In brief
Yes, but not with the VELUX window you would put in a tiled slope. A pitched roof window is designed to sit in a sloping roof, and it is not made for a flat one. VELUX has a separate flat roof range, fitted on its own kerb, and that is the product to use. The kerb is sized and built before the roof covering goes on, so the covering can be dressed up to it.
- A pitched roof VELUX is built for a slope, not a flat deck
- The flat roof range is designed to sit on a raised kerb
- The kerb height and size go to the builder before the deck is covered
- The roofer dresses the membrane up the kerb, then the window is fitted
- Fixed, opening and electric versions exist, so the room decides
Why a pitched roof window does not suit a flat roof
A standard VELUX roof window relies on the slope. Its frame and flashing carry rain past the joints and down the roof, and the maker sets a minimum pitch below which the window should not be fitted. On a flat roof there is almost no slope to do that work, so water lingers at the frame and finds its way into joints that were never meant to hold it.
There is also a difference in how the window meets the covering. On a tiled roof, the flashing kit overlaps tiles or slates. On a flat roof, the covering is a continuous membrane such as felt, single ply or GRP, and it has to be joined to the window by a different method. A pitched window and flashing kit offer no way to do that properly.
If your roof is very low in pitch rather than truly flat, the question is different, and it is covered in our page on roof windows in a low pitch roof.
What the VELUX flat roof range is
VELUX makes a range of windows for flat and very low roofs, separate from its pitched roof windows. They are designed to shed water from a level or near-level deck and to be fixed to a raised kerb, with their own weathering details for a membrane roof. We read the current product literature at the survey and name the model in the quote.
The range includes fixed versions that bring in light, opening versions that let out warm air, and electric versions with a motor and a rain sensor. Sizes are limited to the ones the maker lists, so the opening is planned around the unit rather than the other way round. We check the current choices when we survey, because ranges and sizes change between model years.
Where the room under the window is a kitchen, bathroom or landing, the opening version lets warm damp air escape, though it is no substitute for an extractor fan in a wet room.
The kerb is the part that is built first
The window does not sit directly on the roof deck. It is fixed to a kerb, a raised frame of timber or a made-up base that lifts the window clear of the roof surface so water drains away from the joint. The kerb lifts the unit clear of the roof surface, and the height it needs comes from the maker's data sheet for the unit you have chosen.
The kerb has to be in place before the roof covering is laid, because the membrane is carried up its sides. That is why timing matters. The builder or architect gets the kerb size and height for the exact unit as a written note ahead of the roof going up, so the opening is formed correctly and the roofer is not working to a guess.
On an existing flat roof, the covering is opened up around the new opening, the kerb built, and the membrane made good up the sides. That is more work than a new roof, as the covering has to be joined cleanly to the old one.
The order of work on a flat roof
The sequence on site runs in a set order, and each trade needs the one before to be finished.
- The joists are trimmed and the kerb is built to the unit's measured size.
- The roofer lays the insulation and the membrane, dressing it up the kerb.
- We fit the window onto the kerb and weather it as the maker instructs.
- The electrician connects an electric unit, having run the cable at first fix.
- The plasterer finishes the reveals and the room is decorated.
Holding to that order keeps the roof from sitting open waiting for glass. The window is ordered to arrive when the roof is ready for it, and the delivery date is confirmed to you and the builder once we have ordered.
Other options for a flat roof
The VELUX flat roof range suits many openings, but not all. Large openings, walk-on glazing and the look of a lantern call for other products. If you want more light than a single unit gives, our roof lanterns and flat rooflights are made to measure for a kerb, and we can price them alongside the VELUX option.
For the full list of VELUX work we fit, see our VELUX windows page. The cost of a fitted unit is set out in what decides the price of a fitted VELUX. After the survey, our quote lists the unit, the glass, the kerb size and the delivery date, along with the points your builder must deal with.
Related questions
- Do I need planning permission for a flat roof window?
- Often not, because rooflights are covered by permitted development in many cases. That can be removed by a condition or an Article 4 direction, and does not apply to listed buildings. See our planning permission guide and check with your council.
- Can the window be fitted when the roof is already finished?
- Yes. The covering is opened around the new position, the kerb is built and the membrane is joined back up to it. A roofer is usually involved so that the join is made properly.
