On Sunday, 3 November 2019 12:16:38 UTC, wrote:
As part of a small extension I needed a "hidden" flat roof section. I
got builders to do the groundwork, blockwork and roofing - although
their 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.

You can buy a liquid roof seal that can be applied even underwater.
https://www.rawlinspaints.com/home/r...ak-repair.html

There are other manufacturers too.