A flat rooflight is a glazed unit that sits on a raised kerb above a flat roof, with its glass out of line with the roof surface. That is the definition Approved Document L uses, and it separates a rooflight from a roof window, which sits in the plane of a pitched slope. The difference sounds like jargon, but it decides the U-value that applies, the way the unit is flashed and how water leaves the roof around it.
The kerb, usually called the upstand, is the part people forget. It lifts the glass above the water on the roof, carries the insulation up around the opening and takes the waterproofing up its sides. If it is too low, too thin or poorly finished, the rooflight leaks or runs with condensation however good the glass is.
Our skylights and roof windows overview sets this in context. Below we cover the main types, the thermal rule for the upstand, the falls that keep water moving, the causes of condensation, what walk-on glass can and cannot do, and the fitting choices that sit behind a sound result.
Fixed, opening and walk-on: what each unit is for
Most flat rooflights fall into three groups, and the choice follows from what you need the opening to do.
A fixed rooflight does not open. It is the simplest and the most weathertight, because there is no moving joint in the glazing, and it suits a kitchen extension, a hallway or a landing where light matters and ventilation comes from elsewhere. An opening rooflight has a hinged or motorised sash, so it also ventilates and can release warm air in summer. It brings one more seal into the design, and the opening mechanism needs access for servicing.
Walk-on glass
A walk-on rooflight is built to take foot traffic. It is made with thick laminated glass, often in layers, and it is found over a basement stair, a roof terrace, a light well or a plant room where people may need to cross the roof. Our walk-on rooflight page goes into the loading and slip resistance. The point to hold onto here is that a unit not sold as walk-on must never be stood on. Glass that looks tough can fail without warning, and the same applies to older polycarbonate domes.
Roof lanterns, which rise in a pitched glazed form above a flat roof, are a related type with their own detailing, covered separately under roof lanterns. If you are weighing a rooflight against a pitched-slope window, our rooflight against roof window comparison sets out when each makes sense.
The upstand and its 0.35 U-value
The upstand is where a flat rooflight earns its keep or fails. It is a short kerb built up from the roof deck, normally in timber, insulated on the inside and outside, and clad with the roof membrane. The glass unit is fixed on top of it.
Table 4.2 of Approved Document L Vol 1 (2021 edition incorporating 2023 amendments) caps a new or replacement rooflight in an existing dwelling at 2.2 W/(m²·K). Note 12 adds that an upstand built on site must not exceed 0.35 W/(m²·K). A U-value measures how much heat passes through a building element, so a lower figure is better, and the upstand has to meet its own limit as well as the glass.
Why a poor upstand goes wrong
An upstand with thin or missing insulation is a cold bridge, a path that carries heat out of the room. The inner face of the kerb then runs cold, and warm room air condenses on it. That shows as a damp ring and eventually mould around the opening, and it is easily mistaken for a leak. A kerb that is too low lets water pool against the glazing frame. One that is not wrapped in the membrane lets water creep in at the joint.
We look at three things on the survey: the height of the kerb above the finished roof surface, the insulation thickness around it, and the way the membrane is turned up and finished at the top. Where the roof is being re-covered anyway, rebuilding a poor upstand costs little extra and removes the cause of the trouble. Our page on a roof window in a flat roof covers the pitched-window alternative.
Falls, ponding and where the water goes
Every flat roof is built with a slight slope, called a fall, that drains water to a gutter or outlet, and a rooflight has to work with that fall instead of cutting across it. IKO, citing the 2018 edition of BS 6229 (since revised), recommends a finished minimum of 1:80 and a design fall of 1:40, which leaves room for tolerances in the build and for the deck to flex.
A kerb sits across that slope like a small dam. Water running down the roof meets the upstand on its upper side and has to divert round it. If the fall is too shallow, or the deck has dipped, water collects behind the kerb, dirt settles out, and the lowest corner of the membrane sits wet through the winter. That is the situation in which a sound unit appears to leak.
Cut a tapered fall, or a cricket
The usual fix on a new or refurbished roof is a cricket: a small tapered ridge built on the upslope side of the kerb, sending water to either side rather than letting it pool. On the downslope side, the membrane and kerb finish are kept clear so water can leave freely.
Ponding that lasts days after rain tells us something is wrong with the falls, with the deck, or with the outlet, not just the rooflight. Covering the deck again without correcting the fall repeats the fault. How the finished surface is made is covered in our flat roofing pages, and the rooflight is dealt with in the same survey so the two systems are detailed together.
Condensation: cold glass, cold kerbs and warm rooms
Condensation is the commonest complaint about a flat rooflight, and many reports of a leak turn out to be moisture forming on a cold surface. It forms when warm, damp air touches anything below its dew point. Glass is the coldest part of most rooms, so it is the first place droplets appear.
The glazing specification matters. A double-glazed sealed unit with a warm-edge spacer keeps the inner pane warmer than single glazing, and triple glazing improves on it again. A cheap acrylic dome with a single skin will run with water in a bathroom or kitchen on any cold morning. Check the specification against the 2.2 W/(m²·K) limit for new or replacement rooflights.
Telling condensation from a leak
Moisture on the inner face of the glass, worst on cold, still mornings and clearing by midday, is condensation. Wet patches on the plasterboard splay around the opening after rain are more likely a membrane fault at the upstand. A stain on the ceiling nearby that grows slowly through the winter may be a cold kerb or a leak, which is why we check both. Our roof window condensation guide looks at the causes in more depth.
Reducing it comes down to the room as much as the unit. Extractor fans in kitchens and bathrooms, a trickle of ventilation in the room, and an insulated, sealed kerb all help. A rooflight over a shower or a swimming pool enclosure is a special case that needs a unit specified for it.
How rooflights are fitted into the roof
Fitting a rooflight into an existing flat roof means cutting through the deck and the structure beneath, so the roof is opened up for the work. The opening is framed with timber trimmers, which carry the load of the joists that have been cut, and then the kerb is built up from the deck. Joists cut for a larger opening may need to be supported, and where the opening affects structure the Building Regulations apply, so we make the notification for you wherever it is required.
The waterproofing sequence
The order in which the layers go on is what keeps the opening dry. The kerb is built, insulated and clad. The roof membrane, which may be EPDM (a synthetic rubber sheet), GRP (fibreglass) or felt, is then dressed up the outside of the kerb and finished beneath the glass frame, so the unit sheds water on to the membrane and not behind it. The unit is set on the kerb, fixed through the frame and sealed. Corners are where membranes are most often stressed, so these are the points we photograph for you.
Where the existing membrane is old, patching around a new kerb is a poor plan. The new work then has a weak join at its edge, so the survey also assesses whether the rest of the roof should be re-covered at the same time. Approved Document L treats re-covering a flat roof as renovation of its thermal element: once more than 50 per cent of the surface is renovated, everything in that element has to be upgraded, not just the new layer. We raise that at the survey, so you know it before you accept a quote.
What to ask before you buy and who to ask
A short list of questions at the start saves most of the problems that appear later. Ask for the U-value of the whole unit and of the glass, and check the unit meets the 2.2 W/(m²·K) figure for a rooflight. Ask what the kerb is made of, how high it will stand above the finished roof and how much insulation it carries. Ask how the membrane will be turned up and finished, and what the fall is on the surrounding roof.
If the unit is to be walked on, ask for the manufacturer's stated load rating in writing and check it fits the use. If it is opening, ask who services the mechanism and how it is reached.
Cost and quotes
MyBuilder (updated 21/08/2026) gives typical UK flat roof costs from £2,800 to £7,500, averaging £75 per m², which shows how much of a rooflight project can sit in the roof itself. A rooflight price depends on the size of the opening, the glazing specification, whether it opens, and the condition of the deck and membrane revealed on the survey. Our guide to roof window cost covers the general price picture.
A written, single-figure quote follows the survey, with photographs of your roof beside it. Ask every firm whether its figure covers VAT and the scaffold or access equipment. Our guide to replacing an old roof window is the place to start if you are changing an existing unit, and the minimum pitch page explains why a pitched-slope window is not an option below a certain slope.
