What the two materials are
Glass in a rooflight is not a single pane. It is a sealed unit: two or three sheets with a gap between them, usually filled with gas, with a coating on one surface to hold heat in or keep it out. The edges are sealed so the gap stays dry. When that seal fails, the unit mists between the panes, which is why our page on replacing misted rooflights exists.
Polycarbonate is a plastic. It comes as a flat sheet, as a multiwall sheet with hollow channels running through it, or moulded into a dome. The hollow channels trap air, which gives the sheet a degree of insulation, and a multiwall panel is often fitted in a frame much like glass. It is light, it is tough, and it is cheaper to make into large panels.
The two overlap more than people expect. Both can be framed in aluminium or uPVC, both sit on a kerb, both can be fixed or opening, and from the garden a clean polycarbonate rooflight and a clean glass one look similar. The difference is what each is like from underneath and in ten years.
Where glass wins
Clarity. Glass is transparent. You look through it and see the sky, a tree, cloud moving. Multiwall polycarbonate is translucent: it lets light in, but it scatters it, and the view is a soft white. For a kitchen or a living room, where the point of the skylight is partly to look up, that matters. Some people like the diffusion. Those who sit under it daily often do not.
Ageing. Glass does not yellow, and a good surface resists scratching and cleaning for years. Plastic is more easily marked, and over time it can dull or discolour in sunlight. Makers of polycarbonate apply UV protection to slow this, and the protection is there for a reason.
Coatings and solar control. A sealed glass unit can carry a low-emissivity coating, a solar control coating, a tint, a laminated inner pane for safety or sound. That is how a south-facing kitchen avoids overheating without going dark. The options are set out in our guide to solar control glass for rooflights.
Heat figures. Rooflights in an existing dwelling carry a Part L target of 2.2 W/m²K. A well made double or triple glazed unit can meet that comfortably. A single skin of plastic cannot, and a multiwall sheet needs to be chosen with care. How the figure is worked out is on the rooflight U-values and Part L page.
Feel. A glass rooflight reads as part of the building. In a lived-in room that counts for a lot once you have sat under it.
Where polycarbonate wins
Weight. Plastic is light. One polycarbonate maker puts a polycarbonate unit at less than half the weight of an identically sized glass one. That is a manufacturer's claim, not a rule, but the direction is not in doubt: glass is heavy. A light panel is easier to carry up, easier to set and kind to a roof structure that was not built for much. On an old garage roof, that can settle the matter.
Impact. Polycarbonate takes a knock. A branch, a thrown ball, a dropped tool will mark glass in a way they will not mark a plastic sheet. For a workshop, an outbuilding or a roof beneath a tree, toughness is the point.
Cost and simplicity. Plastic is cheaper to make in large sizes. Where the room does not need the finish, that is a sensible saving, and the saving can go to the better thing, which is usually the roof under the unit.
Diffusion. In a garage or a studio where you want even light without hard patches of sun, the soft spread of multiwall sheet is a benefit, not a defect.
Side by side, it comes down to this.
- Clear view: glass.
- Holds its look over years: glass.
- Coatings and heat control: glass.
- Light to lift and fit: polycarbonate.
- Takes knocks: polycarbonate.
- Gentle, even light: polycarbonate.
Matching the material to the room
The quickest way to choose is to start from the room, not the material.
Kitchen, living room, bedroom, home office. Glass. These are rooms where you sit, look up and care what it looks like. A sealed unit with a suitable coating keeps the room at a sensible temperature.
Bathroom or hallway. Usually glass. Where privacy matters, obscured or frosted glass is the usual answer, and a coating can do the rest. Plastic domes appear here too, in modest sizes.
Garage, utility room, shed, workshop, lean-to store. Polycarbonate makes sense. The roof is often lighter, the room is not a showpiece and the toughness is useful.
Conservatory-style or sun-facing room. Glass with solar control. Polycarbonate gives you less control over heat gain, and a south-facing room under a diffusing sheet can still get hot without looking bright.
Walking on it. Glass only. Walk-on glazing is toughened and laminated, and it is its own category, set out in walk-on rooflights.
A fair worry is whether the cheaper choice is a mistake. Not when it suits the room. A polycarbonate sheet over a garage is a good decision. A polycarbonate sheet over a kitchen you spend three hours in each evening is the one people come back to.
Changing one for the other
Old plastic rooflights often date from the years when a dome was the cheap way to get light into a flat-roofed room. Swapping to glass later is possible, with one condition: the kerb and the structure have to suit the heavier unit. Glass weighs more, and its frame is different, so the survey checks both before anyone promises anything. Our guide to replacing an old plastic dome rooflight covers that route in detail.
The other direction is rarer, but it happens: a heavy glass unit on a roof that cannot take it, replaced by a lighter one. Again, the survey decides, not the catalogue.
How we help you choose
We fit both glass and polycarbonate rooflights. We would rather talk you into the right one than the dearer one, so the question we ask is what the room is for, and the first visit is free. At the survey we look at the roof, the structure, the aspect and how you use the space. The fixed written quote that follows names the glazing in plain terms, so you can compare it with any other quote line for line.
For the full picture of flat roof glazing, including upstands and the fitting sequence, see our flat roof rooflights page. To start, ask for a quote.