alteration of Existing Buildings

4.2.9 In the case of a material alteration of an existing building, the requirements in relation to space separation may be met where:

  • there is no increase in the extent of unprotected areas to the existing external walls of the building; and

  • the building is not altered or extended by the provision of additional floor area(s).

Material Change of Use of Existing Buildings

4.2.10 In the case of a material change of use of an existing building the requirements in relation to space separation may be met where:

  • there is no increase in the extent of un-protected areas to the existing external walls of the building; and

  • the building is not altered or extended by the provision of additional floor area(s); and

  • the new use will not result in an increased fire load density (i.e. the amount of combustible materials from contents and the construction, per unit floor area) in any part of the building.

Where any of the above criteria are not met, it will be necessary to demonstrate that the unprotected areas comply with the requirements outlined at 4.2.7 and 4.2.8.

Where it is necessary to increase the extent of unprotected areas of a wall on an existing elevation, it may be appropriate to employ insulated fire resisting glazing in fixed frames or provide other fire resisting construction remote from any existing openings, to provide the necessary insulation protection.

4.3. Roof Coverings

Introduction

4.3.1 The provisions in this sub-section limit the proximity to the boundary of those types of roof covering which will not give adequate protection against the spread of fire.

Other Controls on Roofs

4.3.2 There are provisions concerning the fire properties of roofs elsewhere. In B3, there are provisions in sub-section 3.2 for roofs that pass over the top of a compartment wall or separating wall. In B2, there are provisions for the internal surfaces of rooflights as part of the internal lining of a room or circulation space.

Classification of Performance

4.3.3 T he performance of roof coverings is determined in accordance with I.S. EN 13501-5: xxxx, Fire classification of construction products and building elements, Part 5 – C lassification using test data from external fire exposure to roof tests or is designated by reference to the test methods specified in BS 476: part 3: 2004, as described in Appendix A. The notional performance of some common roof coverings is given in table A5 of Appendix A.

I.S. ENV 1187: 2002 also refers.

Rooflights are controlled on a similar basis, although there is a different method of classification for plastic rooflights.

Separation Distances

4.3.4 T he separation distance is the minimum distance from the roof (or part of the roof) in question to the nearest boundary, which may be a notional boundary.

Table 4.4 sets out separation distances according to the type of roof covering and the size and use of the building. However, there are no restrictions on the use of roof coverings designated class BRO O F(t4)
(European class) or AA, AB or AC (National class).

In addition, roof covering products (and/or materials) as defined in Commission Decision 2000/553/EC of 6th 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. That is, the roof covering products (and/or materials) defined in this Commission Decision can be used without restriction.

Plastic Rooflights

4.3.5 Table 4.5 sets out the limitations on the use of plastic rooflights which do not meet the basic provisions described in Table 4.4 but which have a lower surface of thermoplastic material with a TP(a) rigid or TP(b) classification (see A14 of Appendix A).

When used in rooflights, a rigid thermoplastic sheet product made from polycarbonate or from unplasticised PVC , which achieves a class C – s3, d2 (European class) or class 1 (National class) rating for surface spread of flame when tested to BS 476 part 7: 1971 or 1987, can be regarded as having BRO O F(t4) (European class) or an AA (National class) designation.

Glass in Rooflights

4.3.6 W hen used in rooflights, unwired glass at least 4 mm thick can be regarded as having BRO O F(t4) (European class) or an AA (National class) designation. Thinner glass should only be used where the separation distance is 6 m or more, unless the glass is over one of the following:

a. a balcony, verandah, open carport, covered way, loading bay or detached swimming pool; or

b. a garage, conservatory or outbuilding, with a maximum floor area of 40 m2.

Important note: The year designation of xxxx is used for pending European standards that are not yet published.

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Table 4.4 Limitations on roof coverings

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Table 4.5 Limitations on plastic rooflights

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Diagram 29 Limits on spacing and size of plastic rooflights having a TP(b) lower surface

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