Your roof window size code is stamped on a small data plate on the top edge of the sash. Open the window, rotate the sash to the cleaning position, and read the label on the upper frame member. The code is a letter followed by a number, such as MK06 or CK02. The letter gives the width, the number gives the height.
Find that plate and a replacement is straightforward, because the codes have barely moved in twenty five years and a 2004 unit can usually be matched to a current model without touching the rafters. Miss it and you are into guesswork, and guesswork on a roof window ends with a frame that will not sit in the opening you already have.
Where the plate actually sits
It is not on the visible frame and it is not on the glass. It is on the top edge of the sash, tucked behind the frame when the window is shut, which is why people who have owned the house for years have often never seen it.
To reach it, open the window and swing the sash right round to the cleaning position, the point where it locks with the outside face turned in towards you. Look at the top rail. On most units the plate is a silver or white label carrying the model designation, the size code, a manufacturing date and a serial number. Photograph it. A clear photo of that plate is enough for us to tell you inside a few minutes whether a like-for-like swap is possible and roughly what it costs.
What the code means
The size code is a letter and a number, and each half tells you one dimension of the structural opening, the gap between the trimmed rafters rather than the size of the glass.
- The letter is the width. C is 550mm, F is 660mm, M is 780mm, P is 940mm, S is 1140mm, U is 1340mm.
- The number is the height. 02 is 780mm, 04 is 980mm, 06 is 1180mm, 08 is 1400mm, 10 is 1600mm.
So MK06 is 780mm by 1180mm. CK02 is 550mm by 780mm. UK08 is 1340mm by 1400mm. If you want the full decode, letter by letter and range by range, our roof window codes explained page lays it out, and the size codes reference is the fuller version.
If the plate is painted over, cooked or gone
On units from the early 1990s the label is often illegible or missing outright, and there are three usual reasons. It has been painted over during a decorating job. It has cooked to a blank on a south-facing slope where years of sun have bleached it. Or it went years ago and nobody noticed.
When the plate is unreadable, do not order off a tape measurement of the visible frame. The visible frame is not the opening, and the difference is roughly 40mm a side. Order to that figure and the new unit will not fit the timber. Instead the opening is measured directly at survey, rafter face to rafter face for the width and trimmer to trimmer for the height, then matched to the nearest current code. That measurement is taken from inside once the lining is understood, not by anybody leaning off the roof.
Do not measure this from the roof. There is no reading worth taking from up there that a surveyor cannot take safely from inside or from a platform. If the plate has gone, that is a survey job, not a ladder job.
Why the code matters before you ring round
Knowing your code changes the phone calls. It tells you whether your unit is a common size that any installer can match quickly, or a wider one like SK06 or UK08 that may have needed a rafter trimmed out when it was first fitted. It also tells you whether the blinds and accessories you already own stand any chance of transferring, though that depends on the window generation as much as the code, which our blind compatibility page explains.
Around Ipswich most loft conversions from the 2000 to 2010 boom used MK04 and MK06 units, often in pairs, so those are the codes we match most. Older terrace conversions off Norwich Road tend to run narrower, on FK sizes, because the rafter spacing was tighter. New-build plots at Kesgrave and Martlesham lean towards the larger PK and UK sizes. Your code places your window in that pattern before a surveyor ever arrives.
There is one more reason to get the code right rather than approximate it. Ordering a unit a size out is not a cheap mistake to unwind. A roof window is made to a structural opening, so a frame that is 40mm too big will not go in and a frame that is too small leaves a gap the flashing kit is not designed to bridge. Neither can be trimmed to fit on the day. That is why we would rather measure the opening properly at survey than take a reading over the phone, and why a clear photograph of the plate is worth more than a description.
Related reading
To see the common families side by side, read what sizes roof windows come in. For the full letter-and-number decode, see roof window codes explained and the fuller size codes reference. All of these sit under our main VELUX window installation guide.
Get a fixed price
If you can photograph the data plate, send it with your enquiry and we will tell you what your options are before anyone visits. If the plate has gone, we measure the opening properly at survey and match it to the nearest current unit. Either way you get one written price, fixed after survey, backed by a 10-year workmanship guarantee from installers with 25 years fitting roof windows.
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Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.