Section 12 Construction of external walls

Construction of external walls

  1. Introduction
    1. Fire resistance standard
      1. Portal frames
        1. External wall construction
          1. External surfaces
            1. Insulation Materials/Products
              1. Cavity barriers

              Introduction

              12.1Provisions 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 Section 13 which sets out limits on the amount of unprotected area of wall. 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 have no fire resistance, except for any parts which are loadbearing (see paragraph B3.ii).

              External walls are elements of structure and the relevant period of fire resistance (specified in Appendix A) depends on the use, height and size of the building concerned. If the wall is 1000mm or more from the relevant boundary, a reduced standard of fire resistance is accepted in most cases and the wall only needs fire resistance from the inside.

              12.2Provisions are also made to restrict the combustibility of external walls of buildings that are less than 1000mm from the relevant boundary and, irrespective of boundary distance, the external walls of high buildings and those of the Assembly and Recreation Purpose Groups. This is in order to reduce the surface's susceptibility to ignition from an external source and to reduce the danger from fire spread up the external face of the building.

              In the guidance to Requirement B3, provisions are made in Section 7 for internal and external loadbearing walls to maintain their loadbearing function in the event of fire.

              Fire resistance standard

              12.3The external walls of the building should have the appropriate fire resistance given in Appendix A, Table A1, unless they form an unprotected area under the provisions of Section 13.

              Portal frames

              12.4Portal frames are often used in single storey industrial and commercial buildings where there may be no need for fire resistance of the structure (Requirement B3). However, where a portal framed building is near a relevant boundary, the external wall near the boundary may need fire resistance to restrict the spread of fire between buildings.

              It is generally accepted that a portal frame acts as a single structural element because of the moment-resisting connections used, especially at the column/rafter joints. Thus, in cases where the external wall of the building cannot be wholly unprotected, the rafter members of the frame, as well as the column members, may need to be fire protected.

              Following an investigation of the behaviour of steel portal frames in fire, it is considered technically and economically feasible to design the foundation and its connection to the portal frame so that it would transmit the overturning moment caused by the collapse, in a fire, of unprotected rafters, purlins and some roof cladding, while allowing the external wall to continue to perform its structural function. The design method for this is set out in the SCI publication P313 Single storey steel framed buildings in fire boundary conditions, 2002 (ISBN: 1 85942 135 0).

              Notes:The recommendations in the SCI publication for designing the foundation to resist overturning need not be followed if the building is fitted with a sprinkler system in accordance with paragraph 0.16.
              Normally, portal frames of reinforced concrete can support external walls requiring a similar degree of fire resistance without specific provision at the base to resist overturning.
              Existing buildings may have been designed to the following guidance which is also acceptable:

              • athe column members are fixed rigidly to a base of sufficient size and depth to resist overturning;
              • bthere is brick, block or concrete protection to the columns up to a protected ring beam providing lateral support; and
              • cthere is some form of roof venting to give early heat release. (The roof venting could be, for example, PVC rooflights covering some 10 per cent of the floor area and evenly spaced over the floor area.)

              External wall construction

              12.5The external envelope of a building should not provide a medium for fire spread if it is likely to be a risk to health or safety. The use of combustible materials in the cladding system and extensive cavities may present such a risk in tall buildings.

              Externall walls should either meet the guidance given in paragraphs 12.6 to 12.9 or meet the performance criteria given in the BRE Report Fire performance of external thermal insulation for walls of multi storey buildings (BR 135) for cladding systems using full scale test data from BS 8414-1:2002 or BS 8414-2:2005.

              The total amount of combustible material may also be limited in practice by the provisions for space separation in Section 13 (see paragraph 13.7 onwards).

              External surfaces

              12.6The external surfaces of walls should meet the provisions in Diagram 40. Where a mixed use building includes Assembly and Recreation Purpose Group(s) accommodation, the external surfaces of walls should meet the provisions in Diagram 40c.

              Insulation Materials/Products

              12.7In a building with a storey 18m or more above ground level any insulation product, filler material (not including gaskets, sealants and similar) etc. used in the external wall construction should be of limited combustibility (see Appendix A). This restriction does not apply to masonry cavity wall construction which complies with Diagram 34 in Section 9.

              Cavity barriers

              Diagram 40

              12.8Cavity barriers should be provided in accordance with Section 9.

              12.9In the case of a an external wall construction, of a building which, by virtue of paragraph 9.10d (external cladding system with a masonry or concrete inner leaf), is not subject to the provisions of Table 13 Maximum dimensions of cavities in non-domestic buildings, the surfaces which face into cavities should also meet the provisions of Diagram 40.

              • 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)