A flat roof skylight is a different product from a roof window. The roof window is built for the slope of a pitched roof, lying level with the tiles around it. A rooflight is made to sit above a flat or near-flat roof, usually on a raised frame called a kerb, and it handles rain by standing proud of the surface. That one difference explains most of the choices that follow: the shape, the way it opens, the glass, and how it is sealed to the roof.
This guide is about product choice. It runs through the main types, what the energy standard asks of a rooflight, how a kerb is built and joined to the waterproofing, and what to weigh before ordering. The height of the kerb is a topic of its own, covered in our answer on rooflight upstands and kerbs. For everything else about windows in the roof, see our roof windows and skylights page.
The main types of flat roof rooflight
Flat roof rooflights fall into a handful of families, and most products are a variation on one of them.
Fixed rooflights
A fixed unit is a sealed pane, or a sealed double or triple glazed panel, in a frame. It does not open, so it has no moving parts to fail and no gaps for rain to find. It is the simplest and usually the least expensive way to bring daylight into a kitchen extension, a hallway or a bathroom. Ventilation then has to come from elsewhere.
Opening rooflights
An opening unit has a hinged sash that lifts on a motor, a chain or a pole, and some are fitted with rain sensors that close the sash when a shower starts. They let hot air out of a room at the high point, which suits a south-facing extension. The moving parts and the extra seals need occasional attention.
Walk-on rooflights
A walk-on rooflight has thick laminated glass, usually with a non-slip finish, and is made so that a person can cross it, for example to reach a terrace or a plant area. It is chosen for a roof that doubles as a floor, such as a balcony or roof terrace. The maker's published load rating should be checked for the exact product before it goes anywhere people will step.
Lanterns and pyramids
A lantern is a raised glazed roof of several panes, set on a kerb. A pyramid is a lantern with four sloping faces meeting in a point. Both bring light from several directions and add height to the room below. They are the most visible of all the options, and they look best where the proportions suit the building.
Modular and ridgelight systems
Long, narrow units fixed end to end can run along a ridge or across a wide extension. They are made as a system, with their own flashings and end details, and are usually ordered to size.
What the energy standard asks of a rooflight
Approved Document L (2021 edition) gives rooflights their own row in Table 4.2. Fitting a new or replacement rooflight to an existing house means meeting 2.2 W/m²K, a looser figure than the 1.4 W/m²K that applies to a roof window. The reason is the geometry: a rooflight stands above the roof on a kerb, so the glazed area and the upstand together form a larger surface.
How the figure is measured
The measuring basis is the outer developed surface area, which means the whole surface seen from outside, including the sloping or vertical sides of the glazing, and not the flat area of the opening. A lantern with tall sides has a larger developed area than the hole it covers, so the unit has to be better insulated than a simple look at the opening suggests.
The kerb counts too
A kerb supplied as part of a single rooflight and kerb assembly is covered by the same 2.2 limit. A kerb built on site, from timber or plywood and insulated by the roofer, has its own, tighter limit: a maximum U-value of 0.35 W/m²K. That is why a factory-made kerb often makes sense, because the manufacturer tests the whole assembly.
What to ask for
Comparing quotes, ask for the U-value of the exact product, the glass specification and whether the kerb is included in the figure. A rooflight that comes with a certified U-value is easier to show to building control than a custom product where the numbers have to be worked out afterwards. Our guide to roof window U-values explains how these measures work for the pitched case.
Kerbs: what the rooflight sits on
A kerb is a short upstand fixed to the roof deck, onto which the rooflight frame is set. Its job is to lift the joint between glass and roof above the level of standing water and to give the waterproofing something to be turned up and fixed to. Kerbs come in two kinds.
Factory kerbs
Factory kerbs are made of GRP, aluminium or insulated composite, often in a single piece with the rooflight. They arrive at a known height with a known U-value, and the maker specifies how the roof membrane joins to them. They are quick to fix, and their dimensions are controlled, which helps with sealing.
Site-built kerbs
A site-built kerb is framed in timber on the roof deck, lined, insulated and clad. It can be built to any shape, which suits an odd-sized opening or a lantern that has to match an existing feature. The cost is more labour on the roof and more risk that the insulation is not continuous. The limit of 0.35 for a site-built upstand is a reminder that the insulation within it matters.
Why the shape of the kerb matters
A rooflight on a flat roof is only as watertight as the join where the roof covering meets the kerb. The membrane climbs the kerb's outer face and ends under a capping or the rooflight's own flange. Falls on the roof around the kerb also matter, because water that pools against the upstand tests the joint. We apply the same logic as for any flat roof: BS 6229 asks for a design fall of 1:40 so the finished roof achieves 1:80, with no ponding against a kerb.
The height question, and why a kerb that is too low invites trouble, has its own answer, rooflight upstand height.
How the rooflight is sealed into the roof
On a flat roof the waterproofing is usually one of a few systems: EPDM rubber, GRP fibreglass, bitumen felt or a single-ply membrane. The rooflight has to be joined to whichever one is on the roof, and the joining method differs.
Single sheets and welded laps
With EPDM and other single-ply membranes, the sheet is carried up the face of the kerb and glued with adhesive or welded to a cloak, with a dedicated corner detail at each angle. A corner is the weak point because a flat sheet has to be folded to turn it, and a neat factory-formed corner is much better than a hand-folded one.
GRP and liquid systems
GRP is laid in place, so it can be formed up the kerb and across the flange in one piece, with the glass-fibre matting worked into the corner. That makes it well suited to awkward shapes. The kerb has to be clean and primed, and the resin needs the right conditions to cure. A GRP finish is rigid, which means the junction can crack if the building moves.
Felt and torch-on
On a built-up felt roof the layers are lapped up the kerb, finished with a cap sheet and sometimes a metal or lead flashing over the top. Felt is more forgiving to detail on site but depends on a clean, careful torch.
Lead and metal trims
Where the rooflight meets a stone or brick parapet, or a pitched roof above, lead flashing is the usual finish. Leadworx names Code 4 as its standard pick for everyday flashing work. The flashing is stepped and dressed into the masonry so water cannot run behind it.
The roof membrane and the rooflight should both be installed by people who know each other's details, and that is easiest when one contractor handles the whole job.
Choosing glass, size and position
Once the type is settled, the next set of choices concerns the glass and where the unit goes.
Glazing
Double glazing with a coated pane and a gas-filled gap is the usual starting point. Triple glazing improves heat retention and reduces noise further, at higher cost. Toughened or laminated outer panes resist impact, and laminated glass holds together if it cracks, which is why it is used wherever someone might fall against or onto it. Solar-control coatings cut the heat that comes in on a bright day. In a south-facing extension that matters, and it is a decision best made before ordering, since the coating cannot be added later.
Size and shape
A larger rooflight brings in more light but also loses more heat. A common mistake is to fit a very large pane over a small room, which makes the space hot in summer. A better approach is to size the unit to the floor area and to the walls around it, and to leave enough roof between the glass and the edges for a proper upstand and waterproofing.
Position
Over a kitchen island, a dining table or a staircase, a rooflight earns its keep. Too close to a wall, it casts a hard shadow line, and too close to a parapet, it makes the flashing difficult to form. The location should also take account of the structure below. The opening needs framing, and a joist cut for it is trimmed in the same way as a rafter would be for a pitched roof.
Loft conversions
Rooflights over a flat-roofed rear dormer or a loft extension are discussed in our guide to windows for a loft conversion.
Fitting, rules and cost
Fitting a rooflight in a flat roof means cutting the deck and framing the opening, fixing the kerb, forming the waterproofing and installing the glass. When the roof is being renewed at the same time, the rooflight goes in as part of the programme. Where the flat roof stays, it is cut into and made good.
Regulations
A rooflight is subject to the thermal limits set out above. If the roof covering is renewed in the same project, Approved Document L treats a new waterproof layer as work on the roof's thermal element, which can mean upgrading the roof insulation as part of the job. Planning-wise, Class C of the General Permitted Development Order covers rooflights as well as roof windows, within limits on how far they stand proud of the original roof surface; a listed house or one under an Article 4 direction needs a word with the council first. A solid roof for a conservatory is different, and needs a Building Regulations application.
Cost
A rooflight is priced by model, size and glazing, and the figure is fixed in the written quote after the survey. MyBuilder's flat roof guide, updated 21 August 2026, puts flat roof replacement at typically £2,800 to £7,500, averaging about £75 per m². Your own quote, issued after the survey, covers the rooflight by model, the kerb, framing, waterproofing, scaffold or access, waste and making good.
Looking after the finished job
Rooflights collect leaves, dirt and bird droppings, and the glass needs washing from time to time. Check the seals each year and keep the roof around the kerb clear. The installation carries a written 10-year workmanship guarantee, and the rooflight itself carries its own manufacturer warranty.