Waterproofing underside of cold flat roof.

[zebedeebedee]

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 I 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. If I put in good ventilation around the sides of the building should I not be too worried about the lack of airflow between the roof and the house? Thanks for your help.

 

[Swwils]

Pictures please

Why is the cold roof not ventilated at the eves.

How is the mositure getting through the vapour barrier?

 

[zebedeebedee]

I just assumed the company that built the structure would include everything essential for it to perform adequately as they build these things frequently. I think now I should have gone into more detail at the design stage, but I am where I am. You can see how the roof buts up to the house in the pictures and what the underside looks like. from what I've read you should just assume that some moisture will get into the void above the insulation, even if you have nothing installed through the ceiling, you will have to fix the vapour barrier somehow and that will entail puncturing it. And just because when you're building something you should assume the worst. I am having a shower in part of the building so there will be a lot of steam trying to escape upwards.