A roof window for a flat or low-pitch roof is a different product from the pitched-roof window most people picture. It is a flat-roof rooflight: a base unit that sits on a raised kerb, with a curved or flat glass top cover over it, designed to shed water at pitches below the point where an ordinary roof window works. You cannot lay a standard pitched-roof VELUX flat, because it relies on gravity and a covering lap to weather, and neither exists on a flat roof. The kerb lifts the glass above standing water, the base unit weathers the upstand, and the falls in the flat roof carry water away around it. This page explains the flat-roof range, how it differs from a pitched unit, and what the installation involves.
The demand for this is steady across Ipswich and Suffolk because flat-roofed rear extensions are everywhere. A single-storey kitchen extension, a rear utility, a flat-roofed dormer cheek: all of them want daylight from above and none of them can take a pitched-roof window. The flat-roof rooflight is the answer, and it is worth understanding as its own thing rather than assuming it is a normal roof window laid down.
Why a flat roof needs a different product
A pitched-roof window weathers by gravity. It sits in the plane of a sloping roof, the covering laps over its flashing skirt, and water runs down and off. Remove the slope and that whole system stops working. Water no longer runs off on its own, it sits, and a window designed to be laid at 15 degrees or more has no way to shed it.
The flat-roof rooflight solves this differently. It is built up off the roof surface on a kerb, which is a raised upstand that lifts the glass above the level any standing or slow-moving water can reach. A base unit weathers the join between the kerb and the flat-roof covering. The glass top cover, curved or flat, sits over the base and sheds rain to its edges. The roof itself is laid to falls so that water is directed away from the kerb rather than towards it. Every part of that is different from a pitched-roof window, which is why the two are separate product families and not interchangeable.
| Feature | Pitched-roof window | Flat-roof rooflight |
|---|---|---|
| Mounting | Set in the roof plane between rafters | Raised on a kerb or upstand above the roof surface |
| Weathering | Flashing skirt lapped by the covering | Base unit weathers the upstand to the roof membrane |
| Glass | Flat sealed unit in the sash plane | Curved or flat glass top cover over a base unit |
| Drainage | Gravity down the roof slope | Glass sheds to edges; roof laid to falls carries water away |
| Minimum pitch | Typically 15 degrees, some to 20 | Designed for flat and low-pitch roofs, often from 0 to 15 degrees on falls |
| Opening options | Centre pivot, top hung, fixed | Fixed, or electric or manual venting on the base unit |
The kerb, the part that does the work
The kerb, or upstand, is the raised frame the rooflight sits on, and it is the single most important part of a flat-roof installation. Its job is to lift the glass clear of the roof surface so that water, leaves and slow-draining pooling cannot reach the glazing seal. Get the kerb height and the weathering of it right and the rooflight is dry for its whole life. Get it wrong and no amount of glass quality will save it.
A kerb can be a builder’s timber upstand formed as part of the flat roof, or a manufacturer’s insulated base kerb supplied with the rooflight. Either way it has to be high enough to sit above standing water, properly insulated to avoid a cold bridge and condensation around the opening, and weathered so the flat-roof covering, whether that is single-ply membrane, GRP or felt, dresses up and over it and terminates cleanly. The kerb build itself is a piece of flat-roofing work as much as a window job, and where it forms part of a wider flat-roof or GRP scheme we cover that on our flat-roof skylight installation page.
The kerb is not an afterthought. On a new extension the kerb should be designed in with the flat roof, at the right height, insulated, and set out so the falls carry water away from it. Retro-fitting a rooflight into an existing flat roof means forming a kerb into a roof that was not planned for one, which is more involved and is where the weathering has to be got exactly right. We survey the roof build-up before we quote so the kerb detail suits what is actually there.
Curved or flat glass top cover
Flat-roof rooflights come with two styles of top cover, and the choice affects both the look and the way the unit sheds water.
- Curved glass. A domed or gently curved top cover. The curve helps rain run off to the edges and sheds debris, and it gives a soft external profile that suits a contemporary extension. It sits slightly proud, so it reads as a feature from the garden.
- Flat glass. A flat top cover for a cleaner, more minimal external line, popular where the extension is designed to look sharp and modern. Flat glass relies more on the fall built into the unit and the kerb to move water, so the detailing has to be right, but the finished look is flush and quiet.
Both are sealed double or triple-glazed units, and both can be specified fixed for pure daylight or as a venting unit for ventilation. On a kitchen or bathroom extension a venting rooflight earns its place, because it lets cooking or shower moisture out through the highest point of the room. Fixed units are simpler, cheaper and fine where ventilation comes from elsewhere.
Drainage and falls
A flat roof is never truly flat. It is laid to a minimum fall, typically around 1 in 40 to 1 in 80, so that water is directed to an outlet rather than sitting in ponds. Around a rooflight the falls have to work with the kerb, not against it. Water should be shed away from the upstand, so the rooflight is not sitting at a low point where water collects against the glazing seal. On a well-designed roof the rooflight sits on a high part of the falls and the water runs away around it to the outlets.
This is why the roof build-up matters as much as the window. If a rooflight is dropped into a flat roof that falls towards it, water will pool against the kerb and eventually find the seal, regardless of how good the unit is. Part of the survey is checking the falls and, on a retro-fit, confirming they suit the position, or adjusting the position to suit them. On a new extension the falls and the rooflight are set out together from the start.
Where a rooflight is not the answer, and a lantern is
A flat-roof rooflight is a single glazed cover on a kerb. Where you want a bigger statement over a flat roof, a full roof lantern is a different structure again: a multi-facet glazed roof, usually pitched glass panels meeting at a ridge, sitting on a larger kerb. A lantern floods a room with light and becomes an architectural feature over a kitchen island. It is more glass, more structure and more cost than a rooflight, and it suits a larger extension. If that is the scale you are thinking of, our roof lantern installation page covers it. For a modest extension a single flat-roof rooflight is usually the right and cheaper answer.
What flat-roof installation involves
The sequence differs from a pitched-roof job because the kerb and the flat-roof covering are central to it. In outline: survey the roof build-up and the falls, confirm the kerb detail and height, form or set the kerb, weather the flat-roof covering up and over the kerb, insulate the upstand to close the cold bridge, set and fix the base unit, fit the glass top cover, then make good the internal reveal. The general principles of setting the unit square and closing the perimeter cold bridge carry over from our VELUX installation process page, but the weathering is flat-roof work rather than covering-lap work. Pricing follows the same fixed-quote-after-survey model as every other job, set out on the VELUX window installation cost page, with the kerb build and roof-covering interface priced as part of the whole.
Flat-roof rooflight spec, at a glance
- Base: insulated kerb or timber upstand, high enough to clear standing water.
- Top cover: curved for shedding and a soft profile, or flat for a minimal line.
- Glazing: sealed double or triple unit, fixed or venting.
- Weathering: flat-roof covering dressed up and over the kerb and terminated cleanly.
- Falls: roof laid so water sheds away from the kerb to the outlets.
- Insulation: upstand insulated to avoid a cold bridge and condensation round the opening.
- Coastal: 316L stainless fixings and upgraded terminations within a mile of open sea.
Common questions about flat-roof rooflights
Can I use a normal roof window on my flat-roofed extension?
No. A standard pitched-roof window weathers by sitting in a slope with the covering lapping over its flashing, and neither the slope nor the covering lap exists on a flat roof. Laid flat it would pond water against the glazing and leak. A flat-roof roof needs a flat-roof rooflight, which sits on a raised kerb with a curved or flat glass top cover and is designed to shed water at low or zero pitch. They are different product families for different roofs.
What is the kerb and why does it matter so much?
The kerb, or upstand, is the raised frame the rooflight sits on. It lifts the glass clear of the roof surface so standing water and debris cannot reach the seal, and the flat-roof covering is dressed up and over it to weather the join. It also has to be insulated to stop a cold bridge and condensation around the opening. The kerb does the weathering work that a flashing skirt does on a pitched roof, which is why we survey the roof build-up before quoting.
Curved or flat glass, which should I choose?
Curved glass sheds rain and debris well and gives a soft domed profile that reads as a feature from the garden. Flat glass gives a cleaner, flush, minimal line that suits a sharply designed modern extension, but it relies more on the kerb and the built-in fall to move water, so the detailing has to be right. Both are sealed units available fixed or venting. It is mostly a look decision, and we talk it through at survey against how the extension is designed.
Do I need a rooflight or a roof lantern?
A flat-roof rooflight is a single glazed cover on a kerb and suits a modest extension at a sensible cost. A roof lantern is a larger multi-facet glazed structure, pitched panels meeting at a ridge on a bigger kerb, and it becomes an architectural feature over a large kitchen. If you want maximum light and a statement over a big island, look at a lantern. For daylight into an ordinary single-storey extension, a rooflight is the right and cheaper answer.
Get a fixed price for a flat-roof rooflight
Send a photograph of the flat roof, one of the room from inside showing where you want the light, and a note of the roof covering if you know it, single-ply membrane, GRP or felt. That gives us enough to talk through kerb, glass style and roughly what the job costs. Then we survey the roof build-up and the falls, confirm the kerb detail, and give you one written price with the kerb and covering interface included, not bolted on afterwards. Every installation carries a 10-year workmanship guarantee behind installers with 25 years fitting roof windows and rooflights.
Request a fixed quote for a flat-roof rooflight, or return to our VELUX window installation hub for the full range.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.