B9 The external walls and roof of a dwelling house shall be so designed and constructed that they afford adequate resistance to the spread of fire to and from neighbouring buildings.
4.1 External Fire Spread
4.1.1 General
The purpose of the requirement of Regulation B9 is to ensure that external walls and roofs have adequate resistance to the spread of fire over their external surfaces, and that spread of fire from one building to another is restricted.
4.2 Performance
The requirements of B9 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.
4.3 Introduction to Provisions
4.3.1 Introduction
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 4.4 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 4.5 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 4.6 for reducing the risk of fire spread between and over roofs.
4.4 Construction of External Walls
4.4.1 Introduction
Under Section 3, provisions are made in subsection 3.4 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 4.5 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. This 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.
4.4.2 Fire Resistance Standard
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 4.5.
4.4.3 External Surfaces
The external surfaces of walls should meet the following provisions in dwelling houses:
(a) less than 15 m in height and less than 1 m from the relevant boundary (the relevant boundary may be a notional boundary) they should be Class B – s3,d2(European) or Class 0 (National);
(b) less than 15 m in height and more than 1 m from the relevant boundary: no provisions.
However, the total amount of combustible material may be limited in practice by the provisions for space separation in par. 4.5.6.
4.4.4 External Wall Construction
The external envelope of a building should not provide a medium for fire spread. The use of combustible materials for cladding framework, or of combustible thermal insulation as an over cladding in drained and/or ventilated cavities, may present such a risk even though the provisions for external surfaces in 4.4.3 (a) or (b) 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 4.4.3 (a) or (b).
4.5 Space Separation
4.5.1 Introduction
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 intensity of the fire is related to the use of the building as a dwelling house.
(b) 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
(c) that no significant radiation will pass through any parts of the external wall that have fire resistance.
4.5.2 Boundaries
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 the boundary makes an angle with the wall of 80° or less (see Diagram 15).
Usually only the distance to the boundary of the site needs to be considered. The meaning of the term boundary is explained in Diagram 15.
4.5.3 Notional Boundaries
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 14.
Diagram HB14 - Notional boundary - Extract from TGD B Vol. 2
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) Flats and Maisonettes, 1(d), 2(a) Residential Institutional, 2(b) Other Residential 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 3 (3.5) as if they were connected together as one building.
4.5.4 Relevant Boundaries
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 14 and 15).
Diagram HB15 - Relevant boundary - Extract from TGD B Vol. 2
4.5.5 Unprotected Areas and Fire Resistance
Any part of an external wall which has less fire resistance than the appropriate amount indicated in Table A1(5) of Appendix A is considered to be an unprotected area.
4.5.6 Status of Combustible Surfaces as Unprotected Area
Besides the restrictions on combustible surfaces in 4.4, their extent may also be limited by the result of the calculation of unprotected area if they are more than 1 mm in thickness. Then:
(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; or
(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.
4.5.7 External Walls within 1 m of the Relevant Boundary
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 in Diagram 16; and
(b) the rest of the wall (if any) is fire resisting.
Diagram HB16 - Unprotected areas which may be disregarded for space separation purposes - Extract from TGD B Vol. 2
4.5.8 External Walls 1 m or more from the Relevant Boundary
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 4.5.8.2 below, and
(b) the rest of the wall (if any) is fire resisting.
4.5.8.1 Canopies
In the case of a canopy attached to the side of a building, provided that the edges of the canopy are at least 2 m from the relevant boundary, separation distance may be determined from the wall (building line) rather than the edge of the canopy (see Diagram 17).
Diagram HB17 - The effect of a canopy on separation distance - Extract from TGD B Vol. 2
4.5.8.2 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 4.5.7 above).
Method 1 (see 4.5.8.3) is only suitable for small dwelling houses which do not belong to Purpose Group 2(a), Residential (Institutional).
Method 2 (see 4.5.8.4) 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" (2nd edition 2014).
The methods outlined in the BRE report are based on the following:
enclosing rectangle (geometric method);
aggregate notional areas (protractor method); and
Fire engineering approaches (Part 4 of BRE Report)
Method 1 Small Dwelling houses
4.5.8.3 This method applies only to a building intended to be used as a dwelling house which is not less than 1m from any point on the relevant boundary. The following rules for determining the maximum permitted unprotected area should be read with Diagram 18 and Table 4.1:
The building should not exceed 3 storeys in height (basements not counted) or be more than 24 m in length.
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 4.1.
Note: In calculating the maximum permitted unprotected area, any areas shown in Diagram 16 can be disregarded.
- Any parts of the side of the building in excess of the maximum permitted unprotected area should be fire-resisting.
Diagram HB18 - Small dwelling houses - Extract from TGD B Vol. 2
Table HB4 - Permitted unprotected areas in small dwelling houses - Extract from TGD B Vol. 2
Method 2 Larger Dwelling houses
4.5.8.4 This method applies to a dwelling house not less than 1m from any point on the relevant boundary. The following rules determine the maximum permitted unprotected area and should be read with Table 4.2.
- 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 of unprotected area set out in Table 4.2.
Note: In calculating the maximum permitted unprotected area, any areas shown in Diagram 16 can be disregarded.
- Any parts of the side of the building in excess of the maximum permitted unprotected area should be fire-resisting.
Note: For any dwelling house or compartment more than 15 m in height, the methods set out in the BRE Report Building separation and boundary distances’ can be applied.
Table HB5 - Permitted unprotected areas in larger dwelling houses - Extract from TGD B Vol. 2
4.5.9 Material Alteration of Existing Dwelling houses
In the case of a material alteration of an existing dwelling house 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).
Note: Where the above criteria are not met, it will be necessary to demonstrate that the unprotected areas comply with the requirements outlined at 4.5.7 or 4.5.8.
4.5.10 Material Change of Use of Existing Buildings
In the case of a material change of use of an existing building to a dwelling house 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);
Note: Where either of the above criteria are not met, it will be necessary to demonstrate that the unprotected areas comply with the requirements outlined at 4.5.7 and 4.5.8.
4.5.11 Extensions
Where an extension to an existing dwelling house is proposed the requirements in relation to space separation may be met where: there is no increase in the extent of un-protected areas to the new external walls of the building or the total unprotected area of the new and existing walls of the dwelling house comply with section 4.5.7 or 4.5.8.
4.5.12 Increase in Extent of Unprotected Area
Where it is proposed 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 protection.
4.6 Roof Coverings
4.6.1 Introduction
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.
4.6.2 Other Controls on Roofs
There are provisions concerning the fire properties of roofs elsewhere. In S3, there are provisions in sub-section 3.5 for roofs that pass over the top of a compartment wall or separating wall. In S2, there are provisions for the internal surfaces of rooflights as part of the internal lining of a room or circulation space.
4.6.3 Classification of Performance
The performance of roof coverings is determined in accordance with I.S. EN 13501-5: 2006+A1 2009 - Fire classification of construction products and building elements, Part 5 – Classification 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 A4 of Appendix A.
I.S. ENV 1187: 2002 - Test Methods for external fire exposure to roofs also refers.
Rooflights are controlled on a similar basis, although there is a different method of classification for plastic roof lights, see paragraph 4.6.5.
4.6.4 Separation Distances
The 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 4.3 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 BROOF(t4) (European class) or AA, AB or AC (National class).
Table HB6 - Limitations on roof coverings - Extract from TGD B Vol. 2
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.
4.6.5 Plastic Rooflights
Table 4.4 sets out the limitations on the use of plastic rooflights which do not meet the basic provisions described in Table 4.3 but which have a lower surface of thermoplastic material with a TP(a) rigid or TP(b) classification (see A16 of Appendix A).
Table HB7 - Limitations on plastic rooflights - Extract from TGD B Vol. 2
Diagram HB19 - Limits on spacing and size of plastic rooflights having a TP(b) lower surface - Extract from TGD B Vol. 2
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 7:1997, can be regarded as having BROOF(t4) (European class) or an AA (National class) designation.
4.6.6 Glass in Rooflights
When used in rooflights, unwired glass at least 4 mm thick can be regarded as having BROOF(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, veranda, 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.
