Section 9 Concealed spaces (cavities)

Concealed spaces (cavities)

  1. Introduction
    1. Provision of cavity barriers
      1. Pathways around fire-separating elements
        1. Junctions and cavity closures
          1. Protected escape routes
            1. Double-skinned corrugated or profiled roof sheeting
              1. Cavities affecting alternative escape routes
                1. Separation of bedrooms
                2. Extensive Cavities
                  1. Maximum dimensions of concealed spaces
                  2. Construction and fixings for cavity barriers

                    Introduction

                    9.1Concealed spaces or cavities in the construction of a building provide a ready route for smoke and flame spread. This is particularly so in the case of voids in, above and below the construction of a building, e.g. walls, floors, ceilings and roofs. As any spread is concealed, it presents a greater danger than would a more obvious weakness in the fabric of the building.

                    Provision of cavity barriers

                    Diagram 33

                    9.2Provisions for cavity barriers are given below for specified locations. The provisions necessary to restrict the spread of smoke and flames through cavities are broadly for the purpose of sub-dividing:

                    • acavities, which could otherwise form a pathway around a fire-separating element and closing the edges of cavities; therefore reducing the potential for unseen fire spread; and
                      Note: These should not be confused with fire- stopping details, see Section 10 and Diagram 33 (see also paragraphs 9.3 to 9.7).
                    • bextensive cavities (see paragraphs 9.8 to 9.12).

                    Consideration should also be given to the construction and fixing of cavity barriers provided for these purposes and the extent to which openings in them should be protected. For guidance on these issues, see paragraphs 9.13 to 9.16 respectively.

                    Pathways around fire-separating elements

                    Junctions and cavity closures

                    9.3Cavity barriers should be provided to close the edges of cavities, including around openings.

                    Cavity barriers should also be provided:

                    • aat the junction between an external cavity wall (except where the cavity wall complies with Diagram 34) and every compartment floor and compartment wall; and
                    • bat the junction between an internal cavity wall (except where the cavity wall complies with Diagram 34) and every compartment floor, compartment wall, or other wall or door assembly which forms a fire-resisting barrier.

                    It is important to continue any compartment wall up through a ceiling or roof cavity to maintain the standard of fire resistance – therefore compartment walls should be carried up full storey height to a compartment floor or to the roof as appropriate, see paragraphs 8.21 to 8.24. It is therefore not appropriate to complete a line of compartmentation by fitting cavity barriers above them.

                    Protected escape routes

                    9.4For a protected escape route, a cavity that exists above or below any fire-resisting construction because the construction is not carried to full storey height or, in the case of a top storey, to the underside of the roof covering, should either be:

                    • afitted with cavity barriers on the line of the enclosure(s) to the protected escape route; or
                    • bfor cavities above the fire-resisting construction, enclosed on the lower side by a fire-resisting ceiling which extends throughout the building, compartment or separated part (see Diagram 35).

                    Double-skinned corrugated or profiled roof sheeting

                    9.5Cavity Barriers need not be provided between double-skinned corrugated or profiled insulated roof sheeting, if the sheeting is a material of limited combustibility and both surfaces of the insulating layer have a surface spread of flame of at least Class 0 or 1 (National class) or Class C- s3, d2 or better (European class) (see Appendix A) and make contact with the inner and outer skins of cladding (see Diagram 36).

                    Notes:See also paragraph 8.29 Note 2 regarding the junction of a compartment wall with a roof.
                    When a classification includes “s3, d2”, this means that there is no limit set for smoke production and/or flaming droplets/particles.

                    Cavities affecting alternative escape routes

                    9.6Cavitiy barriers may be needed where corridors are be sub-divided to prevent alternative escape routes being simultaneously affected by fire and/or smoke (see paragraph 3.26 and Diagram 16).

                    Separation of bedrooms

                    9.7In Institutional and Other Residential buildings, a cavity that exists above or below partitions between bedrooms because the enclosures are not carried to full storey height, or, (in the case of the top storey) to the underside of the roof covering, should either be:

                    • afitted with cavity barriers on the line of the partitions; or
                    • bfor cavities above the partitions, enclosed on the lower side by a fire-resisting ceiling which extends throughout the building, compartment or separated part.

                    Extensive Cavities

                    Table 13

                    9.8Cavity barriers should be used to sub-divide any cavity, including any roof space, so that the distance between cavity barriers does not exceed the dimensions given in Table 13.

                    Maximum dimensions of concealed spaces

                    9.9Table 13 sets out maximum dimensions for undivided concealed spaces. With the exceptions given in paragraphs 9.10 to 9.12, extensive concealed spaces should be sub-divided to comply with the dimensions in Table 13.

                    9.10The provisions in Table 13 do not apply to any cavity described below:

                    • ain a wall which should be fire-resisting only because it is loadbearing;
                    • bin a masonry or concrete external cavity wall shown in Diagram 34;
                    • cin any floor or roof cavity above a fire- resisting ceiling, as shown in Diagram 35 and which extends throughout the building or compartment subject to a 30m limit on the extent of the cavity; or
                    • dformed behind the external skin of an external cladding system with a masonry or concrete inner leaf at least 75mm thick, or by overcladding an existing masonry (or concrete) external wall, or an existing concrete roof, provided that the cavity does not contain combustible insulation and the building is not put to a residential or institutional use; or
                    • ebetween double-skinned corrugated or profiled insulated roof sheeting, if the sheeting is a material of limited combustibility and both surfaces of the insulating layer have a surface spread of flame of at least Class 0 or 1 (National class) or Class C-s3, d2 or better (European class) (see Appendix A) and make contact with the inner and outer skins of cladding (see Diagram 36) ; or
                    • fbelow a floor next to the ground or oversite concrete, if the cavity is less than 1000mm in height or if the cavity is not normally accessible by persons, unless there are openings in the floor such that it is possible for combustibles to accumulate in the cavity (in which case cavity barriers should be provided and access should be provided to the cavity for cleaning).

                    Notes:When a classification includes “s3, d2”, this means that there is no limit set for smoke production and/or flaming droplets/particles.

                    9.11Where any single room with a ceiling cavity or underfloor service void exceeds the dimensions given in Table 13, cavity barriers need only be provided on the line of the enclosing walls/ partitions of that room, subject to:

                    • athe cavity barriers being no more than 40m apart; and
                    • bthe surface of the material/product exposed in the cavity being Class 0 or Class 1 (National class) or Class C-s3, d2 or better (European class).

                    Notes:When a classification includes “s3, d2”, this means that there is no limit set for smoke production and/or flaming droplets/particles.

                    9.12Where the concealed space is an undivided area which exceeds 40m (this may be in both directions on plan) there is no limit to the size of the cavity if:

                    • athe room and the cavity together are compartmented from the rest of the building;
                    • ban automatic fire detection and alarm system meeting the relevant recommendations of BS 5839-1:2002 is fitted in the building. Detectors are only required in the cavity to satisfy BS 5839-1.
                    • cthe cavity is used as a plenum and the recommendations about recirculating air distribution systems in BS 5588-9:1999 are followed;
                    • dthe surface of the material/product used in the construction of the cavity which is exposed in the cavity is Class 0 (National class) or Class B-s3, d2 or better (European class) and the supports and fixings in the cavity are of non-combustible construction;
                    • ethe flame spread rating of any pipe insulation system is Class 1 or Class C-s3, d2 or better (European class) (see Appendix A);
                    • fany electrical wiring in the void is laid in metal trays, or in metal conduit; and
                    • gany other materials in the cavity are of limited combustibility or Class A2 or better (European class) (see Appendix A).

                    Notes:When a classification includes “s3, d2”, this means that there is no limit set for smoke production and/or flaming droplets/particles.

                    Construction and fixings for cavity barriers

                    9.13Every cavity barrier should be constructed to provide at least 30 minutes fire resistance. It may be formed by any construction provided for another purpose if it meets the provisions for cavity barriers (see Appendix A, Table A1, item 15).

                    Cavity barriers in a stud wall or partition, or provided around openings may be formed of:

                    • asteel at least 0.5mm thick;
                    • btimber at least 38mm thick;
                    • cpolythene-sleeved mineral wool, or mineral wool slab, in either case under compression when installed in the cavity; or
                    • dcalcium silicate, cement-based or gypsum-based boards at least 12mm thick.

                    Notes:Cavity barriers provided around openings may be formed by the window or door frame if the frame is constructed of steel or timber of the minimum thickness in a) or b) above as appropriate.

                    9.14A cavity barrier should, wherever possible, be tightly fitted to a rigid construction and mechanically fixed in position. Where this is not possible (for example, in the case of a junction with slates, tiles, corrugated sheeting or similar materials) the junction should be fire-stopped. Provisions for fire-stopping are set out in Section 10.

                    9.15Cavity barriers should also be fixed so that their performance is unlikely to be made ineffective by:

                    • amovement of the building due to subsidence, shrinkage or temperature change and movement of the external envelope due to wind;
                    • bcollapse in a fire of any services penetrating them;
                    • cfailure in a fire of their fixings (but see note below); and
                    • dfailure in a fire of any material or construction which they abut. (For example, if a suspended ceiling is continued over the top of a fire- resisting wall or partition and direct connection is made between the ceiling and the cavity barrier above the line of the wall or partition, premature failure of the cavity barrier can occur when the ceiling collapses. However, this may not arise if the ceiling is designed to provide fire protection of 30 minutes or more.)

                    Notes:Where cavity barriers are provided in roof spaces, the roof members to which they are fitted are not expected to have any fire resistance – for the purpose of supporting the cavity barrier(s).

                    9.16Any openings in a cavity barrier should be limited to those for:

                    • a. doors which have at least 30 minutes fire resistance (see Appendix B, Table B1, item 8) and are fitted in accordance with the provisions of Appendix B;
                    • bthe passage of pipes which meet the provisions in Section 10;
                    • cthe passage of cables or conduits containing one or more cables;
                    • dopenings fitted with a suitably mounted automatic fire damper (see paragraphs 10.11 to 10.15); and
                    • educts which (unless they are fire-resisting) are fitted with a suitably mounted automatic fire damper where they pass through the cavity barrier.

                    Notes:If a cavity barrier is provided above a partition separating bedrooms in accordance with paragraph 9.7 which do not need to be fire resisting partitions then a to e need not apply. However, opinings in the barrier should be kept to a minimum and any penetrations should be sealed to restict the passage of smoke.