Compartment walls and floors are specifically intended to ensure that fire is contained in the compartment of origin, and is not allowed to spread horizontally or vertically through a building. The objectives are:
- To prevent rapid spread of fire and products of combustion which may endanger occupants.
- To prevent a small fire growing to threaten occupants, people in the vicinity of the building, and fire fighters who may have to enter a building to extinguish the fire. The compartmented structure provides demarcated lines of safety for fire-fighters and occupants.
The allowable size of a compartment will vary with the height and use of a building, the fire load contained in the building, and the ability of fire-fighters to intervene effectively. In some cases, the availability of a dependable sprinkler system may allow larger compartment sizes, but it should be recognised that sprinkler systems require adequate water pressures and regular maintenance practices to ensure reliable performance.
Any compartment wall below a service void should run continuously up through the void to prevent the spread of fire through the void. Where the void is a roof void, the wall should reach roof level or pass through the roof to a specified height to prevent spread of fire across the roof. The junctions of compartment walls or floors with each other, with external walls or roofs must provide continuity of the expected fire-resisting performance.
Any element (including structural elements) passing through compartment walls or floors should have associated fire stopping at the point of penetration and the aperture should be kept as small as practicable.
The design should ensure that the failure of a penetrating structure because of fire in one compartment will not cause failure in the adjacent compartment. The same comment applies to the passage of building services, and special provisions are required for protected shafts.
Most guidance documents to building regulations provide recommendations for allowable compartment sizes and guidance on junctions with other walls and roofs. It should be noted that resilient fire stopping systems are recommended where compartment walls meet roofs, and double skinned roof sheeting should incorporate bands of material of limited combustibility centred over each compartment wall.
Insurers may have higher requirements than building regulations to minimise the damage to the property and to the business itself. In addition, the compartment wall may require enhanced fire resistance to the walls and roof within the 'protected zone' on each side of the compartment wall.