Dome Rooflight or Flat Glass: Which Should You Fit?
For a habitable room, flat glass beats a dome on almost every measure: it is clearer, much warmer, it reads as a clean rectangle of sky rather than a cloudy blister, and it does not yellow. A modern flush glass unit sits around 1.1 to 1.4 W/m²K against a dome’s 2.8 to 3.5, and it passes clear light with a view where a multi-skin dome diffuses it into flat brightness. The dome wins on exactly two things: it is cheaper, and it sheds water by shape rather than by fall, which is genuinely useful on a poorly drained roof. So the rule is simple. Habitable room, flat glass. Garage, outbuilding, utility room, or a low-budget swap on a roof that drains badly, a dome is still the sensible choice.
This page compares the two properly, measure by measure, and gives the decision rule. The detail on each is on flat glass rooflight installation and yellowed and brittle dome rooflights. The wider job is on flat roof skylight installation.
The comparison, measure by measure
| Measure | Polycarbonate or acrylic dome | Flush flat glass |
|---|---|---|
| Light quality | Diffused, no view, dims as it yellows | Clear, a view of the sky, stays clear |
| Whole-unit U-value | Around 2.8 to 3.5 W/m²K | Around 1.1 to 1.4 W/m²K |
| Drainage | Sheds by shape, forgiving on a flat roof | Sheds to a shallow fall, needs the fall to be right |
| Longevity | Yellows and goes brittle, 15 to 20 years | Glass does not yellow, decades |
| Appearance | A raised blister on the roof | A flush rectangle, nearly frameless |
| Impact strength | Tough when new, brittle when old | Toughened outer, laminated inner throughout life |
| Cost, up to 900mm | Lower | Higher |
| Best for | Garages, outbuildings, poorly drained roofs | Kitchens, living space, any room you use |
Light quality
This is the biggest everyday difference. A multi-wall polycarbonate dome diffuses light through its flutes, so the room gets brightness with no view of the sky, and as the plastic yellows over the years that brightness fades to a dull, dim glow. Flat glass passes light cleanly, so you get both the daylight and the sky, and it stays that way because glass does not degrade under UV. In a kitchen or living room, where the point of a rooflight is the quality of the light, this alone decides it.
Thermal performance
A dome at 3 W/m²K loses heat more than twice as fast as a flush glass unit at 1.2. Over a Suffolk winter, in a room you are heating, that is real money going through the roof, and it is why an old dome makes a room feel cold as well as look dim. Flat glass, with argon fill, a warm-edge spacer and a low-emissivity coating, holds heat in and meets Part L comfortably. On thermal grounds glass wins clearly for any heated room.
Drainage, and the dome’s genuine advantage
Here is where the dome earns its keep. It sheds water by its curved shape, so water runs off it regardless of the fall of the roof around it. That is forgiving on a poorly drained roof or a dead-flat one where correcting the fall is not on the table. Flat glass is laid to a shallow fall and needs the roof around it to drain properly, or water sits against it. On a roof that ponds and cannot be corrected, a dome is the more honest fit, though we would rather correct the fall, which is covered on why flat rooflights pond water.
Longevity
Polycarbonate yellows and crazes under UV, going cloudy at 12 to 15 years and brittle by around 20. Glass does not degrade under sunlight, so a flush glass unit lasts decades and the only wearing part is the perimeter seal. Over any sensible timeframe glass is the longer-lived unit, and a dome is a shorter-cycle product you will replace sooner.
Appearance
A dome is a raised blister on the roof and reads as one from the garden and from any overlooking window. A flush glass unit sits nearly flat and, from inside, presents as a clean rectangle with almost no frame. On a house where the roof or the ceiling is on show, appearance often settles it in favour of glass on its own.
Cost
The dome is cheaper, and on small sizes noticeably so. A like-for-like dome swap into a sound kerb runs £1,150 to £1,650 up to 900 by 900mm, while a flush glass unit in a new opening starts around £2,200. On a garage or outbuilding where the extra spend on glass buys nothing you will benefit from, that cost difference is the dome’s strongest argument. The full breakdown is on flat roof rooflight installation cost.
The decision rule
Habitable room, flat glass. Utility space or a badly drained roof, a dome. If it is a kitchen, dining room, living space or any room you spend time in, fit glass: the light, the warmth and the longevity all earn their cost. If it is a garage, outbuilding or utility room, or a low-budget swap on a roof that drains badly and cannot be corrected, a new dome is the sensible, honest choice, and we fit them.
A few refinements on that rule. On a north-facing room where solar gain is never the issue, glass still wins on light quality and warmth. On a very large opening, glass gets heavy and expensive faster than a dome does, though a dome that big is rarely what anyone wants to look at. And whatever you choose, the kerb underneath has to be right, because a dome on a rotten kerb leaks exactly like glass on a rotten kerb.
What both share
Whichever unit you pick, the same things decide whether it lasts. The kerb has to be built square, tall enough for the exposure and insulated, the covering has to be dressed over the head rather than sealed, and the falls have to run water away from the unit. Those details, covered on rooflight kerb construction, matter more than the choice between dome and glass. A well-chosen unit on a bad kerb fails. A modest unit on a good kerb lasts.
Common questions about dome versus flat glass
Is flat glass always better than a dome?
For a habitable room, almost always, on light, warmth, longevity and appearance. A dome keeps two real advantages: it is cheaper, and it sheds water by shape, so it is forgiving on a poorly drained roof. On a garage, outbuilding or utility room, or a badly draining roof that cannot be corrected, a dome is the sensible choice. Elsewhere, glass earns its cost.
Why is a dome warmer to buy but colder to live under?
A dome costs less up front but loses heat at more than twice the rate of a flush glass unit, around 3 W/m²K against 1.2. So a heated room under a dome feels colder and costs more to keep warm through a Suffolk winter. The saving at purchase is spent back on heating, which is why glass is the better long-run choice for any room you heat.
Does a dome drain better than flat glass?
On a poorly drained roof, yes, because it sheds by shape regardless of the fall around it. Flat glass is laid to a shallow fall and needs the roof to drain properly. On a roof that ponds and cannot be corrected, a dome is more forgiving, though correcting the fall and fitting glass is the better outcome where the roof allows it.
Can I replace a dome with flat glass in the same opening?
Usually yes, and it is a common job. The opening stays, the kerb is inspected and rebuilt if the timber has gone, and a flush glass unit is set on a covering dressed over the head. The main variable is the kerb condition, which is confirmed once the old dome is off. On a habitable room it is a change well worth making.
Get advice and a fixed price
Tell us the room, the opening size, whether it is a new opening or a replacement, and where the property sits. We will survey it, tell you honestly whether glass or a dome suits the space and the roof, specify the unit, and give you one written fixed price. Every installation carries a 10-year workmanship guarantee, and the installers have 25 years fitting rooflights behind them.
Request a fixed quote in Ipswich, Suffolk or north Essex, or read glass or polycarbonate for a flat rooflight if you are weighing the materials rather than the shapes.
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.