Can a VELUX window be fitted to a flat roof?

When someone asks for a VELUX in a flat roof extension, they are usually thinking of the classic centre-pivot window: the white-framed unit set into a tiled or slated pitch, opening on a hinge halfway up. That window relies on gravity. Its flashings, its drainage channels and its whole weathering design assume rainwater is running down a slope and away. Lay it flat and the water has nowhere to go. It pools on the glass, sits against the seals, and finds its way in.

The flat roof range is a distinct set of units. The frame is deeper, the weathering is built to work at a very shallow angle, and the glazing sits proud of the surrounding roof rather than flush with it. VELUX sells two visible tops for these: a domed or curved polymer cover that sheds water by its shape, and a flat glass finish that looks cleaner from the garden but needs the structure beneath set to a slight fall so water still clears. Both are designed to be walked around by a roofer for cleaning, not walked on, unless you specify a dedicated walk-on unit, which is a heavier and more expensive proposition.

So the honest answer to the question is that you do not fit a pitched VELUX to a flat roof. You fit a flat roof VELUX, and it is a different order code, a different price and a different installation.

A flat roof window is never set straight into the deck. It sits on an upstand, also called a kerb, which is a raised frame lifting the glass 150mm or more above the flat surface around it. That height is what keeps standing water, driven rain and the odd blocked-gutter flood away from the opening. It is the single most important detail on the whole job, and it is where a poorly done installation gives itself away.

You have two routes to that kerb. VELUX supplies its own insulated upstand designed to take the window directly, which gives a known, tested build-up and the right height straight out of the box. Or a builder forms a timber kerb on site, which can be the right call when the roof structure is unusual or the opening size does not match a standard unit. Either way the kerb has to be square, level along its top, and properly tied into the roof covering, whether that is felt, single-ply membrane, EPDM rubber or a liquid system. The window weathers to the kerb; the kerb weathers to the roof. Get the second joint wrong and the best window in the world will still let water track underneath it.

No flat roof is truly flat, and neither is the glass on top of one. A curved or domed cover sheds water by its own shape, so it tolerates a genuinely level deck. A flat glass unit has to be set to a small fall, typically in the region of 5 to 15 degrees depending on the model, so rain runs off rather than sitting in the middle and drying into a dirty ring. VELUX publishes a minimum pitch for each flat roof product, and going below it is not a detail to fudge, because a puddle that never clears will eventually work at the seals and mark the glass.

This matters most on the big rear extensions common across the south Essex housing stock. A deep kitchen-diner behind a 1930s semi often gets a flat roof because a pitched one would foul the first-floor windows, and the roof is built with a shallow fall towards a rear gutter. The window needs to work with that fall, not against it. On a roof that already drains one way, a flat glass unit set square to a slope that runs the other way will hold water at its lower edge. It is the kind of thing that looks fine on the day and shows up two winters later.

Here is the part that gets missed. A pitched window faces a compass direction, so north, east, south and west each call for different glass. A flat roof window faces straight up at the sky, which means it collects the high summer sun almost square on no matter which way the house is turned. In solar terms, a flat roof is a south-facing roof, and then some.

That is a real issue along the estuary. Southend faces south across open water, so there is little to shade a roof and plenty of reflected light off the sea adding to the load. Put a clear-glazed VELUX flat roof window over a kitchen that already makes its own heat from cooking, and on a July afternoon you have built a solar collector over the hottest room in the house. The answer is the glass, not the blind. Ask for a low G-value, solar-control unit: something around 0.3 or below lets in the daylight while turning away most of the summer heat, where standard clear double glazing at around 0.6 pushes roughly twice the energy into the room. This is the whole reason we specify orientation and glass before we ever talk about the opening, and it runs through all of our energy-efficient skylight installation work. If you are choosing a flat roof VELUX and nobody has mentioned the G-value, that is the question to put to them.

Flat roof VELUX windows come in a set range of sizes rather than made to any dimension, so the opening in your roof needs to suit a stock unit or you commission a custom rooflight instead. They are available fixed, or opening by electric motor and remote, or opening on a solar-powered motor that needs no wiring, which suits a room where running a cable across a finished ceiling would be awkward. An opening unit high in a flat roof earns its keep, because hot air collects at the top of a room and a vent at the highest point lets it out.

A new or enlarged rooflight is notifiable under Building Regulations whichever product you choose. Part L sets a ceiling on the U-value for winter heat loss, and Part K covers the safety glazing where the unit is within reach. On a house, a rooflight is usually permitted development if it projects no more than 150mm above the roof plane and sits below the ridge, though flats, listed buildings, conservation areas and Article 4 zones are the exceptions, so it is always worth a check with the local authority before you order. If your project is a full flat roof rather than a single window, our flat roof skylight installation page covers the wider build-up.

If your roof is flat, look for the VELUX flat roof range rather than a pitched window, plan for a kerb of at least 150mm, and settle the glass specification before anything else, because a flat roof is a solar-gain job whichever way your house points. Decide whether you want a fixed unit or an opening one, and whether a domed cover or flat glass suits the fall of your roof.

If you would like us to take it from the opening, tell us the rough size and what sits below, and we will come back with the right unit, a glass specification for the light and a price. You can read more about the range on our VELUX window installation page, ring 01702 898232, or request a quote and we will look at the whole roof rather than just the hole in it.