Flat roofs and rooflights suit each other. A kitchen extension with a wall of glass at one end and a dark middle is the classic case: one well-placed unit in the ceiling can light the part of the room the windows cannot reach. The difficulty is that a flat roof sheds water slowly, so every opening in it is a place where water can wait and then find a way in.
This guide covers the parts that matter before you buy: how the rooflight is raised above the waterproofing, how its glazing is made, whether it opens, and what the rules say. If your existing unit is already dripping, our guide to a leaking flat skylight deals with that, and the skylights hub shows the wider range of roof windows and rooflights we fit.
A rooflight is not a roof window
People use the words skylight, roof window and rooflight as if they meant one thing, but the trade separates them, and the difference shapes the rules. A roof window is made for a pitched roof. It lies flush with the slope, hinges or pivots, and is flashed into the tiles or slates around it. A rooflight is the broader term for glazing set into a roof, and on a flat roof it almost always means a unit fixed on top of a raised frame, with the glass sloping gently or forming a low dome.
That frame matters. On a pitched roof the surrounding covering overlaps the window frame, so gravity does most of the sealing. On a flat roof water can pool, so the glazing has to stand clear of the waterproofing on a kerb and the membrane must run up the kerb and be finished at the top. The unit then reads as a small raised box, not a hole.
Why the distinction affects the paperwork
Part L of the Building Regulations draws a line between them. Fit a roof window in an existing house and it needs a U-value of 1.4 W/m²K or better, or a Window Energy Rating of Band B, while a rooflight may be 2.2 W/m²K. Flat-roof units normally land in the rooflight group, so check which heading a brochure figure sits under before you compare two products.
Kerbs, upstands and where the water goes
The upstand is the vertical lip that raises the glazing above the roof surface. It can be a factory-made kerb that comes with the rooflight, or a timber upstand built on site around the opening. Either way, the waterproof membrane is turned up the face of it, and a cover or cap finishes the edge so water running off the glass cannot get behind the membrane.
Approved Document L says a rooflight and kerb supplied as one assembly is judged as a whole, while a timber upstand made on site is capped at a U-value of 0.35 W/m²K. That is the building control view of a point every roofer already knows: an uninsulated timber box in the middle of an insulated roof is a cold bridge. It chills the plasterboard around the opening, and the result is condensation staining on the lining within a winter or two.
Fall around the rooflight
Water that runs off the roof has to get past the kerb. If the deck is flat or dips towards the unit, the upstand dams the flow and a pool forms on the high side. A good installer builds a small cricket, a sloped shape that sends water around each side, on the up-slope edge of the kerb. For the wider drainage picture, see our guide to rooflight upstands.
Glass or polycarbonate: choosing the glazing
Most domestic flat rooflights are glazed in one of two ways. Sealed double or triple glazed units are the same family of product as a window, with a low-emissivity coating, a gas-filled gap and a laminated inner pane for safety. Multi-wall polycarbonate sheets are lighter, cheaper and more forgiving of an uneven opening, and they are common on older garages and lean-tos.
For a lived-in room, glass wins. It is quieter in rain, holds its clarity and is held to the thermal limit in Part L. Polycarbonate is a fair choice over a store, workshop or porch where heat loss and noise matter less. It scratches more readily and can yellow with age, and when it is replaced it should be changed with the frame in mind, not just the sheet.
Safety glass and what sits overhead
Anything that people stand beneath should have an inner pane that holds together if it breaks. Laminated glass does that, because the interlayer holds fragments in place. Toughened glass breaks into small pieces, which is why it is rarely used alone overhead. Ask what the inner pane is, not just what the outer one is.
Coatings and heat
South-facing glass over a small kitchen can overheat. A solar control coating cuts the heat reaching the room, and many people also add a blind. Our guide to skylight overheating goes through the options.
Fixed, opening and walk-on units
A fixed rooflight is the simplest and has the fewest places to leak. Because nothing moves, there are no hinges or seals to wear, and the thermal performance is usually the best of the range. Its limit is ventilation: a fixed unit gives light only.
An opening rooflight lifts on a hinge or a pair of arms. Opening ones help in a kitchen or bathroom, where steam and cooking smells collect at ceiling height, and a motor lets you reach a unit out of arm's length. They cost more, and the working parts need an occasional look. We cover motors in our guide to electric roof windows.
Walk-on rooflights
A walk-on unit is flat glass set level with a terrace or a roof deck, so that light drops into a room below while people stand on it. It needs a proper structural specification, since the glass is bearing load, and it is a different product from a standard rooflight. Do not step on any ordinary rooflight, however solid it looks. Even a unit designed for access carries a rating, and the maker's data sheet gives the figure.
Roof lanterns and rooflights are not the same
A lantern is a larger, taller glazed structure with its own ridge, usually over a bigger opening. If you have in mind a feature over an open-plan kitchen, our guide to roof lanterns is the better starting point.
Planning and Building Regulations
Most flat-roof rooflights are small enough that planning permission is not the first concern, but it is still worth a check. Class C of the General Permitted Development Order covers roof alterations to a house, and it carries two limits: the alteration may not stick out more than 0.15 metres from the slope it sits on, and it may not rise higher than the top of the old roof. A rooflight on a tall factory kerb can break the first limit. Ask Swindon Borough Council's planning team before ordering, and ask sooner again if the house has a listing or stands in a conservation area.
Class C does not stop at a conservation area or National Landscape boundary, but an Article 4 direction can remove the right, and listed buildings need listed building consent for anything that alters their character. Our guide to planning rules for roof windows goes into the detail.
Building Regulations on a flat roof
Fitting a rooflight is building work. The Planning Portal gives a test for simple re-covering: less than 25 per cent of the total building envelope and less than 50 per cent of the roof affected. A new opening is a different matter, since it means trimming the deck and joists. Should the job be notifiable, we go through what consent is needed and handle the notification. The thermal limits above apply to whichever product goes in, and our page on roof window building regulations sets the rest out.
Cutting into an existing flat roof
Fitting a rooflight into a roof that is already there is a small structural job. The deck is cut, the joists either side of the opening are doubled up with a trimmer at each end to carry the load, and the opening is boxed with the upstand. The waterproofing then goes on around it. The membrane, whether felt, EPDM or fibreglass, has to be joined to the existing covering, and that join is the part to ask about.
Where the flat roof covering is already ageing, the sensible time to add a rooflight is while the covering is renewed, not before. Cutting a hole in a failing roof and patching around it leaves two ageing junctions on one deck. A new covering and a new rooflight installed together can be sealed as one piece of work, and the guarantee on the work is easier to follow. For more on how coverings compare, see our flat roof materials guide.
Ceiling light wells
On a deep flat roof the opening in the ceiling below is rarely directly under the rooflight. The space between is a light well, lined and angled to spread light into the room. Wider at the bottom, it gives a softer spread. A well built straight down is simpler, but it throws a hard patch of light on the floor.
The written quote arrives after a survey, with photos of your own roof, and the work carries a 10-year workmanship guarantee in writing. On the visit we check the deck, the fall, the existing covering, and how the opening will be trimmed, because those decide the job more than the unit itself.
Conservation areas and older properties
Older houses in Swindon's conservation areas, and cottages around the villages, often call for a particular look: slim frames, a flat profile and nothing that stands proud of the roof line. Some manufacturers make rooflights designed for these roofs, with a low kerb and a frame that follows the pitch, and our guide to conservation rooflights explains what to look for.
Flat roofs on these buildings are usually over extensions, dormer cheeks or old outbuildings and rarely face the street. That helps, but the planning rules look at the whole building, not just the face you see from the road. It is worth asking the council's conservation officer before you commit. The 27 conservation areas on Swindon Borough Council's list include Swindon Old Town, the railway area, Wroughton and Highworth, and Article 4 directions apply in the railway village and Bishopstone.
One useful consequence of a flat roof in these settings is that a rooflight can sit well back from the front elevation, where it is hardly visible. Where it must be seen, a flat-glass, low-profile unit usually sits more quietly than a domed one. Our enquiry form is the quickest way to tell us about the building, and we will say what to ask the council.