GRP gives you a seamless, walkable, hard wearing flat roof, but it is unforgiving of poor preparation and cold weather. Here is how the system works and when we specify it.
A roof built in place, not rolled out
GRP stands for glass reinforced plastic. Unlike rubber or felt, it is not a manufactured sheet. Chopped strand glass matting is laid over an OSB3 deck and saturated with catalysed polyester resin, which cures into one rigid, jointless shell bonded to the boards below it.
That makes the roof and the deck a single structural item. It is why fibreglass copes with foot traffic, planters and balcony use far better than a bonded membrane, and why it needs a deck laid to a much higher standard.
The parts most homeowners never see
The trims matter more than the resin. GRP specific edge trims, drip trims and raised kerbs are fixed first, then the matting is laid over them so the finished shell wraps the perimeter with no separate flashing to work loose.
The final layer is a pigmented topcoat with wax in it, which floats to the surface and lets the resin cure hard rather than staying tacky. Skip that step or thin the topcoat and the roof stays slightly soft and picks up dirt permanently.
Weather and timing rules we will not bend
Resin needs a deck temperature of roughly 5 degrees and rising, with no rain on the surface and none forecast during cure. Catalyst ratios are adjusted for the day's temperature, which is why a competent installer mixes small batches rather than a bucket at a time.
In practice that means we will not lay fibreglass on a damp December morning simply because the diary says so. Waiting two days for the right conditions is the difference between a 25 year roof and a delamination claim.
Costs against the alternatives
GRP typically prices around 15 to 30 per cent above EPDM for the same roof, so a 20 square metre extension roof commonly falls between £3,200 and £5,200 fully stripped, redecked and finished with trims.
You are buying hardness, a walkable surface and a colour choice, not extra years. Where the roof is out of sight and never walked on, rubber usually gives better value, and our comparison of the three flat roof systems sets the trade offs side by side.
Repairing and recoating fibreglass
Hairline crazing in the topcoat is cosmetic and is dealt with by abrading and recoating. A crack that goes through the laminate, usually at a kerb or over a deck joint that has moved, needs cutting out, feathering back and relaminating with fresh matting.
Because repairs bond chemically to the original roof, a well executed fibreglass patch is genuinely invisible and as strong as the surrounding shell. That is not true of any other flat roof system.
Can fibreglass be laid over an existing flat roof?
Only over a sound, dry, properly fixed OSB3 deck. It cannot be laid over felt, rubber, bitumen or chipboard, so an overlay usually means a new deck first.
Will a GRP roof crack as the building moves?
It can if the deck is not laid correctly. Boards must be laid staggered with expansion gaps, screwed rather than nailed, and any junction where movement is expected needs an expansion joint rather than flat matting.
What colours are available?
Topcoats come in a range of greys, greens and slate tones. Mid grey and dark grey are the usual choices in Oxfordshire because they sit quietly against both stone and render.
Can I put a balcony or seating area on a fibreglass roof?
Yes, that is one of its strengths, provided the structure below is designed for the load and a non slip aggregate is added to the topcoat where people will walk.
How long before I can use the roof after installation?
The laminate is walkable within a few hours in warm weather and fully cured within a day or so. We confirm timings on the job because cure depends on temperature.
Want a fibreglass price for your roof?
Tell us how the roof will be used and we will specify the right build up, trims and topcoat, then price it properly in writing.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.