Can a VELUX Window Go in a Flat Roof?

A different product for a different roof

A standard centre-pivot roof window is designed for a sloping roof. It relies on the pitch to shed water quickly over its flashing. On a flat roof there is hardly any fall, so water would linger around the frame. That is why a flat roof asks for a different window, and VELUX makes products built for exactly this.

So the short answer is that a flat roof can take a VELUX window, provided you choose the flat roof version and fit it the way it was designed.

How a flat roof window sits

A flat roof window is raised on an upstand, a kerb of timber or a pre-made base that stands above the roof surface. The glazing sits on top, usually with a slight slope on the glass so rain runs off it. The upstand lifts the frame clear of standing water and snow melt, and it is insulated to avoid a cold bridge into the room.

The finished upstand is usually at least 150mm above the finished roof. Our page on flat roof rooflights shows how the same principle applies to non-VELUX units.

The waterproof layer

The roof covering must run up the upstand and be dressed neatly beneath the frame's edge. Whether the roof is a rubber membrane, felt or a liquid coating, the junction with the upstand is the part that needs the most care. It is hidden work, and it is the work that decides whether the window stays dry in the years that follow.

We do this carefully and photograph it before the upstand cover goes on. You receive the photos at handover, along with the makers' warranties and our 10-year workmanship guarantee.

What the roof has to be like

Not every flat roof is a candidate without some thought. Before fitting, we look at:

• The roof structure and the direction the joists run, which affects where the opening can go
• The depth of the roof build-up, including insulation and any boards above the joists
• The current covering and its condition around where the new upstand will rise
• Where rainwater drains, so the new window does not sit in the path of a pool

A very slight fall is workable, since the upstand deals with it. A roof that holds water across a large area is a different conversation, and we raise it at the survey.

Opening types

Flat roof windows come in fixed versions and in opening versions. A fixed pane brings in light and nothing else. An opening one lets out steam and warm air, and some can be operated by a wall switch or a remote. Our page on electric opening rooflights explains how those work, which suits a kitchen or a bathroom well.

Planning and Building Regulations

For a house, the permitted development test is about height. The glass must not rise more than 150mm over the roof plane, nor stand above the top of the roof. A flat roof window on an upstand can break the first limit, so settle the question before ordering. If it does, an application or a council confirmation may be needed. Flats and maisonettes have no permitted development rights for this, and listed buildings need listed building consent.

Cutting through a flat roof for light also brings in the Building Regulations, because a fresh hole in the deck changes the structure and the heat loss. The model decides the target: roof windows are held to 1.4 W/m²K or Window Energy Rating Band B, rooflights to 2.2 W/m²K. We handle the notification ourselves. Our skylight planning permission guide goes through the rules in full.

Most flat roofs sit over a kitchen, garage conversion or rear extension, so neighbours' windows and boundaries can matter too. Mention both at the survey.

What you see from inside

Looking up, a flat roof window shows the splayed reveal of the upstand, which is finished in plaster or lining. The depth of the roof build-up shapes how that looks. A thicker roof gives a deeper well, and a shallow one gives a crisper edge. We talk through this at the survey so there are no surprises in the finish.

Glass choices and heat

A window that looks straight up gets the full sun in summer, so a south-facing flat roof room can warm quickly. Solar-control coatings on the outer pane, or an external blind, take the edge off. Laminated inner panes are our choice for overhead safety, since a pane above your head should hold together if it ever breaks.

Noise matters too. Rain on a flat roof window can drum loudly on thin glazing, and heavier panes soften it. Mention the room's use at the survey, and we match the glass to it.

Next steps

If you are weighing up a pitched roof window against a flat one, or you simply want to know what your own roof will allow, a survey settles it. It costs nothing, and the quote that follows itemises every line of the work. The fit is then run day by day, with the floors protected and the deck sealed against rain each evening. You can read about our VELUX window installation service, or compare costs in our VELUX window cost guide.