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.

                    • 1 Gallery floors with no alternative exit Diagram 1 Gallery floors with no alternative exit
                    • 2 Flat where all habitable rooms have direct access to an entrance hall Diagram 2 Flat where all habitable rooms have direct access to an entrance hall
                    • 3 Flat with restricted travel distance from furthest point to entrance Diagram 3 Flat with restricted travel distance from furthest point to entrance
                    • 4 Flat with an alternative exit but where all habitable rooms have no direct acess to an entrance hall Diagram 4 Flat with an alternative exit but where all habitable rooms have no direct access to an entrance hall
                    • 5 Multi-storey flat with alternative exits from each habitable room except at entrance level 5 Multi-storey flat with alternative exits from each habitable room except at entrance level
                    • 6 Multi-storey flat with protected entrance hall and landing Diagram 6 Multi-storey flat with protected entrance hall and landing
                    • 7 Flats served by one common stair,png Diagram 7 Flats served by one common stair
                    • 8 Flats served by more than one common stair 8 Flats served by more than one common stair
                    • 9 Common escape route in small single stair building Diagram 9 Common escape route in small single stair building
                    • 10 Travel distance in dead-end condition 10 Travel distance in dead-end condition
                    • 11 Alternative escape routes 11 Alternative escape routes
                    • 12 Inner room and access room 12 Inner room and access room
                    • 13 Exits in a central core Diagram 13 Exits in a central core
                    • 14 Open Connections Diagram 14 Open Connections
                    • 15 Merging flows at final exit Diagram 15 Merging flows at final exit
                    • 16 Subdivision of corridors Diagram 16 Subdivision of corridors
                    • 17 Dead-end corridors Diagram 17 Dead-end corridors
                    • 18 Maximum travel distances in a small two or three storey premises with a single protected stair to each storey Diagram 18 Maximum travel distances in a small two or three storey premises with a single protected stair to each storey
                    • 19 Progressive horizontal evacuation in care homes Diagram 19 Progressive horizontal evacuation in care homes
                    • 20 Refuge formed by compartmentation Diagram 20 Refuge formed by compartmentation
                    • 21 Refuge formed in a protected stairway Diagram 21 Refuge formed in a protected stairway
                    • 22 Max travel distance in a small three storey premises with a single stair to each storey Diagram 22 Max travel distance in a small three storey premises with a single stair to each storey
                    • 23 Max travel distance in a small two storey premises with a single open stair Diagram 23 Max travel distance in a small two storey premises with a single open stair
                    • 24 External protection to protected stairways Diagram 24 External protection to protected stairways
                    • 25 Fire resistance of areas adjacent to external stairs 25 Fire resistance of areas adjacent to external stairs
                    • 26 Lighting diffuser in relation to ceiling Diagram 26 Lighting diffuser in relation to ceiling
                    • 27 Layout restrictions on Class 3 plastic roofflights TP-b rooflights and TP b lighting diffusers Diagram 27 Layout restrictions on Class 3 plastic rooflights TP-b rooflights and TP b lighting diffusers
                    • 27a Layout restrictions on small Class 3 plastic rooflights TP b rooflights and lighting diffusers Diagram 27a Layout restrictions on small Class 3 plastic rooflights TP b rooflights and lighting diffusers
                    • 28 Compartment floors illustration of guidance in paragraph 8.16 Diagram 28 Compartment floors illustration of guidance in paragraph 8.16
                    • 29 Compartment walls and compartment floors with reference to relevant paragraphs in Section 8 Diagram 29 Compartment walls and compartment floors with reference to relevant paragraphs in Section 8
                    • 30 Junction of compartment wall with roof Diagram 30 Junction of compartment wall with roof
                    • 31 Protected shafts Diagram 31 Protected shafts
                    • 32 Uninsulated glazed screen separating protected shaft from lobby or corridor Diagram 32 Uninsulated glazed screen separating protected shaft from lobby or corridor
                    • 33 Provisions for cavity barriers Diagram 33 Provisions for cavity barriers
                    • 34 Cavity wall excluded from provisions for cavity barriers Diagram 34 Cavity wall excluded from provisions for cavity barriers
                    • 35 Fire-resisting ceiling below concealed space Diagram 35 Fire-resisting ceiling below concealed space
                    • 36 Provisions for cavity barriers in double-skinned insulated roof sheeting Diagram 36 Provisions for cavity barriers in double-skinned insulated roof sheeting
                    • 37 Pipes penetrating structure Diagram 37 Pipes penetrating structure
                    • 38 Enclosure for drainage or water supply pipes Diagram 38 Enclosure for drainage or water supply pipes
                    • 39 Flues penetrating compartment walls or floors Diagram 39 Flues penetrating compartment walls or floors
                    • 40 Provisions for external surfaces or walls Diagram 40 Provisions for external surfaces or walls
                    • 41 Relevant boundary Diagram 41 Relevant boundary
                    • 42 Notional boundary Diagram 42 Notional boundary
                    • 43 Status of combustible surface material as unprotected area Diagram 43 Status of combustible surface material as unprotected area
                    • 44 Unprotected areas which may be disregarded in assessing the separation distance from the boundary Diagram 44 Unprotected areas which may be disregarded in assessing the separation distance from the boundary
                    • 45 The effect of a canopy on separation distance Diagram 45 The effect of a canopy on separation distance
                    • 46 Permitted unprotected areas in small residential buildings Diagram 46 Permitted unprotected areas in small residential buildings
                    • 47 Limitations on spacing and size of plastic rooflights having a Class 3 or Class D-s3 d2 or TP b lower surface Diagram 47 Limitations on spacing and size of plastic rooflights having a Class 3 or Class D-s3 d2 or TP b lower surface
                    • 48 Example of building footprint and perimeter Diagram 48 Example of building footprint and perimeter
                    • 49 Relationship between buildings and hardstanding access roads for high reach fire appliances Diagram 49 Relationship between buildings and hardstanding access roads for high reach fire appliances
                    • 50 Turning facilities Diagram 50 Turning facilities
                    • 51 Provision of firefighting shafts Diagram 51 Provision of firefighting shafts
                    • 52 Components of a firefighting shaft Diagram 52 Components of a firefighting shaft
                    • 53 Fire-resisting construction for smoke outlet shafts Diagram 53 Fire-resisting construction for smoke outlet shafts
                    • C1 Measurement of door width Diagram C1 Measurement of door width
                    • C2 Cubic capacity Diagram C2 Cubic capacity
                    • C3 Area Diagram C3 Area
                    • C4 Height of building Diagram C4 Height of building
                    • C5 Number of storeys Diagram C5 Number of storeys
                    • C6 Height of top storey in buildingC6 Height of top storey in building Diagram C6 Height of top storey in building
                    • C7 Free area of smoke ventilators Diagram C7 Free area of smoke ventilators
                    • D1 Classification of Purpose Groups Diagram D1 Classification of Purpose Groups
                    • E1 Recessed car parking areas Diagram E1 Recessed car parking areas
                    • Table 1 Limitations on distance of travel in common areas of blocks of flats - see par 2.23 Table 1 Limitations on distance of travel in common areas of blocks of flats - see par 2.23
                    • Table 2 Limitations on travel distance Table 2 Limitations on travel distance
                    • Table 3 Minimum number of escape routes and exits from a room tier or storey Table 3 Minimum number of escape routes and exits from a room tier or storey
                    • Table 4 Widths of escape routes and exits Table 4 Widths of escape routes and exits
                    • Table 5 Maximum distances of travel in small premises with a protected stair Table 5 Maximum distances of travel in small premises with a protected stair
                    • Table 6 Minimum widths of escape stairs Table 6 Minimum widths of escape stairs
                    • Table 7 Capacity of a stair for basements and for simultaneous evacuation of the building Table 7 Capacity of a stair for basements and for simultaneous evacuation of the building
                    • Table 8 Minimum width of stairs designed for phased evacuation Table 8 Minimum width of stairs designed for phased evacuation
                    • Table 9 Provisions for escape lighting Table 9 Provisions for escape lighting
                    • Table 10 Classification of linings Table 10 Classification of linings
                    • Table 11 Limitations applied to thermoplastic rooflights and lighting diffusers in suspended ceilings and Class 3 plastic rooflights Table 11 Limitations applied to thermoplastic rooflights and lighting diffusers in suspended ceilings and Class 3 plastic rooflights
                    • Table 12 Maximum dimensions of building or compartment - non-residential buildings Table 12 Maximum dimensions of building or compartment - non-residential buildings
                    • Table 13 Maximum dimensions of cavities in non-domestic buildings - purpose Groups 2-7 Table 13 Maximum dimensions of cavities in non-domestic buildings - purpose Groups 2-7
                    • Table 14 Maximum nominal internal diameter of pipes passing through a compartment wall-floor - see par 10.5 onwards Table 14 Maximum nominal internal diameter of pipes passing through a compartment wall-floor - see par 10.5 onwards
                    • Table 15 Permitted unprotected areas in small buildings or compartments Table 15 Permitted unprotected areas in small buildings or compartments
                    • Table 16 Limitations on roof coverings Table 16 Limitations on roof coverings
                    • Table 17 Class 3 or Class D-s3 d2 plastic rooflights limitations on use and boundary distance Table 17 Class 3 or Class D-s3 d2 plastic rooflights limitations on use and boundary distance
                    • Table 18 TP a and TP b plastic rooflights limitations on use and boundary distance Table 18 TP a and TP b plastic rooflights limitations on use and boundary distance
                    • Table 19 Fire and rescue service vehicle access to buildings excluding blocks of flats not fitted with fire mains Table 19 Fire and rescue service vehicle access to buildings excluding blocks of flats not fitted with fire mains
                    • Table 20 Typical fire and rescue service vehicle access route specification Table 20 Typical fire and rescue service vehicle access route specification
                    • Table A1 Specific provisions of test for fire resistance of elements of structure etc Table A1 Specific provisions of test for fire resistance of elements of structure etc
                    • Table A1 continued Table A1 continued
                    • Table A2 Minimum periods of fire resistance Table A2 Minimum periods of fire resistance
                    • Table A3 Limitations on fire-protecting suspended ceilings - see Table A1 Note 4 Table A3 Limitations on fire-protecting suspended ceilings (see Table A1 Note 4)
                    • Table A4 Limitations on the use of uninsulated glazed elements on escape routes Table A4 Limitations on the use of uninsulated glazed elements on escape routes]
                    • Table A5 Notional designations of roof coverings Table A5 Notional designations of roof coverings
                    • Table A6 Use and definitions of non-combustible materials Table A6 Use and definitions of non-combustible materials
                    • Table A7 Use and definitions of materials of limited combustibility Table A7 Use and definitions of materials of limited combustibility
                    • Table A8 Typical performance rating of some generic materials and products Table A8 Typical performance rating of some generic materials and products
                    • Table B1 Provisions for fire doors Table B1 Provisions for fire doors
                    • Table C1 Floor space factors Table C1 Floor space factors
                    • Table D1 Classification of Purpose Groups Table D1 Classification of Purpose Groups
                    • Table Maximum dimensions of building or compartment - non-residential buildings Table Maximum dimensions of building or compartment (non-residential buildings)