GRP is glass reinforced polyester, laid wet onto a board deck and cured into one rigid shell with a coloured topcoat. It gives a hard, walkable, seamless surface that can be moulded around awkward shapes, which is why it is the default for balconies, dormer cheeks and roofs with a lot of detail. Its weakness is the other side of the same coin: a rigid shell on a deck that moves will crack.
Why fibreglass roofs depend on the deck more than usual
A GRP roof is only as stable as what it is laid on. Systems are specified over a particular board, usually a good quality OSB of a stated thickness, fixed at stated centres, with the joints supported. Laying GRP over old chipboard, over a deck that flexes underfoot, or over boards that were not dry, is the single most common cause of early failure. If a quote does not name the deck it is going on, that is the question to ask first.
The disadvantages of fibreglass roofing, honestly
It is a wet trade with a curing time, so it needs dry conditions and a temperature window, which makes British winter installation awkward. It is rigid, so thermal movement and structural movement go into the laminate rather than being absorbed. Repairs are possible but a patch in a coloured topcoat rarely disappears. And the quality depends almost entirely on the person applying it, because it is made on site rather than in a factory.
Where it beats a sheet
On a balcony you can walk on, on a dormer with cheeks and a front to wrap, on a small roof with several pipes and an upstand, GRP wins because it is formed in place and has no seams to dress. Trims are bonded into the laminate rather than clamped on, so the edge detail is part of the same shell. For a plain rectangle with no penetrations, a sheet membrane is usually the easier and cheaper answer.
The cost of fibreglass roof work: what a quote should itemise
The board type and thickness, whether the existing deck is being replaced, the number of laminate layers and the weight of the matting, the topcoat and its colour, the trims, and the access. Ask for the manufacturer's system name, because GRP is sold as a system and a guarantee usually depends on the whole of it being used rather than resin from one supplier and trims from another.
Questions people ask about grp roof
Can you walk on a fibreglass roof?
Yes, that is one of its advantages, and it is why balconies and roof terraces are often GRP. For regular foot traffic ask for a non slip finish, because a plain topcoat is slippery when wet.
Can a fibreglass roof be laid in winter?
Only within the resin manufacturer's temperature and moisture limits, and rain during cure will ruin the surface. A contractor who will not wait for a weather window on a wet trade is taking a risk with your roof.
How do you repair a cracked GRP roof?
A crack is ground back, re laminated and re topcoated. It is a sound repair structurally, but colour matching an aged topcoat is difficult, so expect the repair to be visible. If cracks keep appearing in the same place the deck is moving and the covering is not the real problem.