Fire alarm categories explained: L1, L2, L3, L4, L5, P1 and P2

Why the category matters more than the brand

Two buildings can both have a fully working, correctly serviced, certificated fire alarm system, and one of them can be substantially less safe than the other. The difference is the category — how much of the building automatic detection actually covers.

Category is set out in BS 5839-1. It should come from your fire risk assessment. It should not come from a price comparison.

Life safety categories

L1 — total coverage. Automatic detection in all areas of the building, including voids over 800mm and areas people rarely enter. Gives the earliest possible warning and the longest available escape time. Typically specified where there is sleeping risk or where occupants cannot evacuate quickly.

L2 — escape routes, adjoining rooms, and defined high-risk areas. Everything L3 covers, plus specific rooms identified in the fire risk assessment as high risk — plant rooms, kitchens, laundries, storage.

L3 — escape routes and rooms opening onto them. Designed to give warning while escape routes are still passable. This is the most commonly specified category in ordinary commercial premises.

L4 — escape routes only. Detection in the circulation areas forming the escape route. A fire starting in a room will not be detected until it reaches the corridor, which by definition means less warning time.

L5 — a custom category. Designed around a specific risk or a specific fire engineering objective, defined in the system design rather than by the standard. L5 is legitimate; "L5" used as a way of describing a system nobody designed is not.

Property protection categories

P1 — total coverage for the earliest possible warning to limit property damage.

P2 — detection in defined high-risk areas only, where a fire would be most likely to start or would be most damaging.

P-categories exist to protect the building and its contents, and they are usually driven by an insurer rather than by fire safety law. A system can be both — for instance L2/P2 — and many are.

The trap: assuming the category you have

Most buildings we survey do not know their category. The system was installed by someone, extended by someone else, and has been serviced for years by a third party who was never asked to comment on design.

The common findings are:

• A building operating as an L3 that has had detection removed from a room during a refit and is now, functionally, an L4
• A change of use — offices converted to residential, a warehouse now with a mezzanine office — that makes the original design category no longer appropriate
• Storage areas or plant rooms added without detection
• A system described in the paperwork as L2 with none of the high-risk rooms actually covered

None of these show up on a service certificate, because servicing tests what is installed. It does not ask whether what is installed is the right thing.

How to decide

1. Start with the fire risk assessment. Sleeping risk, occupant mobility, escape route length and building height all push the category up.
2. Ask what the building is used for now, not what it was designed for.
3. Check whether your insurer has specified a P-category. Many policies do, in the schedule rather than the wording.
4. Where a lower category is chosen for cost reasons, record the decision and the reasoning. A documented decision is defensible. An undocumented gap is not.

What it costs to get wrong

The cost of upgrading a category is a known number and can be programmed over a couple of budget years. The cost of a fire that started in an unmonitored room is not.