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5.4.1 GENERAL CONSIDERATIONS

This part of the Design Guide assumes that building separation and boundary distances are determined using methods described in the Building Research Establishment Report: External fire spread (see 5.7, References). In addition,

5 Design Guide for the Fire Protection of Buildings

this Design Guide gives further recommendations necessary to ensure adequate property protection. It should be noted that, for the purposes of this Design Guide, a boundary is assumed to exist between buildings on the same site and the methodology in the above report should be followed to establish the separating distance.

An important consideration as regards all premises is the potential for fire spread via roofs whether from buildings on the same site or from adjacent or adjoining property. Internal compartmentation may be compromised if a fire is able to spread unchecked across a roof. Whilst not being regarded as as severe a hazard as that of radiation, the risk of fire spread via burning brands from an adjacent building must not be overlooked. The effect of high winds should also be taken into account when assessing the potential for fire spread across roofs particularly in areas exposed to the elements.

Note: This Design Guide does not include guidance on other important aspects concerned with roofs, for example, waterproofing, and reference should be made to appropriate technical guidance to ensure that this and other design considerations are addressed.

Roof to meet requirements in 5.4.3

Boundary

See 5.4.1

Boundary distance

Figure 5.2. External fire exposure from adjacent building to external wall (see 5.4).

Figure 5.1. External fire exposure from adjacent building to roof (see 5.4).

External fire exposure 5

5.4.2 W^ALLS

In some instances it may be necessary to adopt both passive and active, that is, sprinkler, protection to ensure that adequate property protection is provided.

The outcome of the risk assessment should identify the extent of the passive and active protection necessary.

5.4.2.1 Walls - passive fire protection

Implementation of the recommendations contained in Parts 2, 3 and 4 of this Design Guide will ensure as far as possible that in many instances buildings are adequately protected from both radiation and fire spreading from exposing buildings or premises. Where there is exposure from an adjacent high risk necessitating the provision of a higher level of fire resistance, attention to the following will assist the provision of suitable protection:

(1) If the separating distance between the buildings is less than 10m or critical business equipment or processes are at risk consideration should be given to the provision of walls meeting the fire resistance and insulation provisions of Table 2.1 of this Design Guide subject to a minimum of 30min integrity and 15min insulation. Doors and windows should have fire resistance of not less than 30min in terms of integrity.

The presence of small areas of non-fire-resisting glazing and doors in the exposed building may be acceptable particularly if apertures in the wall are small and in total do not exceed 10 per cent of the wall area.

Note: In some situations a distance of 10m may be inadequate to provide appropriate protection and consideration should be given to increasing the separating distance to ensure adequate property protection is provided. In some instances it may be appropriate to refer to the provisions of section 20 of the London Building Act 1939 (with addendums) for guidance.

(2) Buildings that overlook adjacent low rise buildings, the roof construction of which does not comply with 5.4.3 or is not fire-resisting, should be protected by ensuring that the envelope of the building has fire resistance as described in (1) above. The protection should extend a minimum height of 10m above the roof of the adjacent building. See Fig. 5.3.

Minimum of 10m fire-resisting external wall (see 5.4.2)

See 5.4.3

Figure 5.3. External fire exposure from roof to adjacent building (see 5.4).

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(3) Exposure from a building at right angles, whether it be a different occupancy or within the same occupancy, requires special consideration. The external wall in both buildings should meet the conditions specified in 3.3.1.2 for a distance of not less than 10m in each direction from the corner. See Fig. 5.4.

Note: See 5.4.2.2 regarding the alternative of automatic sprinkler protection.

5.4.2.2 Walls - sprinkler protection

The following aspects must be born in mind when considering the provision of sprinkler protection in the exposed building:

(i) the sprinkler protection should be suitable for the occupancy hazard in the building;

(ii) it should not be compromised by an external fire in exposing buildings or outdoor storage;

(iii) an external fire may result in exposure along the length, or throughout the height, of the exposed building and cause a large number of sprinkler heads to open thereby rendering the sprinkler installation ineffective;

(iv) sprinkler protection alone will not prevent fire spread along the external surface of walls and therefore exposed buildings should have an external envelope which complies with 2.2 or 5.4.2.1 as regards walls;

(v) where compartmentation, and the protected zone on either side, is at risk of being by-passed by fire spreading from an exposing building on the same site, sprinkler protection of both buildings should be considered as an alternative to the provision of fire-resisting construction;

(vi) where an external hazard may increase the risk of compartment walls being compromised the distances specified in 3.3.1.2 and 3.3.1.3 should be increased as necessary.

Note: Sprinkler systems in adjacent buildings in different ownership should be treated with caution in view of the lack of any continuing guarantee of

Figure 5.4. Fire exposure from building at right angles (plan view).

Wall to comply with 3.3.1.2

Occupancy B Occupancy A

External fire exposure 5

efficiency and maintenance and to account for periods when the installation may be turned off.

5.4.3 ROOFS- GENERAL

The materials used in the construction of roofs of all buildings should preferably conform to the recommendations contained in 2.2 of this Design Guide but at the very least should not permit the rapid spread of fire across the external surface when exposed to radiation or fire emanating from adjacent buildings.

See 5.5.3 for further guidance.

5.4.3.1 Roofs - passive fire protection

Roofs of low rise buildings should, in addition to complying with the protected zone criteria in connection with compartmentation as detailed in 3.4, be constructed in accordance with the following provisions to resist spread of fire by radiation or other means from adjacent or adjoining buildings:

(i) Careful selection of materials, and detailing, particularly, as regards pitched roofs, at eaves and for all buildings at the junctions with roof lights and roof ventilators is essential to ensure that fire is not able to spread via any weaknesses at these points.

(ii) With respect to external fire exposure the roofing system, comprising the roof covering, thermal insulation and roof deck should be classified not less than EXT.F.AC or EXT.S.AC when tested in accordance with BS 476: Part 3: 1958.

(iii) The provision of rooflights or ventilators in any roof may assist rapid fire spread to inside the building or to nearby roofs or buildings and they should therefore be classified not less than EXT.F.AC or EXT.S.AC when tested in accordance with BS 476: Part 3: 1958.

However the following should be noted:

• The following wording appears in Approved Document B 1991, Section 14.6: ‘When used in rooflights, a rigid Thermoplastic sheet product made from polycarbonate or from plasticised PVC, which achieves a Class 1 rating for surface flame spread when tested to BS476 Part 7, 1971 or 1987, can be regarded as having an AA designation.’

• It may not be possible to maintain the prescribed underpressure conditions when testing ventilators. For the purpose of this Design Guide, ventilators used in roofs may be tested and classified without complying with the underpressure conditions prescribed in BS 476: Part 3: 1958.

(iv) In buildings containing high values or critical business equipment or processes, rooflights and ventilators should not have a dimension greater than 1m and be spaced not less than 3m apart in any direction.

This restriction does not apply to rooflights or ventilators having a rating of EXT.F.AA or EXT.S.AA when tested in accordance with BS 476: Part 3: 1958.

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5.4.3.2 Roofs - sprinkler protection

Subject to the risk assessment, it may be possible to relax some of the requirements given in 5.4.3.1 when the building is fully sprinkler protected.

However, the recommendations and restrictions in relation to the protected zone shall still be met and, in addition, the following should be noted:

• the provision of a properly designed automatic sprinkler installation in an exposing building should prevent a fire breaking out through the roof from inside the building;

• it should be noted that an external roof fire will not be controlled by a sprinkler system and the exposed building may be severely damaged as a result of external fire spread. Roofs of buildings which are so exposed should be constructed of materials described in 5.4.3.1;

• reference should be made to 5.5.3.2 where the guidance on fire spreading from inside the building and igniting the roof is equally applicable.

5.4.3.3 Roofs - smoke ventilation

In non-sprinklered single storey buildings, where a properly designed and installed automatic smoke extraction system is installed, serious deformation or collapse of the roof may well be delayed as a result of the release of some of the hot gases.

As non-automatic smoke extraction systems rely on the swift response of the fire brigade, no reduction in the performance requirements given in 5.4.3.1 is recommended unless shown to be justified by the risk assessment. In addition, it is recommended that the design of the ventilator ensures that any combustible materials used in the roof construction cannot be ignited by hot gases passing through the opened vent (see 5.4.3.1).

Where a roof presents a severe exposure hazard to an adjoining building on the same site it is essential that the location of the ventilators does not increase the exposure risk.

Note: This Design Guide recommends that no reliance is made on venting the fire through openings caused by collapsed plastic rooflights. Where the designer wishes to extract smoke and hot gases a properly designed smoke extraction system should be installed.