What sizes do roof windows come in?

Each manufacturer has its own range and its own way of describing it. Most give a size code that stands for a width and a height, and some give the frame size and the glazed area separately. The glass you look through is smaller than the frame, so two windows with the same frame can give different amounts of daylight.

That is why a figure in an advertisement is only a starting point. Width is normally quoted first, then height, and the sash is hung either at the top edge or on a pivot about its middle. The hanging style changes how far the window projects and where the handle sits. Before anything is ordered, the sheet for that particular model is read, because the sheet gives exact dimensions, the minimum and maximum roof pitch, and the size of the matching flashing kit. Nothing about a size should be guessed from memory.

When you are swapping an old window, start with its label. It is usually fixed to the edge of the sash or the inside of the frame and carries the model and the size code. A photograph of that label tells a supplier more than a tape measure can, so write the numbers down and keep both with your quote for later repairs. If the label has gone, measure the frame from outside edge to outside edge, then measure the width and height of the opening in the lining indoors.

Many replacements are ordered in the same size so that the opening and the flashings can stay as they are. Our guide to replacing an old Velux window covers matching in detail, and the answer on flashing kits explains why the kit has to match whichever size you choose, since each kit is made to fit one frame.

The first limit is the rafters. A window that fits between a pair of rafters leaves them untouched. A wider one means cutting a rafter and fitting new timbers either side, which is structural work and may need approval. Rafter spacing varies in older houses, so measure it in the loft before you fall in love with a size. A tape measure costs nothing, and a rafter gap measured twice avoids an expensive mismatch.

The pitch of the roof is the second limit. Each model works within a minimum and maximum pitch, and a window rated for a steep slope will not shed water properly on a shallow one. The third is the covering: a big window in slate or clay means more tiles cut and more care in the flashing. The fourth is the room itself, since an opening that sits too low or too high is awkward to reach, as our guide to roof windows for loft conversions explains.

In a loft bedroom, a window may also have to serve as the way out in a fire. The 4.5 m rule and the minimum opening size come from the Building Regulations, and a new loft room above a two-storey house usually brings a protected stair requirement. Our answer on whether a roof window can be a fire escape sets out how that works.

Under Approved Document L 2021 (Table 4.2) a roof window fitted to an existing home needs a U-value of 1.4 or a Window Energy Rating of Band B, and Table 3.2 of the 2026 edition carries the figure forward from 24 March 2027. Bigger glass loses more heat and gains more sun, so size affects comfort as well as compliance. Daylight is the other test. A bigger window is not automatically better, and a pair of smaller ones can light a room more evenly than one large unit.

For a bedroom, one medium window at a comfortable height often does the job. For a stairwell or landing, a narrow tall window brings light down a long space. For a kitchen or living room in a loft, two windows side by side give more even light than one large unit and are simpler to fit between rafters.

A larger window costs more and weighs more to lift into place, and a made-to-measure unit usually takes longer to arrive. When we visit we measure the rafters and the room, photograph the roof and say which size and how many we would choose, before any price is mentioned. The written quote that follows has a single total. Our skylights and roof windows page describes our other roof glazing work. Whatever you choose, check the opening and the rafters again on the day the window arrives, before the roof is opened up.