The short answer
A flat rooflight should have a toughened outer pane and a laminated inner pane, sealed as a double-glazed unit with a low-emissivity coating and a warm edge spacer. The laminate keeps any broken glass in place overhead. Solar control and an acoustic interlayer are added where the room needs them, such as a south-facing kitchen or a bedroom near a flight path.
Two panes, two different jobs
The standard flat rooflight unit is a sandwich. Each layer is there for a reason, and the two panes are not the same.
The outer pane faces the weather. It is toughened, which means heat-treated so that it is several times stronger than ordinary glass and can take hail, a dropped branch, wind load and a ladder leaned nearby. If toughened glass does break, it crumbles into small blunt pieces rather than long shards. Toughened glass on its own has one weakness overhead: the pieces fall.
The inner pane faces the room, so it is laminated. Two sheets of glass are bonded with a clear interlayer, and if the glass cracks the fragments stay stuck to that layer and the pane stays in the frame. Above a bed, a dining table or a bath, that is the pane that matters. Part K of the Building Regulations deals with safety glazing in doors and low panes, and good practice for overhead glazing is the reason the inner pane is laminated on a rooflight.
Between them sits a cavity, filled with argon in most units and closed by a spacer around the edge. The pair of panes, the cavity and the edge seal together are called the sealed unit, and the unit, not the pane alone, is what gives the rooflight its thermal and sound behaviour.
Some people ask why not two laminated panes or two toughened ones. Two toughened panes give no protection from falling glass. Two laminated panes are heavier and dearer, and rarely needed. The mixed pairing is the sound default, and it is why the frame on our flat units is engineered around it, as set out in the page on frameless flat rooflights.
Coatings that change the light and the heat
A coating is a thin metal or oxide layer on one glass surface. It is invisible in use and it changes what the glass does with heat.
- Low-emissivity coating: reflects heat back into the room in winter, so the inner surface stays warmer and the room loses less heat. It is standard on any unit meant to pass Part L.
- Solar control coating: blocks part of the sun's heat in summer. It gives the glass a slight tint, usually a cool blue or grey, and it cuts glare as well.
- Self-cleaning coating: helps rain wash off dirt on the outer surface. It helps on a flat rooflight, where dirt has nowhere to run to.
A south-facing extension that becomes hot in July is the classic case for solar control, and it can be combined with the other two. A north-facing room wants as much light and warmth as it can get, so it is best with the clearest glass. Our page on energy efficient rooflights explains how these choices add up across the whole roof.
Where the sound comes in
A flat pane of glass is a drum skin. The same rain that drums on a lantern lands on a rooflight, and the build of the glass decides how much reaches the room.
Two plain panes of equal thickness tend to ring in step. Two changes help. One is a different thickness for each pane, so they no longer vibrate together. The other is an acoustic interlayer in the laminated pane, a softer layer that damps the vibration instead of passing it on. Together with firm seals and a solid upstand, that is the glazing build we write into the quiet spec for bedrooms, home offices and nurseries. We describe it by specification, never by claiming a figure.
The outer pane can be thicker as well, since heavier glass moves less when it is struck. The price is weight, which affects the frame and the lift, so the thickness is chosen with the span in mind. For a bedroom, the full reasoning is in our page on quieter glazing for a flat roof rooflight over a bedroom.
Choosing for the room
The same flat rooflight can suit very different rooms, and the glass is where they part ways.
| Room | Leaning | Why |
|---|---|---|
| South kitchen | Solar control, self-cleaning | Summer heat and dirt |
| Loft bedroom | Acoustic laminate | Early aircraft and rain |
| Bathroom | Warm edge, obscure finish | Steam and privacy |
| North home office | Clearest low-e unit | Keeping light and warmth |
The bathroom case is covered in whether a flat rooflight suits a bathroom. Triple glazing is also an option, though the extra pane adds weight and cost, and a well-made double unit with the right laminate often does what a room needs.
The glass is one of the largest costs in the unit, so it shapes the quote, as explained in what affects the cost of a flat roof rooflight. We write the chosen build into the quiet spec so you can see the panes, the coating and the interlayer before the order is placed.
Glass questions, answered
Why not use polycarbonate instead of glass?
Polycarbonate is light and tough, and it scratches and yellows in time. It also carries rain noise more readily. Glass is the better choice where the room below is a living space.
Is toughened glass safe on its own?
It is strong, but overhead it is paired with a laminated inner pane so that nothing falls into the room if the outer pane breaks.
Can the glass in an existing rooflight be changed?
The sealed unit can often be replaced while the frame and upstand stay, if they are in good order. Otherwise the whole rooflight is renewed, which we assess at survey. See also misted and failed rooflight replacement.
Does glass choice affect the upstand?
The weight and thickness of the unit set what the upstand must carry and how it is finished. Our page on flat roof rooflight upstands goes into it.
For a glass build written to your room, get a quote. The whole service is on our flat roof rooflight installation page.