Leaking fibre resin shed roof

31 Mar 2015
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United Kingdom
The original roof was made with felt, but it was stubbornly leaking, so I hired someone to fix it and they replaced it with a fibreglass resin roof.

The new roof was put on in January 2022 and it was made with plywood and resin on top. The resin was this product:


and covered with this, with fibre mixed in:

After the summer drought the roof started leaking in a few spots. I've tried reapplying the topcoat several times, but it keeps leaking. Sometimes after putting more topcoat it's ok for a while, then it starts leaking again.
Now after the recent frosts it's leaking in more spots than ever before, and there are multiple hairline cracks:


Leaks from the inside:

The topcoat clearly isn't working as the cracks keep coming back. Moreover, I don't know exactly where the leaks are, as the wet spots on the underside of the plywood are not necessarily where the leak is on the outside, because the water can travel between the topcoat and the plywood and show as a wet spot inches away.

Any ideas how to fix this for good?
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You was robbed. Plywood isn't the best substrate for GRP, quite likely the nails holding that ply down weren't long enough and have popped through. If that ply is less than 18mm thick it'll flex too much for GRP as well.
You also can't keep slamming topcoat on- you can patch topcoat but you need to grind (or coarse sand) the original topcoat off to expose the clear resin and mat, then clean up, wash with acetone, topcoat. Thin layer - too thick and it'll crack when it's curing.
Best cure for that is a roll of shed felt and some nails, they'll be a sod to get in but it'll keep the rain out.
I know I was robbed. They took the money and didn't even finish the job.
I think I'll keep patching it with a thin layer of topcoat for now, until I have more money to hire someone to put felt on.
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What would be the best tool to use to sand it? I don't think I can grind the topcoat off, it's too thick already, but it's bumpy, so if I can grind the bumps off, maybe I can make it flat enough to put the felt on top of that.
Don't try patching topcoat if it's raining or the timber underneath is wet. Job has to be bone dry for resin to work, if it isn't the resin either won't cure or it will but then it'll blow when trapped water freezes or gets warm.
Angle grinder with a flap wheel on works well, for felt on top a few bumps aren't a problem, you should flatten any spikes or sharp ridges.
EDIT For temporary repair in this weather Acrypol or similar might work.
slap this on to the cracks with a throway brush:

Do you have some plastic sheet to place a temporary layer over the roof?
If you don't, then get some to keep the roof secure til the weather improves.
This week will be mild but that usually means rain and there's no time for the ply to dry at this time of year.
If you put felt on that roof during winter it probably won't last.

Mind you, there'll be a time when strong winds would bring a bigger problem with plastic sheeting, so maybe a lick of Evercryl would help seal the joints, or even Denso tape for a temporary fix til spring brings a better opportunity to put 20mm felt over it.

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