Glass reinforced polyester rooflight sheets degrade from the weather face inwards, and they do it on a predictable path: the surface protection erodes under ultraviolet light, the resin below it breaks down, the glass fibres are left exposed, and light transmission and load-arresting strength fall away together. By the time a sheet looks obviously bad from the floor it has usually been fragile for years. Grading the whole roof position by position, rather than reacting to the sheets that leak, is what turns a vague worry into a plan.
This page is about reading that degradation across a commercial roof and deciding what to do with what you find. It sits under our commercial skylight installation service, and it assumes an audience of facilities managers and building owners who have a roof full of ageing GRP and a budget that will not stretch to all of it at once.
The four things that fail, and in what order
GRP is glass fibre suspended in a polyester resin, with a thin protective layer on the weather face. That layer is a gelcoat, a polyester surfacing film, or a higher performance film, and its only job is to hold ultraviolet light off the resin for as long as it can. When it goes, everything else follows.
1. Surface erosion and fibre bloom
The protective film thins and then fails. With it gone, ultraviolet light attacks the resin at the surface, the resin chalks and washes away in the rain, and the glass fibres beneath stand proud. That is fibre bloom. The surface changes from smooth to rough and fibrous, and once it is fibrous it holds dirt and moisture against the sheet instead of shedding them. Erosion then speeds up because the surface is wet for longer after every shower.
2. Yellowing and lost light transmission
New industrial GRP transmits somewhere around 70 to 80 per cent of visible light. As the resin degrades it yellows, and a bloomed, dirt-holding surface adds its own loss on top. The two together pull transmission down steadily and quietly. Sheets past twenty years commonly pass less than half the light they did new, and thirty year old sheets are frequently down to a small fraction of it. Nobody notices, because the fade takes fifteen years and the artificial lighting simply creeps on earlier each year to cover it.
3. Water tracking into the laminate
Exposed fibres wick water. Once moisture is travelling along the fibres into the body of the laminate rather than running off the surface, the sheet is delaminating from the inside. This is where leaks start, and it is why a leak at a GRP sheet is a symptom of a failed laminate rather than a failed joint. Sealing the joint does nothing about the laminate.
4. Embrittlement and loss of fragility performance
The resin is what holds the fibres in tension and lets the sheet flex and absorb an impact. As it breaks down the sheet gets brittle. This is the failure that matters for safety, because it removes the sheet’s ability to arrest a falling person. It is also invisible. A sheet can look serviceable, still 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, not a permanent property of the material, and embrittlement is that degradation happening. There is more on the classification itself in our non-fragile rooflight specification page.
Never treat a suspect GRP rooflight as a walking surface. A sheet that looks sound from the floor can be well past the point where it will hold a person. If access across the roof is needed before replacement, work off crawl boards, staging or a mobile platform that spans the rooflights, and brief everyone who goes up on which surfaces are fragile. Do not test a rooflight by standing near it.
Grading condition across a whole roof
The useful survey output is not “the roof is old”. It is a plan with every rooflight position marked and graded, so priorities and budgets attach to real positions. We grade each position on a simple five-band scale that anyone in the facilities team can follow.
| Grade | What you see | Light transmission | Fragility position | Action |
|---|---|---|---|---|
| A. Sound | Smooth surface, film intact, clear or lightly tinted | Near original | Non-fragile if documented and within service age | Monitor, record age |
| B. Early ageing | Slight yellowing, film thinning, surface still smooth | Mild loss | Assume fragile if over 20 years or undocumented | Plan into a later phase |
| C. Bloomed | Visible fibre bloom, rough surface, dirt holding | Half or less | Treat as fragile | Prioritise for replacement |
| D. Failing | Heavy bloom, cracking, staining below, damp laminate | Heavy loss | Fragile | Early phase, restrict access below and above |
| E. Failed | Cracked or holed, active leak, brittle to touch | Minimal | Fragile, immediate fall risk | Restrict access now, replace first |
Grades are a condition and risk indicator, not a substitute for a competent fragility assessment of the actual assembly. Age and provenance carry as much weight as appearance, because embrittlement runs ahead of what the surface shows.
The point of grading is that a roof is rarely uniform. Sheets on the south-facing slope of a duo-pitch roof take far more ultraviolet than those facing north, and often grade two bands worse for the same age. Sheets under a taller neighbouring building, or shaded by plant, last longer. A grading survey shows you this pattern, and the pattern is what a sensible phasing plan is built on.
A worked example: reading one roof
Take a distribution unit on an Ipswich estate, roof area 4,000 square metres, in-plane GRP rooflighting at roughly 12 per cent, so about 480 square metres of rooflight, installed in the mid-1990s. Thirty years old, on sheets specified for fifteen to twenty.
- The south slope grades mostly D and E: heavy bloom, two active leaks over racking, brittle underfoot on the surveyor’s spanning staging.
- The north slope grades B and C: yellowed and blooming but not yet cracked, transmission roughly half.
- A run under a taller adjoining unit grades A to B, protected from direct sun.
Reacting to leaks alone would replace two sheets on the south slope and return within a year for the next two. Grading the whole roof produces a different plan: replace the whole south slope in phase one because it is both the safety risk and the leak source, take the north slope in phase two the following financial year, and leave the protected run to be reviewed at the end. The safety liability is cleared first, the leaks stop because the failing laminate is gone rather than sealed over, and the daylight comes back uniformly across the worst slope.
Why phased whole-roof replacement beats sheet-by-sheet
The instinct is to replace only the sheets that have failed. It feels like the frugal choice and it is usually the expensive one.
- Access is the cost, not the sheet. Getting safe access onto a roof, netting the bay below, cordoning the floor and setting edge protection is most of the price of a small job. Doing it four times for four scattered sheets over four years costs far more than doing it once for a slope. Access is shared across every sheet replaced in a visit, so grouping work by bay is what makes the unit rate fall.
- The next failure is already coming. Sheets that were installed together, on the same slope, age together. Replacing the one that failed leaves its neighbours a season or two behind it. Sheet-by-sheet is a standing invitation to return, and each return carries the full access cost again.
- Mixed non-fragility is a mixed message on the roof plan. A slope with three new non-fragile sheets and nine fragile ones is still, in practice, a fragile roof that has to be managed as one. A whole-slope replacement lets you record that slope as documented non-fragile in one clean line.
- Daylight arrives in patches otherwise. One bright new sheet among eleven yellowed ones does not fix the lighting bill. Uniform transmission across a slope is what lets the lighting controls actually turn lamps off.
Phased whole-roof replacement is the middle path between the false economy of sheet-by-sheet and the capital shock of doing everything at once. The roof is surveyed and graded in full, the work is grouped into slopes or bays that can each be completed and closed as a unit, and the phases are ordered by risk so the fall hazard and the leaks go first. Our phased rooflight replacement programme answer sets out how the phases are costed across financial years, and why we replace rather than patch covers the patching argument in full.
What the survey records
A grading survey that is worth building a budget on records, for every rooflight position: material and grade, estimated age and provenance, measured or estimated light transmission loss, evidence of water ingress below, and a fragility position. That set of records is the difference between a defensible roof and an anxious one. A partially replaced roof with a current, graded plan is a position you can put in front of an insurer or an inspector. The same roof with no record is a much worse conversation.
Can a GRP rooflight be fragile even though it is not leaking?
Yes, and this is the trap. Water resistance and fragility fail on different timescales. A sheet can still shed most of the rain while the resin has embrittled to the point where it will not arrest a fall. Non-fragility is a tested assembly classification that degrades with age, so a dry sheet of unknown age and provenance still has to be treated as fragile until a competent assessment says otherwise.
How old is too old for industrial GRP rooflighting?
It depends on the surface film grade and the exposure, but as a planning position GRP over about twenty years old should be treated as fragile regardless of appearance, and coastal or port-side sheets sit at the shorter end. Age and provenance matter more than how the surface looks, because embrittlement runs ahead of the visible bloom. Our when to replace industrial rooflights answer gives the age bands and condition triggers.
Is it cheaper to just replace the sheets that have failed?
Almost never, over any sensible timeframe. The cost of a rooflight job is dominated by safe access, netting and cordoning rather than by the sheet, and that cost is shared across every sheet done in one visit. Replacing failures one at a time means paying the access cost again each time, and returning within a year or two as the neighbouring sheets follow. Grouping the work by slope or bay is what makes the rate fall.
Grade your roof before you budget it
If your building is running on GRP rooflighting from the 1990s or early 2000s, the sheets are past their design life and the roof is ageing unevenly. A grading survey turns that into a marked plan you can phase and cost. We arrange access with your team, grade every position, and come back with a written scope priced by bay and a phasing proposal that fits your capital budget.
Request a grading survey for your roof, or read across to our warehouse rooflight leak replacement and GRP, polycarbonate or glass pages if you are weighing up what to replace the old sheets with. Published budget rates are on the costs page.
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.