Flush glazed and domed flat rooflights compared

How each one is actually made

A flush unit is a piece of window construction laid horizontally. Two or three panes of glass are bonded to a spacer bar, the cavity is filled with argon, and the assembly is set into an aluminium frame with a polyamide thermal break running through it. The frame carries a drainage route, a gasket line and a fixing flange, and the whole thing is a single sealed item delivered to site.

A dome is a sheet of plastic heated and formed over a mould, then trimmed and fixed to a base. Where there are two or three skins they are spaced apart and joined at the edge, and the air between them does the insulating. The base is usually a moulded PVC upstand, sometimes with an insulated core, and the dome is bolted or clipped to it.

That difference in construction sits underneath every other difference on this page.

Acrylic, polycarbonate and what they do differently

Domes are made from one of two plastics and they behave differently enough that it is worth knowing which you are being offered.

Acrylic, sold as PMMA, is optically the better material. It stays clear for a long time in sunlight, it holds its light transmission, and it is stiff. It is also brittle. Hit it hard with something small and hard and it cracks rather than dents.

Polycarbonate is the tougher material by a wide margin, which is why multiwall polycarbonate turns up on rooflights that need impact resistance. Its weakness is ultraviolet. Untreated polycarbonate yellows and goes chalky within a few years outdoors, so any decent sheet carries a co-extruded ultraviolet-blocking layer on the weather face. That layer is the thing that determines how long the dome looks acceptable, and it is worth asking whether it is there and which face it is on.

The glass build-up in a flush unit

Overhead glazing has a rule that vertical glazing does not: whatever is above your head must not fall on you if it breaks. So the inner pane of a flush rooflight is laminated, and the outer pane is toughened.

A common build-up is 6mm toughened outside, a 16mm argon-filled cavity with a warm edge spacer, and 6.8mm laminated inside. The 6.8 means two 3mm panes bonded by a 0.8mm interlayer. Larger units move to 8mm or 10mm outer glass and a thicker laminate inside, because the pane has to carry wind uplift, snow and its own weight across a longer span.

Coatings are added on the cavity faces: a low emissivity coating as standard, and a solar control coating where the room faces south or the glass area is large. We cover the security side of the laminate on flat rooflight security and laminated glass.

The light each one lets through

Clear glass gives you a view. You see sky, cloud movement, the shape of a tree, weather arriving. The light has direction, so it makes hard-edged shadows that move across the room through the day, and the room reads as connected to outside.

An opal or textured dome gives you brightness without a view. The plastic scatters the light, so it arrives evenly and from everywhere, shadows go soft, and the ceiling glows rather than casting a bright shape on the floor. Some people prefer that. In a workshop, a store, a stairwell or a windowless internal room it is arguably the better light, because it does not produce glare on a screen or a bench.

Clear domes exist but the double skin doubles the reflections and the view through them is never crisp. If the view matters, that decision has already been made for you.

Insulation values side by side

This is the gap that has widened most over the last twenty years, and it is the reason so many domed units are now being taken off Oxfordshire extensions and replaced with glass.

Two cautions on reading those numbers. A rooflight U-value can be declared in the horizontal plane or converted to the vertical plane, and the same product gives two different figures depending on which convention the data sheet uses, so compare like with like. And a centre-pane figure is not a whole-unit figure. The frame is the weak part, and it is the whole-unit number that Building Control is interested in.

Rain noise, which nobody asks about until afterwards

A thin plastic skin stretched over an opening is close to a drum. Heavy rain on a single skin dome is loud, and hail on one is louder still. Under a kitchen or a utility that may not matter at all. Under a bedroom or a home office it matters a great deal, and it is the complaint we hear most often about older domed units.

Glass is heavier, stiffer and damped by the laminate interlayer, and rain on it registers as a soft hiss rather than a rattle. A laminated inner pane helps more than the thickness alone would suggest, because the plastic interlayer absorbs vibration that would otherwise pass straight through.

Adding skins to a dome helps a little. It does not close the gap.

Weight, and what the structure below carries

A double glazed flush unit weighs in the region of 30 to 40 kilograms per square metre, and a triple glazed one more. A 1200 by 1800 flush unit is therefore a two-person lift at best and a mechanical lift on anything larger.

A triple skin dome of the same size weighs a fraction of that, often under 10 kilograms in total. One person carries it up a ladder.

That difference reaches down into the job. Glass means the trimming joists around the opening are sized for a real point load at the kerb, it means access for getting the unit onto the roof has to be worked out before anything is ordered, and on a first floor or a rear extension with no side access it occasionally changes the specification on its own. We look at access at survey, not on the day.

How each one ages outdoors

Glass does not degrade. What fails on a glass unit is the edge seal, and when it goes the cavity loses its gas and mists internally. That is a long way out on a well-made unit with a properly drained frame, and it is a visible, unambiguous failure when it comes.

Plastic degrades. Even with an ultraviolet-protective layer, an acrylic dome slowly loses clarity, picks up a fine surface roughness that holds dirt, and takes on a warmer tone. Polycarbonate without protection does the same faster. Domes also craze around fixings where the plastic has been over-tightened, and crazing spreads.

The practical point is that a domed unit tends to look tired long before it stops keeping water out, whereas a glass unit tends to look fine right up until the seal goes. Those are two quite different relationships with the thing above your head.

Size range, and the limit of each

Standard domes are made in a set of tray sizes, commonly from 600 by 600 up to around 1200 by 1800, and beyond that you are into linked units or a made-to-measure moulding, which loses the price advantage that made a dome attractive.

Flush glazed units run further. A single pane to around 2000 by 3000 is ordinary, and larger is available with thicker glass and a heavier frame. Past that point you are into structurally framed glazing with intermediate bars, which is a different product and a different price.

Non-standard shapes work the other way round. A dome can be moulded to a circle or an oval readily. Curved and shaped glass is possible but expensive. Sizes are covered properly on flat rooflight standard sizes and made-to-measure.

The kerb each one wants

Both sit above the roof surface, and both want the same working minimum of 150mm from the finished covering to the top of the kerb. What they sit on differs.

A dome usually arrives with its own moulded PVC upstand, splayed outwards, which is bolted down to a timber kerb or occasionally straight to the deck. The splay is useful because it turns the membrane termination outward, but a thin uninsulated PVC upstand is a cold ring around the opening and it needs insulating behind.

A flush unit sits on a built timber kerb, insulated on its outer face in line with the roof insulation, with the membrane dressed up and over and terminated under the frame flange. That is the detail we describe in full on upstands, kerbs and warm roof build-ups. It is more work and it is the reason flush units behave well in cold weather.

Opening versions, and how they differ

Both families have opening variants and they open in different ways.

Domes typically open on a scissor or chain actuator that lifts the whole dome bodily off its base, giving a gap right round the perimeter. That gives good free area for ventilation and it is the basis of most smoke ventilation units. It is also fully exposed to weather when open.

Flush glass usually hinges on one side and lifts on an electric chain or spindle drive, opening to a few hundred millimetres at the far edge. Free area is smaller for the same footprint, the gear is heavier because the glass is heavier, and the seal line when closed is a proper compression gasket rather than a rubber lip. Rain sensors are standard on both and they close the unit automatically once the sensor is wet.

Which one costs more, and where the money goes

Glass costs more. That gap is real and there is no point pretending otherwise, but it is smaller than people expect on a small opening and larger than they expect on a big one, because glass price rises faster with area than moulded plastic does.

The unit is only part of it. A flush installation carries a built and insulated timber kerb, a heavier lift, more careful membrane termination and usually a splayed and finished internal reveal. A dome on a standard tray size on an existing upstand is a shorter day.

Against that, running cost runs the other way. A single skin dome over a heated room is losing heat at roughly four times the rate of a good glass unit of the same size. Our approach is the same for both: a range by email once we know the opening, then a fixed price after a survey.

Where a dome is still the right specification

Plenty of places, and we say so rather than upselling.

Over an unheated garage, store or workshop, where the U-value is close to irrelevant and what you want is daylight over a bench. On an outbuilding where nobody is going to look at it. Where the roof structure is light and old and would need work to carry glass. Where the opening is awkwardly shaped and a moulding solves it cheaply. Where a large free area of ventilation is the point, as with a smoke vent over a stairwell. And on a roof with genuinely difficult access, where the weight of glass turns a straightforward day into a crane hire.

In those cases a triple skin unit with a protected outer face is the sensible version of the choice, not a single skin.

Where only flush glazing will do

Over any heated habitable room where you care about the winter surface temperature. Over a bedroom, an office or anywhere rain noise carries. Where the neighbours or a planning officer will see the roof from above and a raised opal bubble reads badly. Where the room needs a view of sky rather than a bright ceiling. Where the glass has to resist attack, because a laminated pane and a plastic dome are not comparable there.

And in almost every new flat roof extension in and around Oxford, because the build-up is a warm roof, the room below is heated and lived in, and the whole point of the roof was to bring in daylight you can see through.

Swapping a dome for flush glass on an existing kerb

This is a common enquiry and the answer is a qualified yes.

Three things get measured first. The kerb height from the finished roof surface, because a great many kerbs built under old domes are 75mm or less and will not do. The kerb construction, because a thin PVC upstand designed for a 6 kilogram dome is not a base for a 90 kilogram glass unit. And the opening size, because dome tray sizes and glass sizes are set out differently and the aperture rarely lands on a standard glass dimension.

In practice the kerb is usually rebuilt in timber, insulated and brought to height, and the covering is dressed back into it. That is a day of work rather than an hour, and it is what makes the new unit last.

What we ask before recommending one

Five questions, and the answers usually decide it between them.

• Is the room below heated and lived in, or is it a store?
• Do you want to see out, or do you want the room bright?
• Will anyone be sleeping or working directly under it?
• What is the existing build-up, warm or cold, and what is the covering?
• How does a unit physically get onto that roof?

We would rather talk that through on the phone than send a specification blind, because the honest answer on a detached garage is different from the honest answer on a kitchen extension, and quoting glass for both would be lazy. Call 01865 704245 or email info@heritageskylightsoxford.co.uk with the opening size and what is underneath it, and you will get a range and a straight recommendation.