01Short answer
When the glass alone can be swapped
Often yes, if the frame is sound and the make and model can still be matched. If not, the whole window goes.
Many people ask whether you can replace glass in VELUX windows without paying for a whole new window. The short version is that it depends on three things: what is actually wrong, how old the window is, and whether a matching glazing unit exists for it.
Whether you call it a skylight, a roof window or a rooflight, the logic is the same for any of them.
A roof window has two parts that matter here. The sealed unit is the factory-made sandwich of two or three panes with a sealed gap between them. The frame is the timber or polyurethane casing that holds the sash and bolts to the roof. Mist trapped between the panes, a cloudy film that will not wipe off, or a crack through the outer pane are all problems with the unit. Those are the jobs where swapping only the glass can make sense, because the frame is fine and you keep it.
We say "can", not "will", for a reason. A pane swap is only worth doing if the new unit fits the old frame properly and the frame is not itself past its best. We look at both on the free survey, then put the answer in a written quote so you can compare the two routes side by side.
The glass is the part that fails first, but it is rarely the only part that is tired.
02Matching
How we match a unit to your window
Every VELUX window carries a label that says what it is. That label is the starting point for any replacement glass.
Open the sash and look at the edge or the inside of the frame. You will find a sticker with a code made of letters and numbers. It tells us the model, the size and the type of glazing fitted at the factory. Our page on how to find your VELUX window code shows where to look and how to read it.
With the code in hand we can see whether a matching glazing unit is still made for that window. Older models are where this gets tricky. A window that has been in the roof for decades may have been built on an earlier design, and the maker does not always supply parts for it indefinitely. A made-to-measure unit from another source is sometimes an option, but only when it will sit in the frame and clamp correctly.
Take photos before we visit if you like: the whole window from inside, the label, and any marks on the glass. It saves time on the day, though we will check everything ourselves anyway.
What we check before quoting a glass swap
- Window code
- Model, size and original glazing type from the label
- Frame condition
- Timber or polyurethane sound, no soft spots or splits
- Sash and hinges
- Opens, closes and latches cleanly
- Seals and trim
- Rubber seals supple, not hardened or shrunk
- Unit availability
- A matching unit that fits the sash
03Which route
New glass or a new window
Neither route is always cheaper or better. The right one depends on what the survey finds.
Replace the sealed unit
Suits a frame in good order with a failed or cracked pane. The window stays where it is, so the plaster lining, the flashing and the roof covering are left alone. It is usually quicker, and it is not a notifiable job under the Building Regulations, because the window itself is not being replaced.
Replace the whole window
Suits a tired frame, a window that no longer opens properly, or glazing that is simply too old. You get a new frame and new glass together, plus the chance to choose a warmer specification. Under Part L that is a controlled fitting, and we notify Building Control for you.
Where heat is the worry, think about what you are buying. A new unit in an old frame fixes the glass. It does not change the frame, the seals or how the window meets the roof. A whole replacement lets us fit a modern low-e, argon-filled unit with a warm-edge spacer in a frame built to carry it, and the new window has to meet the current window limit of 1.4 W/m²K or WER Band B. Our page on rooflight U-values explained unpacks that number.
Our answer on replacing a VELUX from inside covers how much of the job happens without going on the roof.
04On the day
What happens when we fit the new glass
A glass swap is a tidy, contained job, and you will know in advance which route you are on.
If the survey says a unit swap is sensible, the written quote says so and sets out what is included. One fitting team does the work from start to finish. We lay protection over floors and furniture below the window before anything is opened, and the sash comes out so the unit can be changed in a safe working position.
- Free survey: we read the window code, inspect the frame and seals, and measure the sash.
- Written quote: the unit swap and a whole-window option are both set out, where both are possible.
- Fitting day: floors and furniture covered, the old unit removed, the new one fitted and the sash rehung.
- Handover: we check the window opens, latches and sits square, and clear away what we have used.
Not sure which of the two your window needs? Start from the misted and failed rooflights page, or ask for a survey. Whole-window replacements are covered further on whether rooflights need Building Regulations.
Glass only, or the whole window?
Does replacing only the glass need Building Control?
No. Swapping the sealed unit alone is not notifiable. A new window with its frame as well as the glass is a controlled fitting under Part L, and where that applies we make the notification.
Can I replace the glass in a very old VELUX?
Sometimes, but the older the window, the less likely a matching unit is still made. We read the label code at the survey, and if the frame is also worn we will say a full replacement makes more sense.
Will a new unit make the room warmer?
If the old one had failed, yes, because a sealed unit that has lost its gas and seal passes heat easily. The frame, seals and kerb still influence the result, which is why we look at all of them.
Does it work on a flat rooflight or dome?
The principle is the same, but plastic domes and many flat rooflights are replaced whole. We tell you at the survey whether a unit swap is possible for yours.
