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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.

Have a look through this and you will know as much as any in he-)

https://www.fixmyroof.co.uk/videos-a...lacement/how-t
o-fibreglass-a-roof/

I take it your construction involves a shallow valley. The video above
advises one layer of glass mat (if it is not a trafficked area) plus a
gel coat on top.

They show how to *bandage* joints in the roof boarding and close off any
apertures. One point of interest to you is that the recommended cleaning
solvent, acetone, is soluble in water which may make drying off the
surface ready for a re-coat easier. Lots of rubbing down with 60 grit
glass paper!

Connecting *pin holes* through three layers seems unlikely and you don't
mention a gel coat.

No leaks found in my d-i-y one, yet!

--
Tim Lamb