Compartmentation

A compartment is a part of a building enclosed by fire-resisting construction - floors, walls and the doors and seals in them - so that a fire starting inside it is contained within it, and a fire outside is kept out, for a defined period. Picture the building not as one continuous volume but as a set of sealed boxes stacked and tiled together. Each floor slab is a horizontal compartment boundary; walls between tenancies, around stairs, along corridors and around hazardous rooms are vertical ones. The aim is simple and life-saving: when a fire breaks out in one box, it and its smoke are held there long enough for everyone in the other boxes - and ideally in the fire box too - to reach a place of safety.

This containment does several jobs at once. It protects the escape routes, which is why escape stairs are almost always compartments in their own right, sealed off from the floors they serve so that smoke cannot follow people down. It protects other occupants who are not yet in the escape route, buying them time to become aware and move. It limits the total damage and the fire load that the fire service must fight, keeping the fire to a size a brigade can tackle. And it buys time for the structure, because a fire held in one compartment puts less of the frame at risk at once.

> A compartment is a promise the building makes: a fire that starts here will stay here, for a while. Everything else in a fire strategy is built on that promise being kept.

Think of compartmentation as the containment leg of the three jobs - detect early, escape fast, contain long - made physical in the fabric of the building. It is the reason a fire in one flat need not empty the whole block, why a kitchen fire in a hotel need not reach the bedrooms, and why a plant room fire need not climb the building. None of that happens by accident; it is designed, line by line, on the plan.

A compartment is only real if its boundary is continuous. That sounds obvious, but it is where most compartmentation fails in practice. A fire-rated wall that stops at a suspended ceiling and leaves the void above open is not a boundary - smoke and fire simply travel over the top through the ceiling void. A compartment wall must run full height, slab to slab (or to the underside of a fire-rated roof), and every floor that forms a compartment boundary must be unbroken. The enclosure has to be thought of as a complete, sealed skin, not a line drawn on the plan at eye level.

Then there are the things that must pass through the boundary - and every one of them is a potential breach. Doors are the obvious case: a fire-rated, self-closing door lets people and goods move through a wall in normal use, then shuts and holds when a fire comes. It only works if it is the right rating, self-closes reliably, is not wedged or propped open, and has seals that resist smoke and, with intumescent strips, expand to close gaps in the heat. Services - pipes, cables, ducts - pierce compartments everywhere, and each hole must be firestopped back to the rating of the wall (the whole of lesson 4.3). Ducts that cross a compartment need fire or smoke dampers that close the opening. Glazing in a compartment boundary must itself be fire-rated glass, not ordinary glass that cracks and falls in minutes.

The materials and construction that achieve the rating - the masonry, the layers of fire-rated board, the concrete - are covered in lesson 4.2, and the sealing of openings in 4.3. The design discipline here is to treat the whole envelope of each compartment as one rated system: the wall, its doors, its glazing, its penetrations and its junction with the floor and the wall beside it all have to hold together for the same period. A 120-minute wall with a 30-minute door, or a perfect wall with an open hole above the ceiling, is only as good as its weakest part.

If compartments contain fire, why not make them as small as possible - or, the usual real-world pressure, why can we not make them as large as the client's open-plan dream? Codes answer this by setting maximum compartment sizes - limits on floor area and sometimes volume - that a single compartment may reach before it must be subdivided. The logic is that a bigger compartment holds more fuel, lets a fire grow larger before it meets a boundary, puts more people and more of the structure at risk at once, and is harder for the fire service to fight. Subdividing keeps each fire event to a manageable scale.

The permitted size is not a single number; it depends on the occupancy (a warehouse full of goods is treated very differently from an office), the height of the building (tall buildings are held to stricter limits because escape and firefighting are harder), and crucially on whether the building is sprinklered. A very common lever in codes worldwide is that fitting an automatic sprinkler system, which controls a fire early, permits significantly larger compartments than an unsprinklered building of the same type - an explicit trade between active and passive protection. This is one of the clearest places where the two halves of fire protection meet.

What drives the maximum compartment size (verify every figure):
  - Occupancy / use .......... more hazardous or higher fuel load -> smaller
  - Building height .......... taller -> stricter limits
  - Sprinklers fitted? ....... yes -> usually a larger limit is allowed
  - Single vs multi-storey ... a whole floor may be one compartment, or not

A blunt but honest warning belongs here: the exact areas, volumes and the sprinkler trade-off are binding numbers that vary by code, occupancy and height, and they change between editions. Treat any figure you carry in your head as a rough sense of scale only, and verify the governing limit for your actual project against the current NBC 2016 Part 4 and the authority having jurisdiction - and, on anything large or unusual, with a fire engineer. Designing an open-plan space right up to a remembered limit is exactly how a scheme ends up non-compliant at approval.

The hardest truth about compartmentation is that it is invisible and fragile in use, and it fails quietly. A fire wall you cannot see behind the plaster, a fire door that looks like any other door, a sealed penetration hidden in a ceiling void - none of them announce themselves, and all of them can be defeated by ordinary, well-meaning building life. The single most common way compartmentation dies is the propped-open fire door: held back with a wedge or an extinguisher for convenience, it turns a sealed compartment into an open doorway at the exact moment it must be shut. A fire door that is not closed when the fire comes is not a fire door; it is a hole.

The other classic failures are breaches made after handover. A contractor drills a new cable through a compartment wall and does not firestop it. A tenant fit-out removes a section of rated partition or cuts a serving hatch through it. A maintenance crew runs a new pipe through a floor slab and leaves the gap open. Each is a small, local act; each can open a path for smoke to travel from the fire compartment into an escape route or another occupancy. This is why compartmentation is not only a design problem but a management and inspection problem for the life of the building - a theme lesson 4.3 and Module 9 return to.

For you as a designer, the instinct to build is this: draw the compartment lines explicitly, early, and defend them. Know where every compartment boundary runs, insist that openings in it are minimised and properly detailed, resist the client and the services engineer when they want to pierce a rated wall for convenience, and specify fire doors with self-closers and hold-open devices that release on alarm rather than wedges. A fire strategy drawing that shows the compartment lines in a bold, unmissable way - and is handed on to those who manage and alter the building - is one of the most valuable pieces of paper the project produces.