Standing under a flat rooflight, you rarely think about the glass as a build of layers. You notice the light, whether the room is warm, and perhaps the sound of rain. Each of those is decided by choices made before the unit is ordered, and each choice has a cost, a benefit and a trade-off.
The usual glazing choices for a flat rooflight follow. None is right for every house. The best glass for a north-facing utility room is not the best for a west-facing kitchen extension that takes the evening sun, and the survey is where we decide which suits yours.
What a sealed unit actually is
A sealed unit is two or three panes of glass held apart by a spacer bar, with the gap sealed and filled with a gas such as argon. The gas slows heat moving across the gap. A thin low-emissivity coating on one pane reflects heat back into the room in winter. The edge spacer matters too, and a warm-edge spacer reduces the cold line you can feel at the rim of the glass.
Because it is sealed, the unit should stay clear for many years. If the seal fails, moisture gets between the panes and the glass mists, which is the cue to replace the unit. Our misted and failed rooflights page explains what that means.
Double glazing
Double glazing is the standard for flat rooflights, and a well-made unit with a low-E coating and gas fill meets the Part L 2021 figure of 2.2 W/m²K for rooflights in existing homes. For a bathroom, a landing or a utility room, it is usually enough.
The weak point of double glazing is how a roof exposes it. Glass in a roof faces the open sky, so on a clear winter night it loses heat to it and the inner pane can run cold. In a humid room, such as a kitchen or bathroom, that can lead to condensation on the inner surface on the coldest mornings. Good ventilation and a warm-edge spacer both help.
Triple glazing
Triple glazing adds a third pane and a second gas-filled gap. The inner surface stays warmer, which cuts the chilly feeling under the glass and reduces condensation. It is also quieter in rain, as the extra layer and gap absorb more sound.
The trade-offs are weight and thickness. A triple unit is heavier, which matters for large rooflights, and it needs a frame deep enough to hold it. It costs more, and it may not be worth it for a small unit over a hallway. For a long extension used all day, or a sitting area directly beneath, it is worth considering.
| Warmth under the glass | Triple is the warmer of the two |
|---|---|
| Noise in rain | Triple is the quieter |
| Weight and frame depth | Triple is heavier and needs a deeper frame |
| Thermal standard | A good double unit meets the Part L 2021 rooflight figure |
Solar control glass
Some coatings let in most of the light while reflecting a good share of the sun's heat. This is solar control glass, and it makes a real difference in a room that faces south or west. Without it, a lightweight extension with a large rooflight can be uncomfortable in a warm afternoon.
It has a visible effect. The glass can look slightly tinted or reflective from outside, and the daylight inside is a touch cooler in tone. That is a fair exchange for the comfort in July. If your room is shaded by a tree or a taller neighbouring building, you may not need it.
It pairs well with blinds. A fitted blind made for the rooflight can cut the glare, and ordering it with the unit means it is made to the exact glass size. Our guide to flat rooflight sizes explains why the measurements matter for that.
Laminated glass and overhead safety
Glass overhead needs more thought than glass in a wall, because if it breaks, it falls into the room. Laminated glass has a clear plastic interlayer bonded between two sheets, so a broken pane holds together rather than dropping in pieces.
Nothing in Part K of the Building Regulations obliges a householder to have laminated glass in a rooflight. We choose a laminated inner pane all the same, because the room beneath is where people stand and eat. The outer pane can be toughened. It is a sensible choice, and you will see it written on the quote.
- Laminated inner pane: holds together if struck.
- Toughened outer pane: stronger against impact, breaks into small blunt pieces.
- Both together: our usual specification for overhead glass.
Tints, frosting and privacy
If the rooflight overlooks your neighbours from a loft, or sits in a bathroom ceiling, you might want some privacy. Obscure and frosted glass lets light in and blurs the view. For a flat rooflight over a ground-floor room the issue rarely arises, as a neighbour would need to be above it to look in.
Tints cut glare and reduce heat gain a little. They can feel heavy in a gloomy room, so we recommend seeing a sample against the light before choosing. A glass sample in your own room is better than a swatch in a showroom.
Self-cleaning and easy-clean coatings
A coating that cleans itself, applied to the outside face, breaks down dirt in daylight and lets rain sheet over the surface so it dries without streaks. It helps most on a roof that is hard to reach. It does not replace a wash, but it stretches the time between them, which is useful when the glass sits over an extension reached only by ladder.
Trees, birds and nearby roads shorten the benefit. We suggest a gentle rinse a few times a year with water and a soft brush, never a harsh abrasive, and a look at the surrounding gutters for blocked outlets that send water across the glass.
Condensation, and what the glass can and cannot do
Many people ask whether a better pane will stop misting on a winter morning. Glass helps, but moisture on the inner face is mostly about the air in the room. Warm, damp air from cooking, showers and drying clothes meets the coldest surface it can find, and in a roof that surface is often the glass.
A warm-edge spacer and a third pane keep the inner face warmer, so the air has to be damper before it condenses. Running an extractor, opening a window for a few minutes after a shower and keeping the heating from falling very low overnight all lower the risk further. Surface condensation that wipes away is a nuisance. Moisture trapped between the panes is a different thing, and it means the seal has failed.
Questions to put to any supplier about the glass
- Is the outer pane toughened and the inner pane laminated?
- Which spacer is used at the edge of the unit?
- What is the centre-pane U-value, and the whole-unit figure?
- Is the low-E coating on the correct face for a roof?
- How long is the maker's warranty on the sealed unit?
Whole unit figures and the frame
Glass makers like to quote the centre-pane figure, which is the best the glass can do away from its edges. What counts for a rooflight is the whole unit, frame and glass together, because heat leaves quickly through a poor frame or a cold edge. A pane with an excellent centre figure in a thin aluminium frame can end up worse than a plainer pane in a well-insulated one.
For that reason we look at the figure for the finished rooflight, set it against the 2.2 W/m²K that applies to rooflights in existing homes, and write both on the quote. The frame, the upstand it sits on and the glass all take part, and none of them can be judged in isolation.
Opening rooflights and their glass
Where the rooflight opens, for ventilation, the glazing is the same, but the frame is different, and so is the weight. Hand-opening units are simplest. Electric units are lifted by a motor and can close on their own when a sensor detects rain. If you are weighing an opening unit, our electric opening rooflights service explains the controls.
Opening is not just a comfort. A rooflight that vents a kitchen carries steam out before it condenses on the glass, and that helps the glazing as much as the insulation does.
Living with the glass in the years after
The glass you pick should still be the right one in ten years. Children grow up and a play room becomes a study. A planted garden fills out and shades the extension that once took full sun. A south-facing wall with a rooflight above it may be much hotter after a neighbour removes a tall hedge.
It is easier to choose a specification with some margin than to change a sealed unit later. Solar control that proves unnecessary costs little in daylight. Plain glass that proves too hot in July means a blind, or a replacement unit. Keep the handover paperwork and the maker's details somewhere you will find them, because the pane reference on the spacer bar is what a replacement is ordered from.
Glass and the noise of a roof
Rain on glass is louder than rain on a tiled roof, and some people mind. The three things that soften it are a thicker outer pane, a laminated layer, which damps vibration, and a wider cavity between panes. A kitchen or living room with a conversation under the glass is where rain noise is noticed, and a utility room that sees no one in a downpour can ignore it.
Asking for a quieter specification adds a small cost and a little weight, but it is easy to put in at the order stage and impossible to add after fitting. Ask at the survey and we will say what each option would change, in sound, in warmth and in price. If rain noise is your main worry, we can also bring a laminated sample so you can tap it and compare it with plain glass.
Choosing between them
A fair way to decide is to ask three questions. Which way does the room face? How is it used across the day? What does it sit under: a quiet bedroom corridor, or the dining table?
A north-facing utility room can have double glazing and no coating beyond the standard. A kitchen extension facing the garden to the south often benefits from solar control. A sitting area under a large unit may reward triple glazing. We write the specification on the quote pane by pane, so you can compare it with any other, and our flat rooflight cost page shows how each choice moves the price.
If two options seem close, look at how long you sit under the glass. A room used for an hour in the morning rarely justifies the dearest specification, while a kitchen table under the unit through every meal usually does.
The glass is fitted on an insulated upstand, and the work around it is done with the same care, as set out on our flat roof rooflights page. Book the free survey with the form below and we will bring glass samples to hold against the light in your own room.