What glass goes in a flat rooflight?

The pane facing the sky takes everything the weather throws at it: hail, windblown twigs, the odd dropped slate from a neighbour's chimney work, and years of sun. For that reason it is toughened, which means heat-treated so that it is several times stronger than ordinary annealed glass and, if it does break, crumbles into small blunt pieces rather than long shards.

Because a flat rooflight sits at a very shallow angle, dirt and water do not run off it as quickly as they would from a steep roof window. A self-cleaning coating on the outer face helps here. It breaks down organic dirt in daylight and lets rain sheet across the surface rather than bead. It does not make the glass maintenance-free, but it keeps a flat unit looking clear for longer between washes.

The pane on the room side is usually laminated: two thin sheets of glass bonded either side of a clear interlayer. If it cracks, the pieces stay stuck to the interlayer instead of dropping into the room.

It is worth being clear about where this comes from. Part K of the Building Regulations deals with glazing in doors, the panels beside them and low-level glass, and it does not require laminated glass in rooflights. A laminated inner pane overhead is good practice and industry guidance, and it is what we would put over a kitchen island or a hallway where people stand beneath it. Many manufacturers fit it as standard for that reason.

Between the two panes sits the part that does the insulating. Three details matter.

Low-emissivity coating

A thin metallic layer on one of the inner faces that reflects heat back into the room in winter, while still letting daylight through. Without it, a rooflight loses heat quickly, because warm air rises and the glass above is the coldest surface in the room.

Gas fill

Argon is the common choice for the cavity. It conducts heat more slowly than air, so the unit insulates better at the same thickness.

Warm-edge spacer

The strip that holds the panes apart and seals the cavity. A metal spacer conducts cold around the rim of the unit, and a warm-edge one made from a lower-conducting material reduces that, which helps keep the edge of the glass free of condensation.

New and replacement rooflights must meet the limiting U-values in Approved Document L, Volume 1 (2021 edition), so a reputable unit will carry a declared U-value that you can check against that standard. Ask for it on the quote.

For most flat rooflights over a house, double glazing with a good coating and gas fill meets the regulations and does the job. Triple glazing adds a third pane and a second cavity. It improves the insulation, but it also adds weight and thickness, so it affects the frame and the upstand it sits on. Our guide to double or triple glazed rooflights goes through when the extra pane is worth paying for.

Other choices depend on the room:

• Solar control glass has a coating that reduces the heat of direct sun, useful on a south-facing extension with a lot of glass.
• Acoustic laminated glass uses a thicker or specially formulated interlayer, and can be chosen for a flat roof under a flight path or beside a busy road.
• Walk-on glass is a different product altogether, made from several laminated layers and specified to structural loading guidance, so it is never a simple upgrade to an ordinary unit.
• Tinted or fritted glass can take the edge off glare, but it changes the colour of the daylight in the room.

Some very small domed flat rooflights use acrylic or polycarbonate instead of glass. That is a separate product with its own look and limits, and the glazed units covered on this page are the ones you meet in framed and frameless flat rooflights on modern extensions.

Heat matters on the underside too. A flat roof over an extension has a lantern or rooflight sitting on a kerb, and the cold face of the glass is the first place condensation shows on a damp winter morning. A better spacer and coating keep that pane warmer, which protects the paint on the lining below it.

The glass is what you see from above, but the edge of the opening is what you see from inside the room. A deeper upstand and a thicker glazed unit change the depth of the lining below it. Where the lining is deep, it can be splayed to open up the ceiling; where the roof build is thin, a clean straight lining with a sharp bead is usually better.

Every Brightview quote comes with a finish schedule that records the glazing specification next to the lining and finish, so the glass, the roof build and the ceiling are all described on one page before work starts.

A sealed unit that has lost its seal shows as misting between the panes. Replacing only the sealed glass unit in an existing frame is not notifiable to Building Control, whereas replacing the whole rooflight, frame and glass, is. The frame has to be in sound condition and a matching unit has to be available, so a survey comes first. Our guide to replacing just the glass in a rooflight covers how that decision is made.

If you are comparing quotes, ask each one to name the outer pane (toughened, any coating), the inner pane (laminated), the cavity gas, the spacer and the declared U-value. Those five lines show quickly if two quotes describe the same thing. A price gap often hides a difference in one of them.

For the wider picture on sizes, upstands and roof types, see our flat roof rooflights page, or read how big a flat rooflight can be before choosing glass for it. When you are ready, ask for a quote and we will survey the roof and ceiling before we recommend a specification.