The short answer
Velux roof windows come in a long list of standard sizes, and each is named by a short code of letters and numbers that stands for the frame's width and height. The code is on the data plate fixed to the sash or frame. When you order a replacement, match the code rather than a tape measure. A new window is fitted between the rafters, so rafter spacing limits the width.
How the size codes work, where to find the data plate, and how rafters decide what will fit in your roof.
How the size codes work
Roof windows are not sold by a plain width and height. Each standard size has a code, made of letters and numbers, and the code points to a frame of a set size. The letters stand for the frame width and height band, and the numbers refine them. You do not need to decode them yourself. What matters is that the same code always means the same frame, so a supplier can find a matching window from the code alone.
The range runs from small, narrow windows suited to a stairwell or a tight bay between rafters, up to wide, tall windows for a loft room. Within it, the same code is made in different types, such as a centre-pivot, a top-hung, a fixed light, an electric opener or a window with a different glazing. The type and the size are separate choices, so a single code can come with a dozen specifications.
Because ranges change, an older window may carry a code that is no longer made in the same form. The maker or a stockist can tell you the nearest current equivalent, and whether it will sit in the existing opening. Our answer on replacing a Velux window explains how a like-for-like swap works.
Reading the data plate
The data plate is a small label fixed to the window, and it holds the facts you need to order. On most roof windows it sits on the sash or on the inside of the frame, often near the top edge. You may have to open the sash and look at the frame edge, or take off a cover. Photograph it with the flash on, because the print is small and may have faded.
It usually carries the size code and the type, and often the glazing and a date or batch mark. Send those details, together with a photo of the whole window from inside and out, to whoever is supplying the new unit. If the plate is missing or unreadable, measure the outside of the frame at its widest and tallest points and note the pitch of the roof, then let the supplier match it.
Do not trust the glass size. The visible glass is smaller than the frame because the sash and the seals take up space, and measuring it will lead to an order that is too small. The frame code is the safe starting point. Our answer on replacing the glass in a Velux window shows why the plate also matters for panes.
Fitting between the rafters
Rafters are the sloping timbers underneath that hold up the tiles or slates. A roof window is normally made to fit between two of them, and the gap between rafters limits how wide a standard window can be. In many roofs the timbers are spaced at a regular distance, but older roofs vary, so measure the clear gap between the actual timbers in the loft and do not assume.
If the window you want is wider than that gap, one or more rafters must be cut and the load carried by new timbers across the top and bottom of the opening. That is structural work. It needs sound timber, a sensible layout and, in many cases, a check by an engineer. Cutting without support can sag a roof.
The height of the window is limited by the room below and by how the opening lines up with the ceiling and the floor. A window that begins too high is hard to see out of, and one that begins too low can be a hazard. Pitch limits the choice too, and our answer on the minimum pitch for a Velux explains it. We measure rafters from inside the loft at the survey, before any size is fixed.
Going bigger, smaller or staying put
The least costly swap is the same size in the same place, because the existing opening, the flashing area and the lining all stay as they are. Even then the flashing kit is renewed with the window, so that it matches the new frame.
Going smaller is simple. The opening is closed in with a little timber and the kit and lining are fitted to suit. Going bigger needs the rafters opened up, a larger area of covering cut away and a new lining, so it costs more. Our answer on replacing a Velux with a bigger window goes through what that involves.
Rules apply to new openings. In Crawley, the borough's building control team wants to approve altered openings for roof windows, and we check with them. Elsewhere, speak to your own building control body. On a side slope, the planning order says the glass must be obscure. The window's data sheet shows its U-value, and Approved Document L 2021 sets 1.4, or Band B, for a new or replacement roof window.
As a national yardstick, Checkatrade's October 2026 guide gives £1,250 for a new roof window and £1,025 to replace one. We give one written total after measuring. Our roof windows and skylights page has more.
More on this in Velux Roof Windows & Skylights in Crawley
Two more sizing questions
What if the data plate has worn off?
Measure the outer frame at its widest and tallest points, note the roof pitch and take photos from inside and out. A supplier can often match those to a current size. Look at the hinges and the opening type too, because they narrow the choice. We can confirm the match at the survey before any order is placed.
Is there a standard size most loft rooms use?
There is no single size that suits every loft. The right one depends on the rafter gap, the pitch, the height of the floor below and how much light the room needs. A narrow window may suit a stair, and a wide one a bedroom. We measure from inside the loft and talk through the options with you.