GRP rooflight replacement on an industrial roof

Glass-reinforced plastic is a polyester resin with a glass-fibre mat set into it. On an industrial roof it is pressed to the same profile as the surrounding metal or fibre-cement sheeting so it drops into the cladding as a like-for-like translucent sheet. It is light, it is cheap per square metre, and when it is new it lets through a lot of daylight. That is the whole appeal for a warehouse, a factory unit or an agricultural building where the alternative is running lights all day.

The problem is built into the material. Polyester resin is not stable under ultraviolet light. Over years of sun the surface resin breaks down, the glass fibres underneath become exposed, and the sheet goes from clear to yellow to brown. It picks up a rough, fibrous texture that holds dirt and algae. Light transmission that started around 70 to 80 per cent can fall below 40 per cent, so the floor beneath goes gloomy and the lights come on anyway. At the same time the sheet loses its flexibility. A GRP panel that would flex under a footfall when new becomes brittle with age, and brittle is the dangerous word on a roof.

Most GRP industrial rooflights are given a service life in the region of ten to twenty years, and the ones fitted without a surface gel coat or protective film sit at the shorter end of that. If the metal sheeting around them still has years left, and profiled steel cladding often does, the rooflights reach the end of their life first. Replacing the translucent sheets while leaving the sound metal in place is routine, and it is the work this page describes.

You rarely replace a single sheet on its own, because degradation is driven by sun and age, and every panel in a run has had the same exposure. When one has failed, the rest are usually close behind. A few signs that a run is due for replacement rather than another winter:

• Yellowing or browning right across the panels, not just at one end. This is the resin breaking down and it only goes one way.
• A fibrous, matt surface where the smooth outer resin has worn off and the glass mat is showing through. Once the fibres are exposed the panel weathers much faster.
• Star cracks and splits, often around the fixings, where the sheet has gone brittle and the fixing has worked at it through years of thermal movement.
• Daylight looking grey or green on the floor below at midday. That is transmission that has dropped far enough to matter.
• Water tracking in at laps and fixings, usually because the sheet has embrittled and lost its seal at the overlap.

If a single sheet has been cracked by a one-off impact and the rest of the run is genuinely sound and clear, that is a different situation and a roofing contractor can advise. Where the whole run has aged out, replacement of the run is the sensible call, because the labour and the access are much the same whether you swap one sheet or ten, and the ones you leave will fail next season.

An industrial rooflight has to sit in the roof as if it belongs there, which means it has to match the exact profile of the sheeting on either side of it. Profiled cladding is not generic. The rib height, the rib pitch, the shape of the crown and the way the sheets lap are specific to a system, and a translucent sheet that does not match will not lap down properly, will not shed water at the side laps, and will leak.

The families you meet on south Essex industrial stock are broadly these. Trapezoidal box profiles, the modern standard on steel-clad units, described by rib height and cover width, for example a 32mm rib at 1000mm cover. Sinusoidal corrugated sheeting on older buildings. And the fibre-cement profiles, most commonly the traditional six-rib sheet, still very common on mid-century industrial and agricultural roofs across the estates around Basildon and Rochford. A great many of these fibre-cement roofs contain asbestos, which changes the job completely, and there is more on that below.

Getting the profile right is a survey job, not a catalogue job. The rib profile is measured off the actual roof, the cover width confirmed, the lap arrangement checked, and the replacement translucent sheet ordered to that exact profile so it laps under the upper sheet and over the lower one and seals at the side. Get this wrong and you have bought a leak. Get it right and the finished run reads as a continuous roof with clear panels in it.

This is the part of an industrial rooflight replacement that a domestic skylight has no equivalent for, and it is the most important part. A large proportion of the people killed or seriously injured falling through roofs in this country fall through a rooflight, very often an old, weathered GRP one that looked solid and was not.

A new industrial roof assembly is tested for fragility to ACR[M]001, the industry drop test that classifies a roof or rooflight assembly as non-fragile or not. A sheet classed non-fragile will arrest a person and the tools they are carrying if they fall onto it, at least for the design period. An aged GRP panel has lost that property entirely. It has gone brittle, and a foot or a knee will go straight through it. This is why any work on or near an industrial roof treats the existing rooflights as fragile until proven otherwise, and why the replacements are specified as non-fragile assemblies where people may need to be on the roof afterwards.

Non-fragility is a property of the whole assembly, not just the sheet. It depends on the sheet thickness, the profile, the purlin or rail spacing the sheet spans between, and the fixings and their spacing. A sheet that is non-fragile spanning 1.2 metres between purlins can be fragile spanning 1.8 metres. This is why the survey records the structure under the roof, not only the roof surface. The replacement is then specified to hold its non-fragile classification at the actual span on your building, and installed with the fixing pattern that classification depends on.

The practical upshot for a building owner: a proper replacement removes a genuine safety liability from the roof, not just a leak. If maintenance staff, roofers or engineers ever go up to plant, gutters or solar panels, the difference between an old fragile GRP run and a new non-fragile one is the difference between a walkable roof and a hazard that needs boards, nets and a permit every time anyone goes near it.

Older industrial and agricultural roofs across the region are clad in fibre-cement sheeting, and sheeting made before the ban commonly contains asbestos. The translucent panels themselves are GRP and do not contain asbestos, but they are fixed into and lapped with sheets that may. You cannot lift, cut, drill or disturb an asbestos-cement roof without the right controls, and on many of these roofs that work is licensed.

So on any fibre-cement industrial roof the first step is not the rooflight at all. It is establishing what the surrounding sheeting is, through the building’s asbestos records or a survey by a competent surveyor. If the cladding is asbestos-cement, replacing the rooflights that lap into it is managed as work on an asbestos roof, with the appropriate contractor and controls, and that governs the programme, the cost and who is allowed on the roof. This is not a corner anyone should cut, and a reputable installer will insist on the paperwork before quoting. Where the roof is modern steel or non-asbestos fibre-cement, the job is far simpler, and knowing which you have is the first thing the survey settles.

Like for like is not the only option, and often it is not the best one. Three broad choices come up.

Single-skin GRP is the direct replacement. It matches the profile, it is the cheapest option per square metre, and it keeps the roof simple. Modern GRP with a protective surface film lasts longer than the sheets it replaces. Its weakness is thermal: a single skin of GRP is barely better than a sheet of glass with nothing behind it, and on a heated building that is a lot of heat leaving through the rooflights. It suits unheated stores, canopies and buildings where daylight matters and the inside temperature does not.

Multiwall polycarbonate gives you more skins and therefore a much better U-value, along with very high impact strength. It is the material of choice where the building is heated and the rooflights are a meaningful share of the roof. It can be supplied in profiled form to lap into cladding, or as part of a factory-assembled unit.

Factory-assembled rooflights, sometimes called FARs, are double or triple-skin units built off site with the skins, spacers, seals and upstands assembled under controlled conditions rather than layered up by hand on the roof. They give the most reliable weather seal and the best thermal performance, and on a re-roof or a significant refurbishment they are usually the specification worth paying for. On flat-roof sections of an industrial building the equivalent is a proper kerb-mounted unit, and the same logic that applies to a flat-roof skylight on any building applies here: the upstand and the seal do the work.

These are industry-typical figures for common industrial rooflight build-ups. Actual values depend on the manufacturer, the number of skins, the profile and the coating, so treat them as the shape of the market rather than a fixed price list. The point is the pattern: skins buy you thermal performance, and every material trades light transmission against insulation.

Read the U-value column and the message is stark. A single skin of GRP sits at around 5.7 W/m²K, which is very close to letting the heat straight out. Move to a triple-skin or a decent multiwall and you are down near 1.5 to 1.9, a heat loss cut of two-thirds or more through that part of the roof. On a heated building, replacing an aged single-skin run with an insulated one pays back in reduced heating load, quite apart from the daylight and the safety. Building Regulations Part L sets a limit on the U-value of a rooflight in a heated building, and a modern insulated unit meets it while a bare single skin does not, so on any heated shed the thermal specification is not really optional.

There is a second number that industrial rooflight specifications almost never mention, and on a large low-pitch roof it is the one people on the floor below actually feel. A warehouse or factory roof carries a much bigger area of rooflight than a house ever does, and it faces the sky at a shallow pitch, so it collects solar energy nearly square-on through the middle of the day. Down here on the Essex coast that matters more than it does inland, because the roofs face an open estuary horizon and the region gets some of the higher sunshine-hours figures in England.

Clear GRP and clear polycarbonate let a large fraction of that solar energy through. On a south-facing industrial roof the result is a working floor that overheats through the afternoon and picks up hard glare across screens, benches and pick faces, which is uncomfortable and, in a workplace, a genuine productivity and safety issue. The same physics that drives us to specify solar-control glazing on a south-facing house extension applies to a shed roof, only over a far larger area. Where overheating is the problem, the replacement can be specified with a solar-control or opal diffusing grade that cuts the transmitted heat and spreads the light evenly instead of throwing a bright stripe across the floor. Diffused daylight is usually what a working space wants anyway, because it kills the glare and lights the aisles more evenly than clear panels do.

This is the orientation-first thinking the business is built on, carried onto an industrial roof. Work out which way the roof planes face, then pick the grade of translucent sheet to match. North-facing slopes want the clearest, brightest sheet available. Large south-facing and west-facing areas want the transmitted heat and the glare taken out. Specifying one grade of panel across a whole mixed-aspect roof means one half of it is wrong.

A GRP rooflight replacement on an industrial roof runs in a clear order. Survey first: the profile, the span between purlins, the fixings, the surrounding sheeting and its asbestos status, and the aspect of each roof plane. Then a specification: material, number of skins, non-fragility classification at the actual span, U-value against Part L, and the light or solar-control grade to suit the orientation. Then access and safety planning, which on any industrial roof means edge protection, fall arrest and often boards or nets over fragile areas until the new non-fragile sheets are in. Then the replacement itself, sheet by sheet, matched and lapped into the cladding and sealed at every side lap and fixing.

A few things worth confirming on any quote for this work:

1. Is the replacement assembly classified non-fragile at the span on my roof? Non-fragility depends on the purlin spacing, so the answer should reference your building, not a generic sheet.
2. What is the U-value, and does it meet Part L for a heated building? If the building is heated, a bare single skin will not, and that should be said plainly.
3. Has the surrounding sheeting been checked for asbestos? On any fibre-cement roof this comes before everything else.
4. What grade of sheet on the south-facing planes? If overheating or glare is a problem now, the clear panel that caused it should not be the one going back in.
5. Who notifies Building Control? New and replacement rooflights are notifiable under the Building Regulations. On our installations we make that notification to the relevant local authority as part of the job.

Industrial rooflight replacement sits within our commercial skylight installation work, and it starts the same way every job does, with a look at the roof and the compass rather than a page in a catalogue. If you have a run of yellowed, brittle GRP over a unit somewhere in south Essex and want it specified properly, with the profile matched, the fragility sorted and the right grade of sheet for the aspect, ask us to survey it and set out the options with the numbers on the page.