The short answer
For most rooms people live in, yes: flat glass is warmer, quieter in rain, clearer and ages better than a polycarbonate dome. A dome still earns its place over a store, a garage or a tight budget, where light matters more than looks or sound. The right choice depends on what sits under the roof, so decide by room, not by product.
Two products, two different jobs
A flat glass rooflight and a dome share a roof opening and little else. They are built from different materials for different expectations.
A flat glass rooflight is a sealed glazing unit, usually two or three panes, set in or over an insulated frame on a kerb. The top pane is toughened, the inner pane is laminated, and the surface is a true plane with a slight fall built into the roof around it. Our page on fixed flat rooflights for extensions describes the make-up in full.
A dome is a moulded bubble of polycarbonate or acrylic, single, double or triple skin, bonded to a kerb. It sheds water because of its curve, and it bends light inward from a wide angle. The plastic is light, easy to lift and quick to fit, which is exactly why it became the standard cheap answer for flat roofs. The basics are in our plain guide to dome rooflights.
Neither is wrong. They are aimed at different rooms.
Comparing them on the things you will notice
Four differences show up once the work is finished and the weather arrives.
| You notice | Flat glass | Dome |
|---|---|---|
| Warmth | Coated, gas-filled units | Air gap between skins |
| Rain | Damped by laminated pane | Thin shell, can drum |
| Clarity | Clear, true colour | Slightly hazy, may tint |
| Ageing | Stays clear for decades | Can yellow or craze |
Warmth comes first in the rules as well as in the room. A glass unit with a low-emissivity coating and a warm-edge spacer loses far less heat than a moulded double skin, and a replacement unit has to meet the thermal limits in Part L of the Building Regulations. A triple-skin dome narrows the gap, and we compare the skins in whether a double or triple skin dome is warmer, but a dome has no low-emissivity glass to draw on.
In rain, the difference is physical. A thin plastic shell is light and springy, so a downpour makes it ring. A laminated glass pane, thicker and bonded to a damping interlayer, moves less. Under a bedroom or a home office, that is the difference between hearing the weather and not. We compare the glass builds in our page on quieter glazing for a flat roof rooflight over a bedroom.
Clarity and ageing are the quiet ones. Plastic scratches more readily than glass, collects a dull haze when cleaned with the wrong cloth, and can yellow in sunlight over the years, though the quality of the plastic and its UV protection vary. Glass holds its clarity. The finish you chose on day one is the finish you have in year ten.
Where a dome is the sensible choice
A dome is not a poor product. It is an honest one, and there are roofs where it is right.
A garage, a workshop, a store cupboard or a porch does not need acoustic damping or a designer frame. It needs daylight at a modest level of outlay. A dome gives that, and its curve sheds leaves and water well. Where the opening is small and sits above a corridor, a landing or a utility space, a dome can do the job for a long time.
Domes are also lighter, so they suit a roof structure that has little spare capacity for the weight of glass. Our page on flat rooflights on a garage roof goes through that case.
Where you want a dome to open for ventilation, some versions can be fitted with a hinged or motorised frame, which sits in our electric and opening rooflight work.
Where flat glass is worth the step up
Choose glass when people will sit, sleep or work under the opening, or when the room sits where the eye falls.
A kitchen extension is the usual case. Everyone sees the glass every day, and rain on it is heard over dinner. A flush, flat rooflight looks like part of the ceiling plane, and some can be made frameless, as described in our page on frameless flat rooflights. The brand angle applies here. We specify every rooflight for the light you want and the quiet you did not think to ask for, and glass gives us far more to specify than a plastic bubble.
Flat glass also suits a room where you want a slim reveal and a lined, finished look beneath it. Domes tend to sit on a deeper kerb with a bare lining.
If you are not sure, compare a sample of each in your own daylight. The difference is easy to see when both are held up to a window. For the money side, our what affects the cost of a flat roof rooflight page covers what drives the difference, and costs gives a general range.
Dome or glass questions, answered
These are the points that decide it for most owners.
Can I swap my dome for flat glass on the same kerb?
Often yes, but the kerb has to be checked. Glass units are heavier and need a stiffer, better insulated upstand, and the roof opening may need a fresh look. We assess it at the survey before promising anything.
Is a dome more likely to fail?
Plastic ages differently from glass. A dome can cloud, craze or yellow, and its bonded joint to the kerb is the part to watch. Glass units can fail too, by misting when the seal goes. The difference is that a glass unit is replaced as a sealed item, as in our misted and failed rooflight replacement work.
Which lets in more light?
A dome gathers light from a wide angle, and a flat pane passes light straight down. In an ordinary room the difference is small. The glass pane is clearer, so colours look truer, and a dome is often better at spreading a little light into a dim space.
Which is easier to keep clean?
Glass, by a long way. Our guide to cleaning a flat rooflight gives the routine, and it is gentler than a dome needs.
To compare the two for your own roof, get a quote or go back to flat roof rooflight installation.