GRP rooflights degrade because ultraviolet light attacks the thin protective film on the weather face, then the resin beneath it. Once the film goes, the resin chalks and washes away, the glass fibres are left exposed, water wicks along them into the laminate, and the sheet yellows. Light transmission falls away steadily long before the sheet cracks or leaks, so a GRP rooflight is usually failing invisibly for years before it visibly fails.
This answer sits under our commercial skylight installation service and explains the mechanism, because understanding it is what makes the case for replacement over patching.
What GRP is, and where it fails
GRP is glass fibre suspended in a polyester resin, with a thin protective layer on the weather face: a gelcoat, a polyester surfacing film, or a higher performance film. That surface layer is the whole defence. Its only job is to hold ultraviolet light off the resin. When it fails, everything downstream follows in order.
- The surface film erodes. Ultraviolet light and weather thin the film until it fails. A basic gelcoat may manage ten to fifteen years, a good film considerably longer, but it is a finite defence.
- The resin degrades. With the film gone, ultraviolet light attacks the resin at the surface. It chalks and washes off in the rain.
- The fibres are exposed. As the resin erodes, the glass fibres beneath stand proud. This is fibre bloom, and it changes the surface from smooth to rough and fibrous.
- Water tracks in. Exposed fibres wick water along themselves into the body of the laminate rather than shedding it. The surface holds dirt and moisture, and erosion accelerates because the sheet stays wet longer.
Why the light goes first
The failure a facilities manager notices, eventually, is darkness. New GRP transmits around 70 to 80 per cent of visible light. Yellowing from resin degradation, plus dirt held in a bloomed surface, pulls that down steadily. Sheets past twenty years commonly transmit less than half what they did new, and thirty year old sheets are often down to a small fraction. But nobody notices, because the fade takes fifteen years and the artificial lighting simply creeps on earlier each year to cover it. The lighting bill rises so gradually that it is never traced back to the roof.
The dangerous part is invisible. The same resin degradation that dims the sheet also embrittles it, and embrittlement removes the sheet’s ability to arrest a falling person. This happens ahead of any visible cracking or leaking. A GRP rooflight can look serviceable, keep most of the rain out, and no longer hold a person, all at once. Non-fragility is a tested assembly classification that degrades with age, and this is that degradation happening. Never treat a suspect GRP rooflight as a walking surface.
Why it seems quick
GRP does not degrade quickly in absolute terms. A good sheet lasts fifteen to twenty-five years, and a high performance film longer. It seems quick because the failure is back-loaded and invisible. For most of its life the sheet looks fine, then the film gives out and the last stages, bloom, water tracking, embrittlement, dimming, run together over a few years. And crucially, the lowest tender price usually buys the cheapest film, so a lot of installed GRP is the basic gelcoat grade that fails at the short end. The film grade is the single biggest lever on GRP life, and it is routinely chosen on price. Our GRP rooflight degradation page covers grading the failure across a roof.
Why exposure makes it faster here
On the Suffolk and north Essex coastal belt, near Felixstowe and Harwich, high ultraviolet exposure combines with salt deposition and wind-driven abrasion, and GRP surface life runs at the shorter end of the published band. Salt-laden air scours the surface and holds moisture against it, and the sheets bloom sooner. Inland, around Stowmarket, the driver is ultraviolet alone, and sheets tend to reach the longer end of their band. Either way, the industrial stock built across the region in the 1980s and 1990s is now at or past the point where this degradation has run its course.
How long do GRP rooflights last?
Between about ten and twenty-five years, set mainly by the grade of the surface film. A basic gelcoat manages ten to fifteen years, a polyester surfacing film fifteen to twenty-five, and a high performance film longer. Coastal and port-side sheets sit at the shorter end. The film grade is the biggest single factor, and it is often chosen on the lowest tender price.
Can a degraded GRP rooflight be restored?
No. Once the surface film has eroded and the resin and fibres are degrading, there is no coating or treatment that restores the laminate or the light transmission. Coatings over aged GRP cut the remaining light further and do not restore the load-arresting strength. The failure is in the body of the sheet, and replacement is the only remedy. Our replace-rather-than-patch answer explains why.
Replace it before it fails invisibly
Because GRP fails invisibly, the safe position is to replace it on age and condition rather than waiting for a crack or a leak. A survey grades every sheet so you know where each stands.
Request a survey. Read across to why replace rather than patch, when to replace industrial rooflights, and the GRP rooflight degradation guide.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.