"Tim Mitchell" wrote in message
...
In article , Paul C. Dickie
writes
In article , nightjar
writes
"Tim Mitchell" wrote in message
...
We are building some more offices on a mezzanine floor above some
existing
offices. The mezzanine floor is wooden joists with chipboard flooring.
Beneath the joists is a suspended ceiling with 600x600 tiles.
snip

The fire resistance of the panels is not the only factor. In a fire, a
strong updraught can develop, which would lift the tiles and allow the
fire
past them. We did get a suspended ceiling approved some years ago, but
the
grid had to be steel and we had to fit special clips to hold the tiles
down.

Bearing in mind that the heat from the fire could (and would) cause the
steel grid to expand and buckle -- possibly to the point where the clips
could not hold anything down -- might it not be a wise precaution to
paint the joists and the underside of the floor above with a "fire
resistant", intumescent paint?

Well I am only going on what the ceiling manufacturers say. They say that
the ceiling itself, without the tiles being clipped, will give 30 mins
fire protection to the floor above.

The alternative is to put a layer of plasterboard above the ceiling tiles
but if the ceiling itself is OK, this should not be necessary. The offices
above have a fire exit direct to outside and can be evacuated in about 2
minutes, so 30 minutes should be ample.

The requirement for fire resistance is not based on how long it takes to
evacuate the compartment under ideal conditions. It is intended to provide
adequate protection for someone trapped inside while the Fire Brigade arrive
and make a search. There have been numerous cases where a fire exit has
proved to be useless for one reason or another and a surprisingly large
number of people either panic or freeze when faced with a real fire. Our
mezzanine was deemed to require two hours' fire resistance.

Colin Bignell