Wales

Hello SteamyTea, Thanks for your response. The patches you can see on the back near corner are Bostik Flashband that I've put on, and it did seem to work. Over the last few days we've had high winds, hailstones and a lot of heavy rain. This is Wales! Water pools just in front of that Flashband, and runs over the edge just where you see that rope coming off the roof. So it could be that I haven't sorted the corners with the Flashbend, although there is no longer a leak inside at that top corner; or it could be the running water is going under the tiles and running along a joist. The current leak, inside, is equivalent to about halfway down the slope of the dormer, and about 200mm out from the dormer wall, towards the camera. Hopefully not, but it could even be a problem caused by ineffectively fitted flashing of the dormer wall to the roof tiles. That's why I thought that a slight slope and proper side trims might take the water away from that area.

As part of a roof replacement, about 3 years ago, we had a flat roofed dormer done with fibreglass. I knew nothing about flat roofs at the time, but now know that it was a poor job. There's a slight rise around the 3 edges, and a 'belly' in the middle. So the water pools just in from the edges, and at the back by the roof tiles. It wasn't installed with a fall towards the front, and there are no side trims to guide the water that way; and no trough at the front. Also, the flashing at the back corners were poor, and have allowed water ingress. Would I be better trying to upgrade the rear corner flashing and retrofitting side edging trims - if possible. Or should (and could) I overlay this roof with OSB board and EDPM, with a forwards fall and proper edgings?