The short answer
Yes, if the product is made for a flat roof. A standard pitched-roof window will not work, because a flat roof has no slope to shed water. A flat-roof window or rooflight sits on a raised kerb, and the waterproof membrane is dressed up the upstand and sealed under the cap. The roof also needs enough fall for water to leave.
Flat roofs and roof windows can work well together, but the window must be a flat-roof product, and the waterproofing at the kerb decides whether it stays dry.
Why a flat roof needs a different window
A pitched roof window relies on the slope. Rain runs down the glass and over the flashings, and gravity does most of the work. A flat roof has almost no slope, so water lingers around anything that sticks up through it. A glazed unit therefore needs its own raised edge to keep standing water away from the joint.
What is a kerb, and why does it matter?
A kerb, called an upstand in the Building Regulations, is a low frame built up from the roof deck around the opening. The glazed unit sits on top of it, so the join between the glass and the roof is lifted clear of the wet surface. The roofer dresses the waterproof covering up the outside of the kerb, and a cap or flashing closes the top edge. If that is done well, the window can sit on a flat roof for many years. If the upstand is too low or the membrane is poorly lapped, water finds the joint after the first hard rain. That is why the product choice and the build-up around it come before the colour of the frame.
Flat-roof windows and rooflights
The names overlap here. A flat-roof window is made to open, usually with a hinged sash or a motorised one, and sits on a kerb. A rooflight is a glazed unit that may be fixed, a dome or a glass pane on an upstand. Both are common on extensions, kitchens and garage conversions.
Which U-value row does a flat-roof window follow?
Approved Document L, Volume 1, gives roof windows the window row of 1.4, or WER Band B, and rooflights a row of 2.2. A product sold as a flat-roof window may fall under either one, depending on how it is made and classed. Ask the supplier which row the product data sheet answers to, and confirm it with your building control body. For a kerb built on site, Table 4.2 allows a maximum of 0.35 W/(m²·K), which is why the kerb needs insulating as well as waterproofing. A bare timber box would waste heat all year.
Our answer on skylights and roof windows explains the two names. When a quote names the exact product, you can see what is being priced.
Fall, drainage and the membrane
A window cannot cure a roof that holds water. The deck needs fall, a gentle slope towards the outlets. BS 6229:2025 is the code of practice for flexible coverings laid on roofs of 10 degrees or less. It holds to a finished fall of 1:80 as the minimum, and one manufacturer advises designing the deck to 1:40 so the built roof still reaches that figure. Falls matter most around a window, because the kerb blocks the natural run of water.
What if my flat roof ponds around the opening?
Ponding next to a kerb is a warning sign. Standing water tests the joint after every shower and speeds up wear on the membrane, and a puddle that stays for days is rarely a one-off. If the roof is ageing, the sensible time to add a window is when the covering is renewed, so the roofer lays the new membrane up to the kerb as one operation.
Approved Document L treats a fresh waterproofing layer on a flat deck as renovation, so insulation questions can follow. Our page on flat roofing explains the coverings, and the main roof windows and skylights page shows how the two jobs fit together.
What happens on site
After the survey, the opening is marked out and the covering is cut back. If the deck is timber, the roofer frames the opening with joists on all four sides, builds the kerb, insulates it and then lays the new membrane over the lot, turning it up the upstand. The unit is fixed, the cap goes over the top edge, and the lining below is finished.
Is a flat-roof window harder to fit than a pitched one?
It is a different set of skills. The structural work is simpler where no rafters are cut, but the waterproofing is less forgiving, because there is no slope to help. The new opening may still need building control, and Somerset Building Control is the first call for a Taunton address. Ask us for the plan of work day by day before it starts.
We survey, send you photos of the flat deck itself, and write one total for the window, the kerb and the covering around it. Where water is already getting in, we stop it first and then talk through the choices. Our page on Velux and Building Regulations explains when approval applies.
More on windows in flat roofs
Do flat-roof windows leak more than pitched ones?
They can if the kerb and membrane are poorly built, because water has no slope to run off. A well-made upstand, a properly lapped membrane and a sound fall around it give the unit a dry joint. When one does leak, the cause is nearly always the detail at the kerb, not the glass.
Can a flat-roof window go in an existing flat roof?
Yes, but the covering around it has to be sound, and the deck underneath has to be dry and firm. Cutting into an old membrane close to failing invites trouble, so we judge the roof first. Sometimes the right order is to renew the covering and fit the window at the same time.
The full picture: Velux roof window and skylight installation in Taunton · More short answers