Can it be done?
Often yes. A VELUX roof window is a frame, a sash and a sealed glazing unit, and where the sash and frame are sound a replacement pane can be made to the size and generation of your window. In practice the glass is sometimes bonded into the sash, so the sash itself is swapped as a unit. The code on the data plate, such as MK04, tells you which parts fit.
What a VELUX is made of, and which part has failed
A roof window has three parts that matter here. The frame is fixed into the roof, with flashing round it. The sash is the part that opens or pivots, and it carries the glazing. The glazing is the sealed unit itself, two or three panes bonded together round a spacer.
Glass trouble usually means one of two things. A sealed unit may have lost its seal, so moisture and fogging appear between the panes. Or the outer pane may be cracked or chipped. Both are glazing problems, and both raise the same question: does the pane come out on its own, or does the sash come with it? The answer decides what is ordered and how long the job takes.
Replacement panes by size code and generation
The window's size code is on its data plate, found on the sash edge or on the frame. Read a code such as MK04 or CK02 as width first (the letter), then height (the digits). Older windows carry three-digit codes (104, 206 and 304 are examples) that map across to the current letter and digit codes. A replacement pane, or a replacement sash, is ordered against that code.
Generation matters as well as size. Over the years the makers have changed the glass build, the way the pane is held and the details of the sash, so a unit made for one generation does not simply drop into another. A photo of the data plate and of the window from inside and out lets us identify the generation before anything is ordered. Two answers help here: reading a VELUX code plate and old VELUX size codes show where to look and how to convert.
- Find the data plate on the sash edge or the frame
- Note the size code and any date or type marking
- Photograph the window inside and out
- Measure the visible glass width and height as a cross-check
When the sash is swapped rather than the pane
On many roof windows the glazing unit is fixed into the sash with a bonded seal or a pressed-in profile that is not designed to be opened up on site. Swapping the sash, which arrives with new glazing already in it, is then the clean route: the sash comes off its hinges, the new one goes on, and the frame, flashing and roof finish stay as they are.
Where the pane can be removed independently, which depends on the model, the sealed unit alone is replaced. This is the cheaper part to buy, but it is not always the simpler job, and the sash must be in good condition to take it. A warped or split sash, worn seals or a stiff mechanism point to a sash swap, or sometimes more.
Is a pane or a sash notifiable work?
A new sealed glazing unit on its own falls outside Building Regulations notification. A whole new roof window counts as a controlled fitting, so it is notified. A sash swap sits between them: because the frame stays, we treat it on the glazing side, and confirm the position for your window in the written job plan. Our answer on whether skylight replacement is notifiable sets out the rule.
If the job turns out to be notifiable, Building Control is dealt with by us. The new glazing is specified so the unit meets the current thermal standard for its fitting type, and the plan states the figure.
When a whole-window swap makes more sense
There are cases where the glass is the least of the problems. If the frame has rotted, the flashing has been disturbed, the window is hard to open or the sash has lost its shape, a new pane in an old window is poor value. Where the existing unit is much older than the current range, parts may no longer match, and a whole new window in a matching flashing kit gives a more settled result.
A new window is also the moment to review the specification: a better thermal figure, a laminated inner pane, a different opening type or a change of size. Our guide on replacing a VELUX window covers the whole-unit route, and and VELUX window installation describes how a new one goes in.
The price of the glass alone sits in the market range for a glass unit only: typical UK market figures run from £300 to £900, a starting point, not our prices. A VELUX like-for-like replacement typically runs from £600 to £1,500 on the same basis. Both depend on size, glazing build and access, and the survey comes before any written figure.
| Situation | Usual route |
|---|---|
| Sealed unit misted, frame and sash sound, pane removable | Glazing unit only |
| Sealed unit misted, glass bonded into the sash | Swap the sash |
| Sash warped or seals worn as well | Swap the sash, or the window |
| Frame rotted or flashing disturbed | Whole window |
How the job is mapped before you commit
We start with a free survey: code plate, sash and frame condition, and how the roof is reached. Ridgeway Skylights then puts the part to be replaced, the wait for the part and our fitting date into the written plan, so pane, sash or window is settled before money is asked for. Our fitting work is guaranteed for ten years.
On failed units more broadly, misted and failed rooflight replacement lays out the glazing routes, and the answer on whether misted glass will clear covers what to expect from a unit that has fogged.