The purpose of a cavity barrier is is to prevent smoke and flame from penetrating and/or moving from one concealed fire compartment to another.

Documents supporting the regulations provide guidance on where such barriers should be located within hidden voids in a building. They usually give examples of deemed-to-satisfy barriers for voids in stud walls or partitions.

If a barrier in a concealed space coincides with a compartment wall or floor it will normally be required to provide the same fire performance as the wall or floor. If the barrier is located between such walls or floors however, the barrier is defined as a "cavity barrier". As such, the barrier will normally and only be required to provide 30 minutes integrity and 15 minutes insulation.

A large cavity barrier is defined as a barrier across a void in which a square with 1m sides can be accommodated. A small cavity barrier is one in which such a square cannot be accommodated. A large cavity barrier is required to provide 30 minutes integrity and 15 minutes insulation. A small cavity barrier need only provide 30 minutes integrity. Cavity barrier definitions are also dependent on the requirements of local building regulations.