What does VELUX GGL mean?

Most VELUX windows in houses around Exeter carry one of four codes. They come in pairs: two ways of opening, each in two frame materials.

GGL

Centre-pivot, white-painted pine. The sash turns about a pivot near its middle, so the bottom swings out as the top swings in. Pine is a warm, timber frame that can be repainted in a bathroom or kitchen if it ever needs it.

GGU

Centre-pivot, white polyurethane. The same opening action as the GGL, but the frame is a white polyurethane coating over a core, built for damp rooms where timber is more likely to mark. Note that GGU is a pivot window, not a top-hung one.

GPL

Top-hung, white-painted pine. The hinges sit at the top of the sash, so the whole window swings outwards from the bottom edge, like a door opening upwards. It suits rooms where the window sits low enough to reach the handle at the bottom.

GPU

Top-hung, white polyurethane. The opening action of the GPL in the polyurethane frame.

So the answer to the question people most often type, VELUX GGU vs GGL, is simple. The opening action is the same. The difference is the frame: pine for the GGL, polyurethane for the GGU. Neither is better in general. It depends on the room.

Treat the code as a short description of what the window does and what it is made from. The pivot or top-hung action sits in the first part. The frame finish is the last letter: L for the white-painted pine, U for the white polyurethane. That is why swapping the last letter changes the material and leaves the movement alone, while swapping the opening letters changes the movement and leaves the material alone.

The code does not include the window's size. That is a separate pair of characters, a letter for the width followed by two digits for the height, for example MK04 or CK02. A full description of a window therefore reads like a pair, such as a GGL in an MK04. If you only have the type code and not the size, you cannot order from it. Our answer on how to find your VELUX size code shows where the label sits on the frame.

A centre-pivot window opens from the top handle and swings through a half-turn. You can reach the outside of the glass from inside by rotating the sash, which is useful on a roof that is hard to get to. The sash needs clearance inside the room as it turns, so something tall placed under the window, such as a wardrobe or a bookcase, can get in its way.

A top-hung window opens outwards and keeps the sash outside. That leaves the room clear, and with the right size it gives a wide opening for ventilation. The trade-off is that cleaning the outside face takes more steps, because the sash has to be released and turned into a separate cleaning position. Its handle is at the bottom, so it can sit higher in the roof than a centre-pivot window and still be reached.

Neither action is better for light. Both give the same glazed area for a given size. The difference is ventilation and access, which is why the code matters when you swap one for the other.

Pine suits living rooms, bedrooms and landings, where a timber frame looks right against a painted or plastered lining. It can be repainted. In a steamy room, a polyurethane frame is the safer choice, because it is sealed against moisture and wipes clean. Kitchens and bathrooms are where we would expect to look hardest at the U versions.

This is also where the inside finish comes in. The frame is white, but the lining around it is a surface we form: plaster, board or lime. A white-painted pine frame against a freshly skimmed lining looks very different from a mismatched pair, so the frame choice goes into the finish schedule with the lining.

If you want a like-for-like swap, the type code and the size code are the two things you give us. A GGL replaced by a GGU keeps the same movement and the same hole, so the ceiling lining does not need touching. A GGL replaced by a GPL is different: the sash now opens outwards from the top, the handle moves, and the position of the window in the roof may no longer suit it.

Replacing a whole roof window, frame and glass, is notifiable building work, and we handle the Building Control notification. The flashing kit that seals the window into the roof also has its own code. Kits begin ED: EDW for profiled roofing such as interlocking tiles, EDZ for flat roofing such as plain tiles or thicker slate, and EDL for thin slate. The kit is picked for your roof covering, not for the window type, so it is a separate line on the quote. Our guide to VELUX flashing kits goes through the three.

The type and size codes sit on a label on the frame or the sash, usually towards the top, which you can read by opening the window fully. Many people also find them on the original paperwork, the guarantee card or the packaging if it was kept. The same label usually carries a date and a batch number. Photograph the whole label in good light, as an out-of-focus number is the commonest reason a code cannot be read.

Photograph the label on the frame or sash, which carries the type and size, and a wider shot of the room and the roof outside if you can. If the label has worn or been painted over, the size can be measured on a survey and the type read from the frame and the way it opens. A photo of the handle position is enough to tell a pivot from a top-hung window.

Ask for the quote to state the type code, the size code, the frame finish and the flashing kit, so you can compare it line by line with any other. You can ask for a quote and we will confirm each line at the survey.

For the wider picture, from choosing a window to how it is fitted, see our page on VELUX window installation. If you are unsure whether a like-for-like swap is possible on yours, can a VELUX window be replaced like for like covers it.