Puddling on fibreglass flat roof.

What sort of finish is on the FG surface?

It may be simply a matter of you getting a FG kit and laying a sheet over the dips to level it up.

If its not leaking you might be better of just leaving it alone.

If you are going to build it up you'll need to make sure its bone dry and abrade the surface after removing any aluminium painted coating. Don't add more than 2% catalyst to make the resin go off quicker and make sure any CFM is wetted out completely.

IANAE, so how do I identify an "aluminium painted coating"? It is (or was) rather a fetching blue colour. It certainly smelled a very great deal when the guys put it on.

In answer to suggestions above - the reason I am reluctant to leave it alone is because my brother, who has recently qualified as some sort of surveyor, saw the puddling and a bird standing in it, and said that the the bird was eating some variety of small crustacean that serendipity had placed in the puddle, and that the beak could damage the roof. Seemed unlikely to me, but not worth the risk.

I am tempted to build it up because there is a fall from one side to the other, but the problem is that the substrate is not flat and even. So I'm minded to follow EricP and 1501's advice (crazy name, by the way!) and build up the surface to "iron out" the bumps

Edward

"some sort of surveyor"? He doesn't seem to know much about strengths of materials and likely effects of a birds beak. I would forget it

Your choice of course!