Fire Compartmentation Calculator — Compartment Sizes per Part B

A fire compartment is a building space enclosed by fire-resisting walls and floors designed to prevent fire spread for a specified period. Compartmentation limits the size of a fire, protects escape routes, and gives occupants time to evacuate. Under Approved Document B, compartment walls and floors must achieve a defined fire-resistance period based on the building purpose group, height and number of storeys. In non-residential buildings (AD B Vol 2 §8.11) compartment walls are provided where needed to keep within the Table 8.1 size limits; every floor is a compartment floor where the top storey is above 30m, the ground floor over any basement is a compartment floor, and intermediate basement floors are compartment floors where the basement is more than 10m deep.

Maximum compartment dimensions are set in AD B Vol 2 Table 8.1 and depend on the purpose group, not a single blanket figure. Offices have NO compartment floor-area limit (single- or multi-storey). Shops are 2000 m² single-storey and 2000 m² multi-storey without sprinklers (single-storey shops become No limit with sprinklers; multi-storey 4000 m² with sprinklers). Assembly and recreation (and non-shop commercial) are No limit single-storey and 2000 m² multi-storey (4000 m² with sprinklers). Industrial buildings are No limit single-storey and 7000 m² multi-storey up to 18m / 2000 m² above 18m (14,000 m² / 4000 m² with sprinklers) — by floor area. Storage is the only group limited by compartment VOLUME for multi-storey buildings: single-storey storage is 20,000 m² floor area (No limit with sprinklers), while multi-storey storage is 20,000 m³ volume up to 18m / 4000 m³ above 18m (40,000 m³ / 8000 m³ with sprinklers). Sprinklers raise these limits but do not apply to offices, which are unlimited either way.

Building height is the primary factor for the required fire-resistance period in AD B Vol 2 Appendix B, Table B2 (the minimum-periods table; it was Table A2 in the 2006/2013 editions and Table B4 in the 2019+2020+2022 edition). Periods are banded by the height of the top floor above ground: Up to 5m, Up to 11m, Up to 18m, Up to 30m and More than 30m. For an office without sprinklers that is 30 / 60 / 60 / 90 minutes, and a top storey above 30m is not permitted without sprinklers. Shops and assembly buildings start at 60 minutes even at single-storey; industrial and storage are 60 minutes up to 5m then 90 minutes up to 18m, rising to 120 minutes up to 30m. Basements are keyed on the depth of the lowest basement: up to 10m deep needs 60 minutes (90 for industrial/storage), and more than 10m deep needs 90 minutes (120 for industrial/storage).

Yes. A sprinkler system to BS EN 12845 (AD B Appendix E) raises the Table 8.1 compartment floor-area limit and can also reduce the Table B2 fire-resistance period. For example, a multi-storey shop limit rises from 2000 to 4000 m², and an office with sprinklers needs only 30 minutes up to 18m instead of 60 (and 60 minutes up to 30m instead of 90). Sprinklers are mandatory rather than optional in some cases: a block of flats with a top storey above 11m, and any office, shop, assembly, industrial or storage building with a top storey above 30m, must have sprinklers. Sprinklers do not change the office area (unlimited either way) and do not eliminate the need for compartmentation.

Typical 60-minute compartment-wall constructions include 100mm dense concrete blockwork (plastered or rendered), a metal-stud partition with 2 layers of 15mm fire-rated plasterboard each side plus mineral-wool infill, or 200mm cavity blockwork with tied leaves. These are indicative only — fire resistance must be proven by test to BS EN 1364-1 (walls), or by a manufacturer’s tested system (for example British Gypsum White Book or Knauf fire-test data), because the exact board, fixings and stud spacing all affect the rating. All junctions with other fire-resisting elements must be fire-stopped, service penetrations sealed to BS EN 1366-3, and the wall must extend to the structural floor or roof above — not just to a suspended ceiling.