How do I find my VELUX product code?

Every VELUX roof window carries an identification plate. On the centre-pivot and top-hung models that make up most of what is fitted in south Essex lofts, it is fixed to the top of the sash, the moving frame that holds the glass. You cannot see it with the window shut, which is why so many people assume there is no label at all.

Open the window and rotate the sash. On a centre-pivot unit, turn it until the glass is roughly horizontal and the top edge swings down towards you. On a top-hung unit, push the sash out and look along its upper edge. The plate is a strip of foil or thin metal, usually silver or white, printed with the code, a manufacturing date and a barcode. It is often tucked close to the hinge, so run your finger along the top rail if your eye does not catch it straight away.

If the window is above a stairwell or set high in a vaulted ceiling, do not lean out over it from a stepladder to read the plate. Photograph it instead. Hold a phone up to the open sash, take a picture of the top rail, and zoom in on the image afterwards. The code is small and the print fades, so a photograph you can enlarge beats squinting at it in poor light.

The string breaks into three parts, and each one answers a different question.

• The three letters give the window type and finish. GGL is a centre-pivot window with a lacquered pine frame. GGU is the same shape in white polyurethane, which suits a bathroom or kitchen where damp would mark timber. GPL and GPU are the top-hung versions of each. GHL and GHU cover the older top-hung range, and a code beginning GX or GV points to an access or escape window. An electric or solar VELUX INTEGRA carries the same opening letters with extra characters for the motor.
• The size code is a letter followed by two digits, such as MK04 or SK06. The letter fixes the width and the number fixes the height. This is the block you need for a like-for-like swap, because it maps to a fixed opening in the roof.
• The four-digit glazing code, such as 3070 or 2070, describes the glass and the handle generation. The first digits relate to the era and opening system; the last two identify the pane itself. This is the number to look at hardest if the room runs hot, because it is the one you can change without altering the hole in the roof.

Write the whole thing down in order. A supplier can work from GGL MK04 3070. They cannot do much with “a medium VELUX, the pine one”.

The size letter is a width band and the two digits are a height band. These are the standard combinations you will meet most often. Treat the centimetres as the nominal window size and always check the current VELUX size guide before you order, since the range has changed over the years.

The pattern holds across the range. C is 55 cm wide, F is 66, M is 78, P is 94, S is 114 and U is 134. The height numbers climb in steps, so 04 is 98 cm and 06 is 118 cm on the common widths. If your plate reads PK08 you have a 94 by 140 cm window, and so on. Getting this block right is what lets a replacement unit drop into the existing timber without cutting the roof about.

Sunlight fades the print over the years, and on an older window the plate can be unreadable. There are two fallbacks.

The first is the glass itself. Look into the bottom corner of the pane from inside. VELUX etches or prints a small marking on the spacer bar between the panes, and while it is not the full product code, it often carries a date stamp and glazing reference that a supplier can cross-check. The second fallback is the tape measure. Measure the visible glass and then the outer frame, note whether the window pivots from the centre or hangs from the top, and count the panes. Two panes is double glazing; three is triple. Those measurements plus the opening style are usually enough to identify the size code even with no plate at all.

What you should not do is guess the size from the plasterboard opening inside the room. The finished reveal is larger than the window, and ordering to the hole in the ceiling rather than the frame in the roof is how people end up with a unit that does not fit.

The glazing block is where the code stops being admin and starts being about comfort. A great many Southend, Westcliff and Thorpe Bay lofts have their rear roof pitch facing south across the estuary, with an open horizon and reflected light off the water on top of the direct sun. A window fitted twenty years ago with plain double glazing will have a glazing code to match, and that is the room that bakes in July. When such a unit is being taken out and replaced, the code tells us exactly what was there so we can put back the same size in a better pane, one with a lower G-value that turns down the summer heat without darkening the room. That trade-off between glass specification and solar gain is the whole basis of our energy-efficient skylight installation work.

So do three things. Read the plate and note the full code. Note which way the roof faces and whether the room overheats in summer. Then send both to us. With the size code we can match the opening for a straight replacement, and with the orientation we can advise whether the glazing should stay as it was or move to a solar-control pane. You can see the full range and how it is fitted on our VELUX window installation page, get a figure from our skylight installation costs guide, or ring 01702 898232 and read the code down the phone. We will work out the rest.