What is a fire compartment?

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. Every floor in a building above or below ground is generally a compartment floor.

What are the maximum compartment sizes in the UK?

Maximum compartment floor areas are defined in AD B Table 12 and depend on the purpose group. Offices, shops and assembly buildings are generally limited to 2000 square metres per compartment floor. Low-risk single-storey storage buildings can be up to 20000 square metres. High-risk storage is limited to 4000 square metres single storey or 500 square metres multi-storey. Residential buildings (flats, hotels) are limited to 2000 square metres. These limits can be doubled when a BS EN 12845 sprinkler system is installed.

How does building height affect fire resistance requirements?

Building height is the primary factor in determining the required fire resistance period under AD B Table A2. For most purpose groups: buildings up to 5m height need 30 minutes (some need 60); 5-18m height need 60 minutes; 18-30m height need 90 minutes; and over 30m height need 120 minutes. Basements always require a minimum of 60 minutes regardless of depth. These periods ensure structural stability during a fire for long enough to allow evacuation and firefighting.

Do sprinklers reduce compartmentation requirements?

Yes. Installing a sprinkler system to BS EN 12845 allows compartment floor areas to be doubled per AD B Table 12. For example, an office building limit increases from 2000 to 4000 square metres per compartment. Sprinklers can also allow some relaxation of travel distances and structural fire resistance periods in certain building types. However, sprinklers do not eliminate the need for compartmentation entirely. The fire resistance period of compartment walls and floors remains unchanged.

What construction is needed for a 60-minute compartment wall?

A 60-minute compartment wall can be achieved with: 100mm dense concrete blockwork (plastered or rendered), metal stud partition with 2 layers of 15mm fire-rated plasterboard each side plus 50mm mineral wool infill, or 200mm cavity blockwork with tied leaves. All junctions with other fire-resisting elements must be fire-stopped. Penetrations for services require fire stopping to BS EN 1366-3. Compartment walls must extend to the full height of the building or to a compartment floor, not just to a suspended ceiling.

Fire Compartmentation Calculator — Compartment Sizes per Part B

How We Calculate This

This calculator determines fire compartmentation requirements based on Approved Document B of the UK Building Regulations. It considers the building purpose group, height, number of storeys and whether sprinklers are installed to calculate maximum compartment sizes and required fire resistance periods.

How it works

Purpose group: AD B classifies buildings by use. Each purpose group has different compartment area limits and fire resistance requirements.
Building height: The primary factor for fire resistance period. Taller buildings need longer fire resistance.
Sprinklers: A BS EN 12845 sprinkler system allows compartment areas to be doubled.
Basements: Always require at least 60-minute fire resistance, increasing to 90 minutes for multi-level basements.

Key references

AD B Table A2: Minimum fire resistance periods
AD B Table 12: Maximum compartment dimensions
BS 9999: Code of practice for fire safety in buildings (alternative to AD B)
BS EN 12845: Automatic sprinkler systems

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.