Glossary — Building Fire Safety
Fire Compartmentation
Compartmentation is the principle of dividing buildings into fire-resistant cells. When intact, it confines a fire to its room of origin long enough for everyone to escape. When breached — by open doors, unsealed cables, or poor workmanship — it can fail catastrophically.
Definition
Fire compartmentation — the division of a building into separate fire-resistant sections (compartments) using walls, floors, and protected openings that limit the spread of fire and smoke for a defined period. Compartment boundaries are rated by the resistance period they achieve: 30, 60, or 120 minutes being the most common in UK buildings.
Most common compartmentation failure points
Identified in fire compartmentation surveys and fire investigations. Any one of these can render an entire compartment boundary ineffective.
Pipes, cables, and ducts that pass through a compartment wall must be fire-stopped with intumescent products. Unsealed penetrations are the most common failure.
A fire door held open defeats the entire compartment. Self-closing devices must be fitted and functional.
Door-edge strips expand in heat to seal the gap. Missing or damaged strips mean smoke passes freely.
Loft conversions, partition walls, and new openings often introduce un-fire-stopped breaks in existing compartment walls.
Some insulation products within wall cavities burn and allow fire to travel horizontally between compartments unseen.
If a fire compartment wall does not extend above a suspended ceiling to the structural soffit, fire travels over the top.
How compartmentation works
A fire compartment is a three-dimensional cell formed by fire-resisting walls, floors, and ceilings. The principle is borrowed from ship design: if one compartment floods, watertight doors keep the rest of the ship afloat. In a building, if one compartment catches fire, fire-resisting construction keeps the rest of the building habitable long enough for evacuation.
The rated period — 30, 60, or 120 minutes — refers to how long the construction maintains its integrity, insulation, and load-bearing capacity under standard fire test conditions (as defined in BS EN 1363 and related parts). These tests simulate a real compartment fire using a standardised temperature-time curve.
For the technical detail of how resistance periods are expressed and tested, see our fire resistance rating glossary entry.
Passive vs active fire protection
Compartmentation is a form of passive fire protection — it does not require activation or power. It works automatically whenever a fire occurs. Active fire protection — sprinklers, alarms, suppression systems — responds to a fire but relies on detection, power, and mechanical operation. See our fire suppression glossary entry and our overview of sprinkler systems. The best-protected buildings use both passive and active systems in combination.
Compartmentation in flats and HMOs
In purpose-built blocks of flats, the walls and floor/ceiling between each flat form the primary compartment boundary — typically rated to 60 minutes. The flat entrance fire door is a critical element of this boundary. If it is wedged open, damaged, or replaced with a non-fire-rated door, the entire compartment is compromised.
Houses in multiple occupation (HMOs) are required under the Housing Act 2004 to have adequate fire compartmentation. In practice, many older converted HMOs have significant breaches in their compartment walls from previous building work. A fire risk assessment will identify these. Read our 2026 high-rise fire safety update for current regulatory context.
Frequently asked questions
FAQ