Bob Mannix wrote
I don't believe you are correct. A single sheet of skimmed plasterboard is
insufficient to provide the fire protection (unless it is Fireline
plasterboard). The requirement for mineral fibre insulation was part of the
fire protection requirements for my loft conversion and its protection was
added to that provided by the ceiling to make up the required 30 minutes. A
depth of 65mm of mineral fibre insulation, nailed to the joists either side
was required (glass fibre is not allowed as it melts). The floor was not
allowed to be laid until the BCO had inspected the installation of the
mineral fibre.
Two sheets of plasterboard with a 12mm (AFAICR) gap would also do, but
that's a lot more difficult.
I don't know any details about your loft conversion Bob, so I pass no judgement
on whether you are right or wrong. I can say for certain that I have never come
across a BCO requiring thermal insulation when upgrading the fire resistance of
ceilings. Maybe there was an issue about sound insulation in your case?
I checked my copy of the British Gypsum White Book just to make sure my
understanding was correct. There are various different constructions considered
but the two important cases a
a minimum 15mm thick T & G flooring on 38mm timber joists at 450mm centres
with a ceiling of 9.5mm plasterboard with 5mm skim coat gives ½ hour (modified)
FR. The Building
Regs permit this relaxed construction over rooms but not over the escape route.
b ditto but with 12.5mm plasterboard and 5mm skim gives full ½ hour FR. This
is
what the Regs require over escape routes.
There is no requirement for thermal insulation in these details I am looking at.
I've also never heard of using 2 sheets of plasterboard with a 12mm gap. In the
above examples, 2 staggered layers of 12.5mm wallboard (without skim) will give
1 hour FR and 2 layers of 15mm Fireline unskimmed will give 1½ hours FR.
You can see the latest on-line version of the White Book on timber floors and
ceilings on
http://www.british-gypsum.com/PDF/c60_103.pdf
Look at page 4 onwards
Peter