Condensation from cold flat roof, more vents?

No they should be a min of 50 m air gap between the joist and roof deck

Hi John, it sounds like there is an issue of cross ventilation by the way you're describing it. Your suggestion of a fan would absolutely Not be allowed as you will introduce more moisture which will lead to even more condensation and make matters worse. As per the comment of the other roofer there needs to be a minimum of 50mm air gap between the top for the insulation and the roof deck. Obviously the gap will become bigger towards the ridge/hip as the firrings should be installed to create a fall on the flat roof. I have a couple of questions for you? 1. How high is the top of the ridge from the flat roof of the dormer/loft? If there's around 250- 300mm it may be possible to drill some holes in for cross ventilation. This will help a bit! 2. You/ roofer need to introduce 25mm continuous vent strip at the base of the roof tile on the flat roof side? 3. Install a dry ridge system on the ridge with some vented ridge capping as well! 4. You can also install ventilation on the flat roof with mushroom hats, you will need a few of those spread along the roof. 5. You will also need to check if the builder has installed VCL ( Vapour Control Layer) between the insulation and plasterboard. This is crucial as you've a cold roof system and any moisture from internal humid air will condense as soon as it gets in contact with the cold surface which is the underside of the rubber. If you have downlights on the loft this also need to be investigated as humidity with pass through if they're not sealed properly. You should also be aware that if you've noticed stains coming through the ceiling that would mean that the OSB boards under the rubber might be very wet which may have to be removed. It sounds like you should get a professional to make a few inspection holes on the ceiling. Investigate and then get a roofer or different builder ASAP who has a good understanding to resolve this for you before the issue gets more serious. Best of luck DanEco Build Ltd