GRP, Polycarbonate or Glass for Commercial Rooflights?

For most commercial and industrial rooflights the choice is between GRP, multiwall polycarbonate and glass, and there is no single winner. GRP is the cheap, formable industrial default that ages fastest. Polycarbonate holds its light and resists impact better and suits barrel vaults and heated buildings. Glass gives the clearest light and the best sealed-unit thermal performance but is heavy and costs the most. The right choice follows from the building use, the roof type and how long you intend to hold the asset, not from a general ranking.

This page compares the three so you can specify against your building. It sits under our commercial skylight installation service, and it is for facilities managers and building owners choosing what to replace ageing rooflights with. It ends with a specification rule by building type.

The three materials, in short

• GRP, glass reinforced polyester, is glass fibre in a resin with a protective surface film. Light, cheap per square metre, forms to any profile, and installs in-plane with the metal sheet around it. Its surface degrades under ultraviolet light, and its life is set largely by the grade of that surface film.
• Multiwall polycarbonate is a rigid plastic sheet with internal walls that trap air for insulation. Very impact resistant, light, diffuses light well, and holds its transmission better than GRP. It suits curved barrel vaults and heated buildings.
• Glass, in a framed sealed-unit system, gives the clearest light and the best thermal and acoustic performance, with safety glazing where people are below. It is heavy, needs a proper frame, and costs the most.

How they compare

Service life and the film-grade decision

The biggest variable within GRP is the surface film. A basic gelcoat gives perhaps ten to fifteen years before fibre bloom starts. A polyester surfacing film pushes that to fifteen to twenty-five. A high performance film on a good sheet is specified for considerably longer. The difference in installed cost between the grades is far smaller than the difference in replacement interval, which makes the film grade the single specification decision with the biggest effect on lifetime cost, and it is routinely made on the lowest tender price. Our why GRP rooflights degrade answer explains the mechanism.

Polycarbonate and glass hold their transmission far better over life. Polycarbonate’s curve on a barrel vault self-cleans, and glass wipes clean, so both avoid the steady daylight fade that makes an old GRP roof so dark. Over a long holding period that retained light is worth real money in lighting energy.

Light transmission is not the whole story

GRP has the highest transmission when new, but it loses it fastest, so a thirty year old GRP roof can be transmitting less than a polycarbonate one that started lower. What matters is transmission over the life of the asset, not on day one. Polycarbonate’s lower headline figure buys retained light and better insulation. This is why comparing materials on new-transmission alone leads to the wrong choice on a building you intend to hold for decades.

Impact, and why it matters for safety

Polycarbonate is close to unbreakable under impact, which is one reason it suits areas with a risk of falling objects. GRP is moderately impact resistant when new but embrittles with age, and that embrittlement is the same process that removes its non-fragility. Glass has to be laminated wherever people are below, so it holds together and stays in the frame if broken. On any of the three, non-fragility from above is a tested assembly classification that degrades with age, and it is specified and recorded against the actual installation. Our non-fragile rooflight specification page covers that.

Thermal performance

Where the building is heated or cooled, the rooflight’s thermal performance matters, because every square metre of rooflight loses more heat than the roof around it. Single-skin GRP is modest, twin-skin GRP with a liner is better, multiwall polycarbonate improves with each added wall, and a glass sealed unit performs best. On a heated warehouse or an office within an industrial shell, the thermal build-up is set so the lighting saving is not eaten by heating and cooling. Our double or triple skin answer covers that trade-off within GRP and polycarbonate.

Cleaning, maintenance and retained light

The number that matters over a roof’s life is not transmission when new but transmission ten and twenty years on, and that is set as much by how the surface holds up to dirt as by the material. GRP’s surface roughens as it blooms, so it holds dirt against itself and the transmission falls faster than cleaning alone would explain. Polycarbonate on a curved vault self-cleans in the rain because of the curve, and glass wipes clean, so both hold their light with far less intervention. On a roof that is expensive or dangerous to access, and most commercial roofs are, the material that keeps its daylight without regular cleaning is worth a premium, because the cleaning visits carry the same access cost as the installation.

There is a safety dimension to this too. Cleaning an old GRP roof means getting people onto fragile sheets, which is a fall risk in itself. A material that does not need routine cleaning removes a recurring reason to send someone onto a fragile surface, and that is a real, if indirect, argument for polycarbonate or glass on a hard-access roof.

Matching the material to the existing roof

The choice is not made in isolation. It has to fit the roof it goes into.

• Profiled metal roofs take in-plane sheets, and GRP and multiwall polycarbonate both mould to a profile. GRP is the cheaper match, polycarbonate the more durable one. Glass does not fit in-plane and needs a kerb.
• Flat, membrane and built-up roofs have no profile to match, so the rooflight is kerb-mounted, and any of the three can sit on a kerb. Here the choice opens up to include glass for a heated or public-facing space.
• Mixed estates rarely want one material across every building. An unheated store, a heated production hall and an office within the same shell each point at a different choice, and specifying one build-up across all of them wastes money on some and underspecifies others.

Our what in-plane rooflights are answer covers how the material sits in the roof, and what percentage of a roof should be rooflights covers how much of it you need.

Choose the material against your building

The right rooflight material depends on how the building is used, how the roof is built, and how long you will hold it. We survey the roof, weigh the daylight, thermal and access factors, and recommend the material and grade that gives the best value over the life of your asset, then price it by bay.

Request a survey. Read across to factory daylighting and barrel vault and continuous rooflight installation, and see budget rates on the costs page.