I live in the attic of a house which has been converted into flats.
During the decoration (ahem) of the lounge, I've opened up a storage
space under one of the eaves, which was previously closed off. Before
I board out this space, I think I ought to meet current building regs
with regard to creating a fire compartment here. As this is a
three-storey flat conversion, the building regs say that 30 minutes
`could' be acceptable here.
The space has a suspended ceiling (belonging to the flat below) about
1m underneath it.
So, my question is: will 9.5mm of plasterboard and 18mm of chipboard,
plus the suspended ceiling below, give me the 30 mins fire resistance
I require? What about 18mm of chipboard alone? Or is this one for the
local BCO? :-)
On 12 Feb 2004 14:38:38 -0800, firstname.lastname@example.org (Ben
If you are going over someone elses property then no matter what the
BR say think of your own security. Make sure the compartment is
smoke proof for a start - fires rarely kill - their combustion
product beat them to it. It used to be that a double layer of
plasterboard was adequate to give 30 min resistance - I'm not sure if
this is still so but it isn't far out.
The suspended ceiling should be ignored in any fire resistance
Thanks Peter, I hadn't thought of that. I'll fill the gaps between the
joists, that ought to do it smoke-wise...
Googling, I think that the floors in the rest of the flat have been
filled with something called `foamed perlite', which apparently has 60
minutes fire resistance, so I'd better meet that in the new
Assuming the original conversion work complied with Building Regulations, I
would be extremely surprised if the 30 minute FR compartmentation covered only
the area within the eaves voids. Normally it would extend to the outside walls
all round. This, coupled with sound insulation, is probably the reason why
there is a suspended ceiling at all. But, either way, here's a thought - if you
provide 30 minute FR doors to the void areas there will be no need to upgrade
the ceiling anyway.
The British Gypsum website http://www.british-gypsum.com/immediacy-824 gives FR
figures for various configuations of plasterboards/joists/floorboards etc.
I don't think it is strictly necessary to apply for Building Regulations
permission to do this work. Providing access to eaves voids, if only for
maintenance, is normal procedure and this is usually through the short walls at
attic level, rather than through the ceiling below. However, it's probably wise
to ask the Building Control office for their views as there may be special
circumstances here. They may even have kept archived records of the conversion
I would have said that to achieve 30min FR with a ceiling of 9.5mm pb, you
need t & g boarding, and the floor void needs 60mm mineral wool on chicken
wire mesh stapled to the joists. For FR purposes only, in the voids you
could do away with the boarding and use 100mm mineral wool instead (but not
if sound insulation is required).
We also would not accept the FR door into the voids; inlike some BC
authorities, we required the entire floor eaves to eaves to have the full
fire resistance. Nowadays all authorities would require the voids boarded
out for sound insulation purposes (eg when doing a loft conversion).
According to British Gypsum White Book (Section c60 page 5), min 18mm T&G
boarding, 47mm thick joists at 450c/c and 9.5mm wallboard with 2mm skim finish
gives 30 minutes FR. No quilt is needed.
That's what I would expect too. I'd be surprised if any extra fire resistance
is needed in the voids.
Double layer of 12.5mm plaster board does meet 30 min requirement. So
does single layer of 12.5mm firecheck plasterboard which is slightly
pink in colour, but it's very heavy especially in 8x4 sheets.
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