Hi, got a flat roof which had polystyrene tiles which I’ve just removed, they were attached to plaster board, I’ve noticed there’s no insulation above plaster board, I’m thinking of removing the plaster board and putting rock wool or some sort of insulation between the joists then putting up new plaster board, would this be okay? do I need to worry about damp or anything?
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Flat roof insulation query
- Thread starter Chris_909
- Start date
You have what is known as a 'cold' roof. Therefore each joist run will need venting outdoors. If your flat roof is in need of a re-felt, I'd explore fitting a 'warm deck' roof.
Ideally I’d do that but it all looks in good nick, I’ve had a look around I can’t see any obvious vents but I assume they will be there? The roof covers multiple rooms and I’m just changing one ceiling so I can’t see all the way around the roof, as long as I leave an air gap of like 50mm should be fine?
Is the flat roof space (each joist run) venting into the open air, I.e. outside?
Yes it is!
You need 50mm of space above whatever you put between the joists. Stuffing rockwool in is very likely to block the ventilation, which is very likely to lead to structural damage caused by rot.
I retro-fitted insulation to my flat roof by measuring the joist height (at the lowest point, it will have a slight slope), subtract 50mm then buy Celotex or similar in the next size down. Pin a thin wooden batten to each joist at this size upwards from the joist's lower side, shove the celotex against it then seal around it (gapotape, sealant or foam)
You then have the option of adding a full layer of Celotex over the whole lot, losing room height but gaining lots of insulation, then plasterboard over it all. Or use insulated plasterboard (IMO less good but easier).
I retro-fitted insulation to my flat roof by measuring the joist height (at the lowest point, it will have a slight slope), subtract 50mm then buy Celotex or similar in the next size down. Pin a thin wooden batten to each joist at this size upwards from the joist's lower side, shove the celotex against it then seal around it (gapotape, sealant or foam)
You then have the option of adding a full layer of Celotex over the whole lot, losing room height but gaining lots of insulation, then plasterboard over it all. Or use insulated plasterboard (IMO less good but easier).
Thanks, I’ve done as you’ve suggested one slight query,don’t no if it’s a big deal or not, but the air gap is mostly 90mm between roof and insulation however there are a couple of small support beams attached to the roof 50mm wide by 40mm height, this means there’s a small area with only 40mm gap, will this be okay?
Don't worry about it at all, it's a guideline... the world doesn't explode at 49mm!
Just make sure you add battens to prevent it getting pushed up too high. I used whatever wood was THE cheapest in B&Q. Can't remember the dimensions. I pilot drilled for the nails so it wouldn't split. I set my square to the depth of the celotex then used it to position the battens, pressing it against the underside of the joist. Or make a little wooden jig to do the same - probably a better idea as it won't slip out of adjustment.
Just make sure you add battens to prevent it getting pushed up too high. I used whatever wood was THE cheapest in B&Q. Can't remember the dimensions. I pilot drilled for the nails so it wouldn't split. I set my square to the depth of the celotex then used it to position the battens, pressing it against the underside of the joist. Or make a little wooden jig to do the same - probably a better idea as it won't slip out of adjustment.
Cool cheers all insulation finished and sealed just gotta put the plaster board up!
Thanks again
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