Repairing a GRP roof (ed.)

I’m currently in the middle of an extension with a flat roof, parapet wall and a sky lantern. The flat roof has a GRP surface. We have had pretty substantial leaks at the edges below the parapet wall and also the corners of the roof lantern. I think plan A for my builder is currently to put another layer of GRP over the top of what is there, I guess when it’s a dryer day than we are having at the moment. Is that going to cut it? Any other suggestions. I’ve done some reading around the parapet walls already and need to check he added slate and a membrane under the coping stones. thanks

Don't go GRP unless you have already, there are single ply products from Sarna which look like lead, are repairable, and are quiet and don't suffer from thermal expansion and cracking.