GRP is glass reinforced polyester, the same material used for boat hulls, laid wet onto a prepared deck and cured in place. The result is one seamless, rigid shell bonded to the roof rather than a sheet resting on it. Where you need a flat roof you can walk on, GRP is usually the right call.
Hand laid glass reinforced polyester giving a hard, seamless, walkable finish.
Unlike EPDM and felt, GRP is not a flexible sheet. It cures into a hard shell that takes foot traffic, furniture and ladders without denting or puncturing. That rigidity is its strength and also its limitation, because a rigid finish over a moving structure will eventually crack if the deck is not right.
It also finishes with proper GRP trims that become part of the roof rather than a separate edge, so there is no mechanical joint at the most exposed point.
Where GRP is the better choice
Balconies and roof terraces where people will walk on the surface, dormer tops that get used for access, porch roofs that ladders lean against, and any roof where a hard, tidy, high finish matters visually.
Walkable and hard wearing, with anti slip finishes available
Genuinely seamless once cured, including up the upstands
Takes a coloured topcoat, so it can be finished to suit the building
Excellent around complex shapes, outlets and multiple penetrations
The deck matters more than the resin
GRP must be laid on new, dry, correctly fixed OSB3 deck boards with the right expansion gaps. Laying it over old chipboard, damp timber or an existing covering is the single most common reason a fibreglass roof cracks within a couple of years.
It also has weather constraints. Resin needs to cure, so it cannot be laid in rain, in very low temperatures or late in a short winter day. We would rather move a date than rush a cure.
Getting a quote you can compare
GRP quotes vary widely because the specification varies widely. Ask what weight of matting is being used, whether a new deck is included, which trims are specified and what topcoat is going on. A cheap fibreglass roof is nearly always a thin one on an old deck.
Can I walk on a GRP roof?
Yes, that is one of its main advantages. For regular use as a balcony or terrace we specify an anti slip topcoat rather than a standard finish.
Why did my fibreglass roof crack?
Almost always deck movement. If GRP is laid on an old, damp or poorly fixed deck without proper expansion gaps, the rigid shell has nowhere to go and it cracks along the board joints.
Neither is better in general. GRP wins for hard, walkable, seamless finishes. EPDM wins for movement tolerance, speed and value on simple garage and extension roofs.
How long does GRP take to install?
Typically two to four days on a domestic roof, including deck work and cure time between coats.
Can it be laid in winter?
Only in the right conditions. The resin needs temperature and dry weather to cure properly, so we plan GRP work around the forecast rather than forcing it.
We check joist spacing, deck condition and falls, because GRP demands a firmer base than a flexible membrane.
Matting weight, resin system, trim type and topcoat colour all stated in the quote.
Old covering and deck removed, new OSB3 laid with correct fixings and expansion gaps.
GRP trims fitted, matting laid and wetted out, then left to cure fully before topcoat.
Coloured topcoat applied, edges and outlets checked, roof water tested before sign off.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.