A school built Circa 1969, has a common cavity above a false ceiling that is 580mm in height (from the top of the false ceiling tile to the underside of the original roof). The cavity spans the whole school (classrooms + corridors). The HM Education Guide is useless regarding guidance on cavity barriers and I understand the issues with applying BB100 retrospectively; however, BB100 does state the maximum dimensions of a cavity in any direction as 20 metres, which I intend to use for reference purposes - has anybody else used BB100 retrospectively?

Also, I cannot find any reference regarding the cavity height i.e. is there any recognised guidance that takes account of the cavity height? Obviously a cavity height of 100mm compared to 1000mm is a lesser risk to fire spread.

Sounds like it's a CLASP building for which there are limited practical improvements that can be made to the passive FP issues as they are endemic to the means of construction (they were meant to be temporary and all demolished after 10-15 years). The usual approach is L1 fire alarm, get everyone out and sacrifice the building.

The various editions of BB7, the predecessor of BB100, may deal with this, I would have to check my copies in the office.

There are some sector guides dealing with what you can do to improve CLASP buildings fire protection, it would be worth casting your eyes over those too.

Thanks for your input Anthony. Regarding the height of the cavity, are you aware of any recognised guidance?