We have a GRP balcony, and it has been fine for about 7 years now.
Can't remember the supplier - if I find the papers I will give the details.
Also a more simple GRP roof on the room over the garage - this is just GRP matting over exterior ply but it seems to be doing O.K. I replaced the ceiling recently and the underneath of the roof looked fine.
IMHO much better that the traditional roofing felt.
HTH Dave R
P.S. if it is done properly it shouldn't split - there would be a lot of unhappy sailors otherwise.