How is the fire resistance period determined?

Fire resistance period depends on building height, number of storeys and purpose group (use type). Domestic buildings up to 5m: 30 minutes. 5-18m: 60 minutes. Over 18m: 120 minutes. Non-domestic buildings follow similar but stricter thresholds. The period applies to structural elements, floors, walls and fire doors.

When are sprinklers required?

Sprinklers are mandatory in residential buildings over 11m height (about 4 storeys), all buildings over 30m, and certain warehouse/industrial buildings. They may also be required by fire engineering assessment. Voluntary sprinkler installation can allow relaxations in compartment sizes, travel distances and fire resistance requirements.

What are the external wall material requirements?

Since the Building Safety Act 2022 and updated AD B, buildings over 18m must use non-combustible external wall materials (Class A2-s1,d0 or better). This affects cladding, insulation, cavity barriers and fixings. Buildings under 18m have less strict requirements but must still meet appropriate fire performance standards.

What is a purpose group?

Purpose groups classify buildings by use: 1a (dwelling house), 1b (flat), 2a (residential care), 2b (hotel), 3 (office), 4 (shop), 5 (assembly), 6 (industrial), 7 (storage). The purpose group determines fire resistance periods, compartment sizes, means of escape requirements and sprinkler thresholds.

What means of escape are required for a dwelling?

Single storey: direct exit. Two storey: protected stairway, FD30S doors, escape windows in first floor rooms. Three storey: protected stairway from top to ground, all rooms to have access to stairway, alternative escape for top floor may be needed. Mains-wired interlinked smoke alarms on every floor.

TradeCalculator.co.uk

Part B Fire Calculator — Escape Routes & Fire Resistance

Determine Part B fire safety requirements based on building height, purpose group and floor area including fire resistance period, means of escape and sprinkler requirements.

Purpose group

Building height (to top floor)

Number of storeys

Typical floor area

How We Calculate This

This calculator determines Approved Document B (Fire Safety) requirements from building parameters.

Fire resistance

Based on Tables A1/A2 of AD B, cross-referencing building height and purpose group.

Means of escape

Travel distances, stairway protection, fire door requirements and smoke detection are determined by building type and height.

Frequently Asked Questions

Related Calculators

Building Control Fee Calculator

Estimate building control fees for UK domestic projects including plan check, inspection and VAT.

Part L Compliance Calculator

Check element U-values against Part L 2021 targets for walls, floors, roofs and windows.

Part E Sound Test Calculator

Check separating wall and floor dB targets and select systems to achieve Part E compliance.

Part F Ventilation Calculator

Calculate Part F ventilation system selection with extract rates and background ventilators.

Fire Compartmentation Calculator

Calculate fire compartment maximum area and fire wall/door positions by building use.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.