As part of a small extension I needed a "hidden" flat roof section. I got builders to do the groundwork, blockwork and roofing - althoughtheir work in most areas has been good, the GRP roof did not go well and they've run out of ideas, so I need some advice. The flat roof slopes back (and sideways) to an existing tiled roof and water then spills down a valley. The problem is that the GRP seems to be porous where the flat roof meets the old roof, this despite appropriate flashing and 3 layers of mat. The cure would be fairly easy (with another layer of GRP) if the weather was dry and warm, but it isn't now and probably won't be until the Spring. Once the area is dry I think the answer will be to cut-out this section of GRP and to glass-in some new mat, taking care to thoroughly impregnate it with resin. The question is: what can I/they do to make it watertight until it can be fixed properly? It needs something that can be applied to a damp surface.