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Waterproofing underside of orangery flat roof

Postby zebedeebedee » Sun Nov 14, 2021 10:22 am

Hi all, I'm currently building an extension which is an orangery with a large lantern sky light on one side and a bathroom on the other. The structure was completed by a local firm and I'm DIYing the interior fitting. I'm struggling at the first hurdle which is ensuring sufficient ventilation to protect the deck and rafters from rot from condensation. I hadn't considered this at all at design stage, but having looked at some things online I'm reminded of a nightmare I had with a flat roof at an old property so I want to do what I can to get it right.

It's a cold roof system so I have to do everything a can from underneath. Headroom is already extremely tight because of the way it attaches to the main building. There is clearly no ventilation provided where the roof abuts the main structure and it would be a massive job to retrofit this, and on a new and watertight roof I'm inclined to do something else if I can, but I'm concerned there won't be enough ariflow to prevent condensation damage. I will try to maintain the integrity of the damp proof membrane above the plasterboard by avoiding downlighters and maybe putting the extractor in the wall, but one thing I've thought might help but I've seen no mention of is painting the underside of the roof, which is an OSB board and rafters, with some sort of waterproof coating so if anything does condense up there it should just sit on this surface and not penetrate the wood. Has anybody done this, or is there some fatal flaw in my plan? Any other tips on avoiding condensation rot also warmly welcomed.
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Postby stoneyboy » Mon Nov 15, 2021 11:21 pm

Hi zebedeebedee,
If your rafters run side to side over the extension you should be able to provide ventilation by fitting vent strips at each end. If however the rafters run at right angles to the main house you really need to incorporate an abutment strip along the wall with vents on the outer edge.
Painting the structure may lead to condensation bleeding through the ceiling especially over the bathroom.
Regards S
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