Part B Fire Calculator — Escape Routes & Fire Resistance

The period comes from Table B2 in Appendix B of Approved Document B, which is the same table in Volume 1 and Volume 2. You read it by purpose group and by the height of the top floor above ground, in bands of up to 5m, up to 11m, up to 18m, up to 30m and more than 30m, with two further columns for a basement storey read on the depth of the lowest basement. A dwellinghouse is 30 minutes up to 5m and 60 minutes up to 18m, with note 5 against the up to 11m cell relaxing it to a minimum of 30 minutes for a three storey dwellinghouse, increased back to 60 minutes minimum for compartment walls separating buildings; this calculator reports the unrelaxed cell value and tells you when note 5 is available. Table B2 note 4 puts a dwellinghouse with a top storey more than 18m above ground, or with a 10m deep basement, outside the scope of the guidance, so there is no 120 minute figure to quote for a house. A block of flats is 30 minutes up to 5m, 60 minutes up to 18m, 90 minutes up to 30m and 120 minutes above 30m, with sprinklers mandatory above 11m under note 2. Residential institutional and other residential buildings, purpose groups 2(a) and 2(b), run 30, 60, 60, 90 and 120 minutes across the same bands. An unsprinklered office is 30 minutes up to 5m, while an unsprinklered shop or assembly building is 60 minutes in that same band, and unsprinklered industrial and storage buildings are 60 minutes up to 5m and 90 minutes up to 18m. Where there is a basement, Table B2 note * takes the higher of the basement period and the ground or upper storey period. Fire doorset ratings come separately from AD B Appendix C Table C1 and are not always the same period, for example a flat entrance door is FD30S in a 60 minute compartment wall, and a protected stairway doorset in a single family dwellinghouse is minimum E 20 (FD20).

Two triggers are set out in the Approved Documents and this calculator models both. Table B2 note 2: a block of flats with a top storey more than 11m above ground level should be fitted with a sprinkler system in accordance with Appendix E. AD B Volume 2 para 8.14: buildings in the office, shop and commercial, assembly and recreation, industrial and storage and other non-residential purpose groups, that is purpose groups 3 to 7(a), require sprinklers where there is a top storey more than 30m above ground level. A third trigger is indirect: Table 8.1 sets maximum compartment sizes for buildings without a sprinkler system, and its notes say that where those limits are exceeded a sprinkler system to Appendix E should be provided. Sprinklers can also be required by the fire strategy for the building, and fitting them voluntarily can allow a lower period in Table B2 or a larger compartment in Table 8.1.

Regulation 7(2) of the Building Regulations 2010 requires materials which become part of an external wall, or of a specified attachment, of a relevant building to achieve European Class A2-s1,d0 or Class A1, subject to the exemptions in regulation 7(3). Regulation 7(4) defines a relevant building as one with a storey at least 18m above ground level which contains one or more dwellings, an institution, or a room for residential purposes. The threshold is 18m, and AD B gives the same figure at Volume 1 para 10.14 and Volume 2 para 12.15. SI 2022/603 added regulation 7(1A) and removed the exclusion for rooms in a hostel, hotel or boarding house; it came into force on 1 December 2022, with a transitional window that ran to 1 June 2023, and it did not lower the height. The 11m figure that is often quoted alongside regulation 7 is not regulation 7. It is AD B para 10.7 in Volume 1 and para 12.7 in Volume 2, the same provision under two numbers, which requires any insulation product or filler material in the external wall of a purpose group 1 or 2 building with a storey 11m or more to be class A2-s1,d0 or better, subject to an exception for masonry cavity wall construction; and para 10.10, or 12.11 in Volume 2, which restricts balcony materials at the same height. Regulation 7(2) itself starts at 18m and, where it applies, it prevails over that paragraph. Which volume you cite depends on the building: Volume 1 covers dwellings, so a dwellinghouse or a block of flats, while Volume 2 covers everything else including care homes and hotels, which are residential but are not dwellings. Offices, shops, assembly buildings, industrial buildings and warehouses are not relevant buildings under regulation 7(4) at any height and are outside the 11m rule too, but AD B Volume 2 para 12.6 catches any building with a storey 18m or more and requires insulation and filler material in the external wall to be class A2-s3,d2 or better.

Purpose groups classify a building by use. 1(a) is a flat, 1(b) and 1(c) are dwellinghouses, 2(a) is residential institutional such as a care home or hospital, 2(b) is other residential such as a hotel, hostel or boarding house, 3 is office, 4 is shop and commercial, 5 is assembly and recreation, 6 is industrial, 7(a) is storage and other non-residential and 7(b) is a car park. The purpose group is the row you read in Table B2 for fire resistance periods, in AD B Volume 2 Table 2.1 for travel distances and in Table 8.1 for compartment sizes, so getting it wrong changes every figure downstream.

Single storey: direct exit (window can serve as escape). Two storey: no floor is more than 4.5m above ground, so each habitable room (other than a kitchen) needs either an escape window or external door or direct access to a protected stairway, with no fire doorsets required on that route; escape windows in first floor habitable rooms need min 0.33 m² unobstructed openable area, no dimension less than 450mm height/width, sill max 1100mm above floor. Three storey: protected stairway from top to ground in a minimum REI 30 enclosure with minimum E 20 (FD20) doorsets to the rooms opening onto it, no smoke seal and no self-closer in a single family dwellinghouse (AD B Vol 1 para 2.21b, Table C1 item 11(b) and Appendix C5c); alternative escape route required for the third storey if sprinklers not fitted (per AD B Vol 1 Section 2). FD20 doorsets are no longer certified as products, so an FD30 door is normally fitted to meet the E 20 minimum. Mains-wired interlinked smoke alarms (BS 5839-6 Grade D LD3 minimum) on every storey.