Help, I’m a girl and definitely not a builder, possibly taken on more than I can chew but here goes. we are self building a kitchen extension and due to pitch 15 degrees and no knowledge we chose to sark the lean to roof with plyboard before coving with rubber shield and then a rubber slate called Eco slate. This is a warm roof with 3 velux windows. On advice from the building regs we have cut a gap in the top of the sarking under the flashing and and used a hedgehog type plastic layer to allow an air flow under the flashing, the rafter have holes cut in them to allow an air flow from the soffits to the top. We’ve put 100mm celotex in between rafters leaving around 75 mm gap above between insulation and sarking. Our intention was to put a further layer of 50 mm insulation over the whole lot the plasterboard but we hasn’t anticipated this damp on the sarking ply which h is only appearing now I’ve put the insulation in. We screed the floor about 3 weeks ago and haven’t turned on the underfloor heating yet but I guess there may be a lot of damp in the air due to this. Saying that it is a kitchen so will be a fairly moist room. Will it help to have a polythene sheet between the plasterboard and insulation to stop the air getting up there in the first place? Do we just crack on and seal it with plaster and hope it’s ok??? I’m really hoping this isn’t an expensive set back as we are totally out of money.

  • AB Architectural Design Ltd

    Hi. This is actually a cold roof as you have an air gap over the top of the insulation which is there to help vent the roof. The damp could be one of two things. A Leak or Condensation.
    If you don’t have enough insulation between the rafters the warm air from the kitchen below will hit the cold air above the insualtion and will condensate on the under side of the sarking boards. So you can put further insulation under the rafters to help prevent this as it cools the air that does escape so it doesn’t condensate. Having the air gap above the insulation will also help to prevent condensation forming.
    However if you don’t have any heating on in the kitchen there shouldn’t be any warm air rising to cause condensation therefore I would probably put this down to a Leak somewhere.
    Normally leaks will form around openings or on abutments (Where you have the flashing) so I would check these areas first and then wait for the next big downpour and keep an eye on the underside of the roof to see where it might be getting in.

    Hope this helps!

  • Elle Hammond

    Hi, thanks for your reply, I don’t think the roof is leaking as its been completely dry for the past 2 months before we put the insulation board in, even during the big storms. where we cut the hole in the sarking under the flashing it’s actually dry for around 100mm where the air seems to be circulating. The damp is uniform underneath the insulation boards, now I’ve taken 3 pieces out those areas have dried up completely so I’m as sure as I can be its condensation forming where the air is trapped. Your right we haven’t turned the heating on but I guess it’s just slightly warmer than outside due to residual heat from the house. I think we will try covering the whole thing in insulation and see if it’s still as bad once we’ve tried to form some barrier to stop air getting in there in the first place.

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