OK to overboard damp ceiling?

Christyt

Hi there,

I'm having a flat roof replaced and to satisfy building regs I have to overboard the ceiling below with thermal backed plasterboard which is to be skimmed over the top.

However, the existing ceiling is quite damp and a bit mouldy in places due to the bad long term leaks to the existing flat roof.

Is it wise to overboard and skim on top of this?

If not - would it be worth using a humidifier to dry out the ceiling or should we take it down?

And if we should take it down, can we just put the thermal backed plasterboard straight on to joists or do we have put some other plasterboard or other layer up first?

VVV grateful for some expert advice!

ormus

i would take it down, even if just inspect for any further damage/problems.
you dont need any other layer of PB first.

in fact with the right sort of insulation, you can use bog standard PB.

Christyt

Thanks - it's a cold roof system so they're putting 75mm celotex between joists straight onto the ceiling. The whole roof is coming off. (Hoping the joists will be intact). It's not so much hassle for them to take the ceiling down if the thermal plasterboard can go straight on to joists so sounds like that's the best plan.

I could even do it!

glubalub

75mm of Celotex isn't quite enough for modern insulation standards. Current Building Regs require a minium U value of 0.18W/m2.K This is likely to mean about 150mm to 200mm of Celotex. Celotex have a U value calculator on their web site, you can put in the details of your roof construction and they will tell you the thickness of the insulation you need. If you have the head room to fit extra insulation, cross batten the joists and fit the additional insulation between the cross battens before you tack the plasterboard. You must put a vapour barrier such as 1000 gauge polythene fixed to the underside of the ceiling before tacking the plasterboard, to reduce the risk of condensation. Use 12.5mm plasterboard as it only needs perimeter noggins, not noggins at all edges. And please make sure the builders have left a gap that is properly ventilated on top of the insulation between the joists in the flat roof.

Christyt

Thanks very much for advice.

Because it's a non heated room so building regs said we could do the following:

20mm thermal backed plasterboard tacked to ceiling (or to form new ceiling.

75mm between joists.

25mm cross battons on joists to create 50mm airgap.

Soffit vents all round.

Then deck and felt etc.

THey didn't mention a Polythene layer on underside of ceiling - if the 20mm thermal backed plasterboard forms new ceiling should this have polythene layer between plasterboard and joists (with 75mm celotex sitting on it?)

Thanks

keystone

I see nothing wrong with your solution and yes get the old sodden stuff down.

Cheers

Christyt

Just one more question - if I take down the ceiling and put thermal backed plaster straight onto joists as new ceiling, does it need a vapour layer? There will be 75mm celotex between joists resting straight on the plasterboard - and a 50mm vent gap above followed by the deck etc.

glubalub

Yes a vapour barrier is needed on the warm side of the insulation. It's worth checking with the suppliers of the plasterboard as some thermal plasterboards have an integral vapour barrier in which case the polythene is unnecessary. If in doubt though fix the polythene as it costs next to nothing and can save problems with condensation in the future.

Christyt

Thanks - so, sorry to be thick,, but the warm side means attached under joists and over the new thermal backed plasterboard that will form the ceiling?

glubalub

Yes that's right Christyt. The easiest way to fix the polythene is with a staple gun and staple it directly to the underside of the joists then tack the plasterboard sandwiching the polythene between the plasterboard and the joist.

slipperyshoe

If it is a non-heated room then there is NO requirement for a thermal upgrade upon refurbishment. In which case you can do the flat roof, leave everything to dry out and then underboard the existing plasterboard. Building Control is talking nonsense, check Part L1 of current building regs (Oct 2010).

By the way 110mm foil faced PIR insulation will get you the 0.18Wm2K required. Generally I would suggest a warm roof to anyone refurbishing a flat roof due to the difficulty in creating effective ventilation within the void, thus causing condensation.