Roof void sub-division

[Paul1979]

Hi All,

Looking for some advice regarding roof void separation.

We have a long (40m) roof void space which has no sub-division at all. It's an old building (1800's) and is a converted house now used for a mix of commercial and residential purposes. We have plasterboard ceilings throughout though. The roof voids are empty, no persons work up there, and there is void fire detection.

But we are receiving conflicting advice from fire assessors and fire service so I wanted to find out for myself what is the correct thing to do.

Looking at the tech stuff (ADB Vol2), "6.7 The need for cavity barriers in concealed floor or roof spaces can be reduced by installing a fire
resisting ceiling (minimum EI 30) below the cavity, complying with Diagram 9.3." [this is the risk assessors arguement].

However, item 9.9 of same said document also states "Cavity barriers should be used to divide any cavity (including roof spaces)". It also says that the maximum dimension should be 20m, which would suggest we need to put a fire barrier around the midway point. [this is the fire service's argument].

Any clues / tips as to which of the above is correct?! both arguments make sense, but we just need to know which is right!

Thanks as always for any advice!

[witsd]

Thinking of normal residential arrangements, there would usually be a minimum of 60 minutes protection between houses, be that connecting walls or two 30 minute rated flat doors opening into the same lobby. In this case, if your ceilings are at least 30 minutes, then any fire would need to breach the ceiling twice to pass from one house to another, getting you that 60 minutes. A fire might start in the void, but that risk is mitigated by the detection in there.

Your cavity barrier guidance is not retrospective, and therefore the assessor wouldn't necessarily be directing you to follow it if existing controls are good enough.

But.

You said it that the building had been converted. The crucial question for me, is when was it converted, and what did the building standards require at that time.

If the 20m limit existed then, why was it not put in place as part of the conversion?

[Paul1979]

Hi Witsd

Thank you for the detailed reply.

We don’t know the exact date of conversion I’m afraid but know it was pre-1980’s.

I think the assessor is trying to use the guidance in ADB just as a tool to explain the fire risks.