Requirements for ceiling above service cupboard

[bigalxyz]

In our (leasehold) building there’s an electrical service cupboard on the ground floor. Above that there’s a service duct which goes all the way to the top of the building - for electrical stuff, phone lines, etc.

The cupboard and the duct are approx 90cm x 90cm in cross section.

Above the service cupboard there’s no ceiling - if you look up you can see the “naked” joists running across. Our fire safety assessor doesn’t like it - if a fire broke out in the electrical service cupboard at the bottom, very easy for it to spread upwards through the building. He wants us to put a ceiling in, to be made from fire board.

We could do it by attaching the fire board to the joists I suppose, but they’re a very long way up so it would be an incredibly awkward job. I’m wondering if instead we could construct a ceiling a bit lower down. If we did that, could we get away with screwing the board into bits of timber fixed horizontally to the side wall (quite easy), or would we need to construct some kind of frame with wooden studs and fix the board to that (more difficult)?

(how the ceiling looks doesn’t matter at all - hardly anyone will ever see it)

Thank you.

 

[^woody^]

In principle, the perimeter of a service duct should be sealed to stop heat or smoke escaping into the surrounding structure.

The problem with a ceiling lower down is that there will still be cables above in an unprotected duct.

Has the actual risk and fire loading been assessed? If the risk is to the cupboard then yes, put the ceiling there.

You'll need a suitable frame with timber suitably spaced to hold whatever board you propose. It can be fixed, or removal for access. Intumescent sealant to the perimeter.

 

[bigalxyz]

Thank you.

The perimeter of the service duct is already sealed - it was previously just done with MDF but the fire safety guy said no, that's no good, you should use cement fibreboard - so we did that job last year.

Re "has the actual risk and fire loading been assessed" - could you explain what you mean by this pls? Fire safety assessor's explanation was simply that if a fire broke out in the cupboard at the bottom, there was nothing to stop the flames or smoke from heading straight up the duct.

Re sealing gaps - he advised that something like fire resistant expanding foam or sealant would be suitable. So I think we just need to know about what we need re a frame for the ceiling. Because it's so small (90cm x 90cm), wondered if we could get away with something very simple, but perhaps not?

 

[^woody^]

What many fire risk assessors forget to do, is actually assess the risk - the chances of a fire occurring, and the consequence of that.

With cupboards, that typically means how is a fire going to occur in the first place and what is going to burn and to what extent? With cables, that could include what amount of fire could occur from a few cables. That's the fire load.

And you don't want to be spending thousands or doing awkward work if the risk and benefit is low. That's the risk assessment.

What he should do is assess the risk, and deal with that. If the risk is the cupboard and not the duct, just sort the cupboard out.

MDF of certain thickness can provide a certain fire resistance, and consider how this mdf will actually catch light in the first place.

Other options with reduced work bases on low risk would be to install early protection - i.e. a smoke alarm.