Fire stopping and cavity barriers explained: Approved Document B Sections 9 and 10

Buildings are divided into compartments by fire-resisting walls and floors, so a fire is held in one part long enough for people to escape and firefighters to work. But every joint, gap and hole for services is a weak point. Approved Document B says each one should be sealed with fire stopping, so the wall or floor keeps its fire resistance.

And cavities, any concealed space, give smoke and flame a hidden route around those walls and floors, which is why they need cavity barriers.

The two are easy to confuse, and Approved Document B warns against it. Fire stopping is a seal that closes a gap, an imperfect fit, or a hole where a service passes through, to stop fire and smoke getting past. A cavity barrier is a construction inside a cavity that closes it off, or stops smoke and flame travelling along it.

Fire stopping is about the holes. Cavity barriers are about the hidden spaces.

Paragraph 9.3 of Volume 2 sets out where cavity barriers go. At the edges of cavities, including around openings such as windows, doors and where services come in. Where an external cavity wall meets every compartment floor and compartment wall.

And where an internal cavity wall meets a compartment floor, a compartment wall, or any other fire-resisting wall or door. A compartment wall can't be finished off with a cavity barrier above it: the wall itself should go right up to the floor or roof above.

Cavities also matter around escape routes. If the fire-resisting enclosure of a protected escape route stops short of full height, or doesn't reach the roof on the top storey, the cavity above or below it needs cavity barriers on the line of the enclosure. The alternative is a fire-resisting ceiling, of at least 30 minutes, running across the whole building or compartment.

The same applies above or below partitions between bedrooms in residential buildings. And replacing windows can mean adding cavity barriers around the new openings.

Large cavities should also be divided up. Under Table 9.1, a cavity between a roof and a ceiling should be no more than 20 metres in any direction. Other cavities can be up to 20 metres where the exposed surfaces are class C or better for reaction to fire, but only 10 metres where they're worse.

There are exceptions, for example a ceiling void above a single large room, where barriers can follow the room's walls if they're no more than 40 metres apart.

A cavity barrier, tested from each side, should give at least 30 minutes' integrity and 15 minutes' insulation. In a stud wall or partition, or around openings, Approved Document B accepts simpler materials. They include steel at least half a millimetre thick, timber at least 38 millimetres, mineral wool in a polythene sleeve or slab, squeezed into the cavity, or 12 millimetre boards.

Barriers should be tightly fitted and mechanically fixed, and still work if the building moves, if services through them collapse, or if the ceiling they meet falls away.

Openings through a cavity barrier should be kept to a short list: a fire doorset of at least 30 minutes, pipes that follow Section 10, and cables and conduits. Then openings with a suitable fire damper, and ducts that are either fire-resisting themselves or fitted with a fire damper where they pass through. Anything else leaves a hole in the barrier.

Section 10 covers openings for services. A pipe through a fire-separating element should use one of three approaches. A, a proprietary sealing system that's been tested to keep the wall or floor's fire resistance, for any size of pipe.

B, a pipe kept below the maximum diameter in Table 10.1, with fire stopping around it and the hole as small as possible. In most situations, that's 40 millimetres for plastic pipes, and 160 millimetres for pipes of a metal like cast iron, copper or steel. Or C, a pipe of up to 160 millimetres, in a material such as uPVC or aluminium, inside a sleeve of high melting point metal.

Ducts are a bigger hole, so Approved Document B gives four ways to protect a wall or floor they pass through. A fire damper that closes with heat, a fire-resisting enclosure, fire-resisting ductwork, or a fire and smoke damper closed by smoke detectors. Kitchen extract ducts shouldn't rely on dampers, because grease can stop them closing.

Dampers should meet BS EN 15650, last at least 60 minutes or match the wall, and be reachable for inspection, testing and maintenance.

For fire stopping in general, paragraph 10.24 asks for joints between fire-separating elements to be sealed. Openings for pipes, ducts, conduits and cables should be as few as possible, as small as practicable, and fire stopped, allowing pipes and ducts to move with heat. Where the gap is wider than 100 millimetres, or the material isn't rigid, it should be reinforced or supported.

Proprietary, tested fire stopping and sealing systems can be used. Other materials include cement mortar, gypsum plaster, vermiculite or perlite mixes, mineral fibre products, and intumescent mastics. But Approved Document B stresses that not every material suits every situation.

Use a system that's been tested for that kind of wall or floor, that service and that gap, and install it the way it was tested. The Association for Specialist Fire Protection, the ASFP, publishes detailed guidance.

Fire stopping is easily damaged after handover, for example when someone runs a new cable through a fire wall and never seals the hole. The Home Office guide for responsible persons says compartmentation includes the openings in it: fire doors, glazing, service penetrations and ductwork. Check regularly that it's still effective, especially after any work that might breach it, and have damage repaired by a competent person.

Contractors running cables or pipes through compartment walls must co-operate. And the fire safety information handed over at completion should show where the fire-separating elements are, including cavity barriers in spaces people can walk into.