Stopping Hidden Dangers

Particularly important for tall buildings, avoidance of fire and smoke spreading through concealed spaces and gaps is an essential part of fire safety. While this is covered in regulatory guidance such as Approved Document B, some requirements are often overlooked.

Taking effect in April this year, Volume 2 of the latest Approved Document for Part B (AD B) applies to flats and non-dwellings, and covers England and Wales - although other national, regulatory guidelines are similar. There are clear requirements to inhibit the spread of fire and smoke in concealed spaces using cavity barriers and also to maintain continuity of fire protection at joints and openings in fire separating elements with fire stops. Cavity barriers - as distinct from fire stops - are needed in various locations to close cavities and sub-divide ‘extensive cavities'. They can also be used to continue walls protecting escape routes and certain bedrooms into ceiling and floor voids - but not to extend compartment walls into these areas. An important addition to the latest AD B is that extensive cavities in floor voids - as well as ceilings - should now be subdivided with cavity barriers, a provision which was omitted from the previous edition.


Cavity barriers are also required to close cavities in external wall constructions such as rain screen systems. In tall buildings, where the top floor is 18m or more above ground, stringent requirements apply to what goes into external walls to avoid fire spread to adjacent buildings - including the cavity barriers - which must all be of ‘limited combustibility', as defined in AD B Appendix A. Many cavity barriers currently available for rainscreen and other cladding constructions fail to comply with these requirements. It is therefore essential that specifiers and building control officers satisfy themselves that cavity barriers in high rise buildings will perform as intended. In addition, all cavity barriers must achieve a minimum of 30 minutes integrity and 15 minutes insulation, when tested using the principles of BS476 Part 20. This requirement is simply not achievable with some systems - and often not enforced.

For fire separating elements (such as compartment walls/floors) to be effective, every joint, imperfection of fit or opening for services should be protected by fire stopping. This should have the same fire resistance as the compartment or fire separating element it is protecting. AD B gives minimum fire resistance for building elements - and hence fire stopping - related the top floor height above ground and whether or not sprinklers are fitted. This can be as high as 120 minutes with some tall buildings.


It is of fundamental importance that cavity barriers and fire stopping systems are able to accommodate deflections to maintain the required level of fire protection over time and in differing conditions, particularly in a fire. While this requirement is dealt with in AD B, it is far from obvious and often overlooked. For example, fire stops at the top of compartment walls should be able to accommodate the predicted deflection of the floor above - but this can be substantial and ADB assumes up to 40mm unless a smaller value can be justified by assessment. Most fire stopping systems are unable to accommodate deflections of this nature unless made from materials capable of pre-compression to give enough integral resilience.

For more information please see our Lamatherm Fire Protection