How long does a GRP fibreglass roof last?

What GRP is and what the guides say

GRP stands for glass-reinforced plastic, which is fibreglass matting set in polyester resin. On a flat roof it is laid wet on a timber deck, built up in layers and finished with a topcoat, so it cures into one continuous sheet with no laps to open. That absence of joints is its main selling point, and it is why GRP is often chosen for small roofs with awkward shapes, upstands and tight corners, where a sheet covering needs many cuts.

The published lives are close together at the bottom and far apart at the top:

Both put the floor at 25 years. The difference is the ceiling, and the ceiling is where workmanship and conditions show up. A lifespan figure describes a typical roof; it is not a warranty and not a date the roof has to reach. For a side-by-side with other coverings, use the lifespan by material guide.

Poor laminating is the first thing that shortens it

Because a GRP roof is made on site, not unrolled from a factory sheet, the quality of the finished roof is the quality of the person who laid it on that day. The weak points are in the making:

• Resin that went down onto a damp deck, which can leave the laminate poorly bonded and blistered underneath.
• Matting not worked fully into the resin, leaving dry fibre that lets water wick into the sheet.
• Too thin a build, so the cured shell flexes and crazes.
• A rushed topcoat, which is the layer that takes the weather and the wear.
• Bad finishing at the edges, where the laminate has to turn over trims and meet the wall.

Cold and wet weather make all of this harder, because resin needs a dry, adequately warm surface to cure; the cold-weather answer covers that in general terms. A roof laminated in poor conditions can look perfect on the day and show its faults within the first few winters. Ask who laid your roof and in what conditions if you can find out, because early failures often start there.

Movement and foot traffic

Fibreglass is hard and rigid once cured. It does not stretch the way a rubber or felt membrane does, so any movement in the structure underneath has to go somewhere, and it often goes into a crack in the laminate. Timber decks move as they dry out, as boards swell and shrink and as an extension settles against the house wall.

This is why the quality of the deck and its fixings matters as much as the resin. Boards need to be dry, firm and well supported before anything is laid. The GRP cracks guide looks at what different cracks mean and which ones are cosmetic.

Foot traffic is the other regular cause. GRP can be walked on, but constant use wears the topcoat through, and a roof used as a terrace with furniture, plant pots and grit under shoes loses its protective layer sooner than one that is left alone. Where a roof does need to be walked on for maintenance, the walking on a flat roof answer sets out sensible care. Surfaces also age faster when outlets block and water sits there for weeks.

Judging the age and the next step

A GRP roof in its late 20s is within both published ranges, so age on its own settles very little. What matters is what the surface and the edges are doing. Fine hairline cracks at the trim, a chalky or worn topcoat, a patch of yellowing, or a soft spot that suggests wet boards beneath are all worth acting on while they are small.

A worn topcoat can sometimes be refreshed, and an isolated crack can be cut out and relaminated. Widespread cracking, delamination or wet boards point to a new roof, with the deck checked first. Keep the gutters and outlets clear, and avoid storing anything heavy on the surface, since a point load on a rigid shell is how a hairline crack begins. Ladders, planters and paving slabs are the usual culprits, and a rubber mat under a ladder foot costs very little.

If your fibreglass roof is approaching that late-20s mark and you would like it looked at, start from the flat roofing page. Our survey covers the surface, the edges and the underside of the deck where it can be reached, and the written price follows it.