Flush glazed and domed flat rooflights compared

What each product physically is

A flush glazed unit is a double or triple glazed sealed unit in an aluminium frame, glazed at a fall of typically three to five degrees built into the frame itself so the glass sheds water even where the roof around it is close to level. The frame is thermally broken, the outer pane is toughened and the inner is laminated.

A dome is thermoformed acrylic or polycarbonate, one skin, two skins or three, blown or pressed into a curve. There is no sealed unit and there is no glass. The skins are separated by an air gap and the whole shell sits on a kerb, which on older installations is a moulded plastic or glass reinforced kerb rather than a built timber one.

How each one sheds water

The curve is the dome’s argument. Water arriving anywhere on a dome runs off in every direction by gravity, and there is no horizontal surface for it to sit on. Dust and pollen leave with it. That is genuinely useful on a roof with a poor fall.

A flush unit relies on its built-in slope, which is why the fall in the frame matters and why the direction of that fall is set at installation rather than left to chance. Where the roof itself ponds, the water sitting against the kerb is the problem rather than the glass, and that is a roof design question covered on the main flat roof page.

Light quality through plastic against glass

This is the difference people feel without being able to name it.

An opal or textured dome diffuses light. You get brightness without a beam, no hard-edged rectangle of sun tracking across the floor, and no glare at a desk. What you do not get is any sense of weather. The room is lit but it is not connected to outside.

Clear glass does the opposite. Sunlight arrives as a shape, moves through the day, and the room reads as being under sky. In a kitchen or a living space that is usually the point of the exercise. In a north facing utility or a hallway, diffusion may be the better outcome, and a dome delivers it without a blind.

Whether you want a view of the sky

Clear domes exist, but the curve distorts. Looking up through a curved acrylic shell gives you a bent horizon and a soft edge to clouds, which is fine for daylight and poor for looking at anything.

Flat glass gives a true view. Under a rear extension in Chelmsford that means rain on the surface, the shape of the trees on the boundary, and at night an actual sky. It is the reason flush glazing has taken most of the domestic market even where a dome would perform adequately.

Solar gain, and the one place a dome wins

A diffusing dome scatters incoming light before it becomes a hot spot, which softens the peak. It does not, in itself, reject much solar energy, because acrylic passes a high proportion of it.

Glass can do better, but only if it is specified to. A solar control coating on a flush unit brings the g-value down to around 0.3 or 0.4 against roughly 0.6 for clear, and that is a specification decision at order rather than something added afterwards. Over a south facing flat roof in Essex, on a unit of any size, it is close to compulsory. The wider question is covered under energy efficient rooflights.

Rain noise on each surface

Domes are louder, and the reason is mass. A thin acrylic skin has very little of it, so a drop of water sets the whole shell vibrating and the curve acts as a drumhead. Under a bedroom, a single skin dome in a heavy shower is genuinely disruptive.

Glass is heavier and damps more. A laminated inner pane adds an interlayer that absorbs vibration rather than transmitting it, and an asymmetric make-up, where the two panes differ in thickness, breaks up the resonance instead of reinforcing it. A double skin dome sits between the two, because the inner skin is decoupled from the outer.

Visual impact on a rear extension roof

From ground level in a garden you rarely see a flush rooflight at all. That is the intent. The roof is a flat plane and the glass is part of it.

A dome stands 150mm to 300mm above the roof, has a visible kerb below it, and reads as a piece of equipment. On a utilitarian roof, a garage or an outbuilding, nobody minds. On a rear extension overlooked by the neighbouring first floor windows, and in Essex that is most rear extensions, the difference is obvious from every upstairs room on the terrace.

How each ages over twenty years

Different materials fail differently, and knowing which failure you are looking at tells you what you have.

Acrylic yellows. Ultraviolet works on the polymer, the surface crazes into fine cracks, and light transmission drops away steadily. A dome that was clear in 1998 is amber and cloudy now, and the crazing traps dirt so it cleans up worse each year. Polycarbonate resists impact better and yellows too, though usually more slowly where it carries a UV-stable coating.

Glass does not degrade. What ages on a flush unit is the perimeter seal of the sealed unit, and when that goes the failure is unmistakable: misting between the panes that no cleaning will touch. Twenty to twenty five years is a normal working life, and at that point the unit has reached its end and wants replacing.

Impact resistance and what sits overhead

Both can be specified to be safe overhead, but they get there differently.

Polycarbonate is extremely tough and will take a dropped roof tile or a branch off a garden tree without breaking. Acrylic is more brittle and can crack. Glass overhead is always laminated on the inner pane, which means that if it is broken the interlayer holds the fragments in the frame rather than dropping them into the room. That is the requirement that matters, and it is why a flush unit specified for a domestic roof is not a straight sheet of toughened glass.

Debris, moss and what collects on each

On a flat roof with trees nearby, and Essex gardens are full of them, leaves and moss accumulate around anything standing proud.

A dome kerb collects a ring of debris at its base, which holds moisture against the junction and slowly closes off the drainage path. A flush unit has less of a ledge but the frame perimeter still catches material, particularly on the low side where water leaves. Neither is difficult to keep clear. Both want ten minutes each autumn, and the roofs where problems appear are almost always the roofs nobody has looked at since the scaffold came down.

Condensation behaviour on the room side

The inner surface temperature decides this, and it is where the two products separate most sharply in a real house.

A single skin dome has an inner surface that tracks the outside temperature closely, so on a January morning in Chelmsford it will be running with water in any room that produces moisture. A double skin dome improves it, though the skins are thin and the air gap between them is unsealed on many designs, which limits how far it goes. A modern flush glazed unit with a low emissivity coating, argon fill and a warm edge spacer holds its inner pane within a few degrees of room temperature, and stays clear through most of the winter with normal ventilation.

The difference shows first over a kitchen, a utility or a bathroom, where the vapour load is highest and the ceiling is the surface it finds.

Buying price against working life

A dome is cheaper to buy and that gap is real, particularly at small sizes where a single skin unit is a fraction of a glazed one.

The gap narrows once the dome is specified properly. A triple skin unit on an insulated kerb with a decent thermal break is not a cheap product. And it closes further when the working life is factored in, because a dome replaced twice in the period a glass unit lasts once has cost more in labour alone. The installation cost is broadly similar for both: the kerb, the covering junction and the internal reveal are the same work whichever lands on top.

Where flush glazing is the right answer

For a habitable room under a flat roof, flush glazing is usually the better outcome and the reasons stack.

It hits Part L comfortably, it gives a real view, it is quieter under rain, it does not discolour, and it reads as architecture rather than equipment. On a rear kitchen extension, a side return infill, or a bedroom over a flat roof, that combination is difficult to argue with. Where the room needs air as well as light, the same frame is available as an opening unit, manual or electric.

Opening versions of each

Both are made to open, and the mechanisms differ enough to matter.

A hinged flush unit lifts on a chain or spindle actuator along one edge, giving a modest free area and a clean look. A dome typically opens on a scissor mechanism or lifts vertically on a base unit, which can deliver a larger free area for the same footprint and is why they persist where ventilation is the driver rather than daylight. Manual poles work to about two and a half metres of reach; beyond that an electric or solar actuator is far less irritating, and the cable is trivial to run at first fix.

Replacing an old dome with flush glass

This is one of the most common jobs on Essex flat roofs, and it usually goes well because the hard part is already there.

The existing kerb is inspected first, since a timber kerb under a seventies dome may be sound or may be soft at the corners. Where it stands up, the height is checked against the covering and the finished roof surface, the size is measured corner to corner and diagonally, and a unit is ordered to suit. Where the old kerb is imperial, a made to measure unit avoids re-forming it. The room below usually changes character on the same day, because a clear glazed unit of the same size delivers a great deal more usable light than a yellowed double skin dome that has been quietly getting darker since the Blair government.