Can you replace just the glass in a VELUX window?

People ask it when the glass has gone wrong but the window still works. One cause is mist between the panes, which means the sealed unit has failed, while the frame, the opening mechanism and the flashing all look fine. Replacing the whole window feels like a lot to do for a pane of glass.

It is a fair instinct, and sometimes it is the right one. A roof window is a frame carrying a glazed sash, and the glass is one part of it. Whether that part can be changed on its own depends on the model, on its age and on the condition of everything it sits in. The answer for your window is a matter for the survey, not for guesswork from the room below.

In VELUX's table every roof window has a size code, set against a width and a height. An MK04, for example, is 78 by 98 cm, an MK06 is 78 by 118 cm and a UK04 is 134 by 98 cm. VELUX shows that code on a metal plate, the type sign, along the upper edge of the sash, where you can see it with the window open, and any VELUX roof window from after 1968 carries one.

That code is the first thing anyone needs before new glass is ordered or a replacement window is priced. A photograph of the type sign, sent to us on WhatsApp at 07880 284133, is the most useful single picture you can take. The type sign and its code get a fuller treatment in our VELUX size codes guide.

The case for it is strongest when the frame is sound, the sash opens and closes properly, the flashing and the seal to the roof are in good order and glass can be sourced for the model. In that situation the new glass goes into the window you have, and the roof covering round it can be left alone.

It also helps if you want to change the glass itself. VELUX lists a laminated inner pane among its glazing options, for example, and what is available varies with the model, so the choice of glazing is one more thing the window decides.

It is worth asking too when a window is a good size for the room and in the right place. If the opening is the position you would choose again, there is little reason to move it.

A new window is the better answer when the frame has rotted, when the sash no longer closes square, when the flashing has failed or when glass for an older model cannot be found. New glass in a tired window can leave you paying for the same visit again later.

It is also the better answer if you want something the old window cannot give: an electric opening, a blackout blind for a loft bedroom, or a different size. At night the glass reads black and mirrors whatever is lit in the room, which is why a blackout blind is worth planning at the same time.

For a whole-unit job, replacing a rooflight is covered in its own answer, and the hub on misted rooflights covers the wider picture. Whether a failed sealed unit or the whole window is the better route is compared in sealed unit or whole rooflight. Where one loft has more than one roof window, a second look at the type sign on each is time well spent.

Under Approved Document L (Wales), swapping the glass while the old frame stays is not building work for the energy rules. A like-for-like replacement window or roof window still is. Approved Document L is guidance, and it says a replacement roof window should meet 1.4 W/m²K or Window Energy Rating band B. Where the work is notifiable, the Building Regulations notification is ours to make, through local authority building control or a registered building control approver.

We look at the window from inside and from the roof, read the type sign, check the frame and the flashing, and tell you whether glass alone will do. Either way, a fixed written quote follows the survey. The survey form books the visit at no charge, or you can write to info@supremeskylightsnewport.co.uk with a photo of the type sign.