Building Standards technical handbook 2017: domestic buildings

In order to reduce the danger to the occupants of other buildings, one building should be isolated from another by either construction or distance. The distance between a building and its relevant boundary is dictated by the amount of heat that is likely to be generated in the event of fire. This will be influenced by the extent of openings or other unprotected areas in the external wall of the building.

The guidance for fire spread to neighbouring buildings should not be assessed in isolation and reference should be made to the guidance to Standard 2.4 for fire spread in cavities and the guidance to Standard 2.7 for fire spread on external walls.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

Apart from unprotected areas, as described in clause 2.6.2, external walls should have:

However short fire resistance duration is sufficient even if not more than 1m from the boundary, for the following:

Fire resistance duration need not be provided for a building ancillary to a dwelling, comprising a carport, covered area, greenhouse, summerhouse, or swimming pool enclosure unless the building contains oil or liquefied petroleum gas fuel storage (see section 3 and section 4).

An unprotected area means any part of an external wall (including a door or window opening) which does not attain the appropriate fire resistance duration as recommended in the table to clause 2.6.1.

An unprotected area does not include a fixed unopenable window where the frame and glazing has the appropriate fire resistance duration. Any wallhead fascia, soffit or barge board, or any cavity vents or solum vents may also be excluded from the unprotected area calculation (for external wall cladding see clause 2.6.4).

Up to 500mm from boundary - where the external wall of a building is not more than 500mm from the boundary there should be no unprotected area, other than any wallhead fascia, soffit or barge board, or any cavity vents or solum vents.

Up to 1m from boundary - where the external wall of a building is more than 500mm but not more than 1m from the boundary, the level of unprotected area is limited to:

More than 1m from boundary - where the external wall of a building is more than 1m from the boundary the amount of unprotected area (in square metres) may be equivalent to six times the distance (in metres) to the boundary. Therefore if the distance to the boundary is at least 1m, the unprotected area should not exceed 6m², if the distance to the boundary is at least 2m, the unprotected area is 12m², and so on.

Where the external wall of a building is more than 6m from the boundary, the amount of unprotected area is unlimited.

The use of the simple geometry method described above is limited to buildings which are more than 1m from the boundary, not more than 9m in height, and the length of the side of the building facing the boundary is not more than 24m. Any external side of a building which makes an angle of more than 80⁰ with the plane of reference can be ignored in the calculation of unprotected area. The diagram below provides a typical example of a conservatory attached to a house.

Figure 2.3. Conservatory example

Where a domestic building exceeds these limits, reference could be made to the Enclosing Rectangle Method (Table B) contained in the non-domestic guidance document or the Building Research Establishment Report ‘External fire spread: building separation and boundary distances’ (BR 187, 1991). In these cases, for the purpose of calculating the enclosing rectangle, a separating wall or separating floor should be regarded as a compartment wall or compartment floor.

External wall cladding includes all non load-bearing external wall cladding systems attached to the structure, for example, clay or concrete tiles, slates, pre-cast concrete panels, stone panels, masonry, profiled metal sheeting including sandwich panels, weather boarding, thermally insulated external wall rendered systems, glazing systems and other ventilated cladding systems.

External wall cladding not more than 1m from a boundary should have a non-combustible classification.

Fire within the building may break out through a window or door opening and as a consequence, the cladding, once ignited, would contribute to the heat generated from the fire. Therefore where the cladding is more than 1m from a boundary and is constructed from combustible material more than 1mm thick that has a low, medium, high or very high risk (as described in annex 2.B), the cladding should be included in the calculation of unprotected area.

However combustible cladding need not be included in the calculation of unprotected area where:

In addition, a cavity formed by external wall cladding should be protected in accordance with the guidance to Standard 2.4 and fire spread on external walls in accordance with the guidance to Standard 2.7.

Houses – External wall cladding to a house need not have a non-combustible classification where the external wall has the appropriate fire resistance and the cladding achieves a low risk reaction to fire classification. In such a case the cladding may be excluded from the unprotected area calculation regardless of openings.

Every part of an external wall including external wall cladding (see clause 2.6.4 for exceptions) not more than 1m from a boundary or the external wall of a high rise domestic buildings, should be constructed of non-combustible products. This does not apply to insulation exposed in a cavity that is between two leaves of masonry or concrete at least 75mm thick, and which has a cavity barrier around all openings in the wall and at the top of the wall-head.

However a structural frame which is low, medium, high or very high risk (see annex 2.B) may be used not more than 1m from a boundary provided:

See the guidance to Standard 2.7 for additional recommendations on external wall cladding and the alternative to the need for non-combustibility mentioned above.

Where an element of structure provides support to an external wall (including external wall cladding) which has a fire resistance duration (as recommended in the guidance to clauses 2.6.1 and 2.6.2) the supporting element of structure should also have at least the same fire resistance duration.