The short answer
Sometimes. Where the glass is a sealed unit and the maker still supplies a matching one, the pane can be changed without replacing the frame. On an older or discontinued window it often cannot, and a new window is the sensible fix. The model label, the state of the frame and the reason the glass failed decide which route suits you.
A new pane costs less than a new window only if the frame is sound and the right unit exists. Find the label on the frame first.
What the glass in a roof window really is
The glass in a roof window is usually a sealed unit, two or three sheets of glass held apart by a spacer and sealed at the edge, with a gas or air gap between. It sits in the sash, the part that opens, and the sash hinges on the frame, which is fixed into the roof.
Why does a sealed unit fail?
Over time, the edge seal can break down. Moisture then gets into the gap, and the glass mists or shows a cloudy film that will not wipe off from either side. That is a failed unit, and it cannot be cleaned out. Our answer on misted Velux windows explains the difference between a failed unit and ordinary condensation.
A cracked pane is a separate fault, usually caused by impact from a branch, a hailstone or something thrown up from the ground. The unit is damaged and has to be changed. Either way, the first question is the same: can that unit be supplied for this window, and is the rest of the window in good enough order to be worth keeping?
When a pane replacement makes sense
Replacing the glass alone is worth considering when the frame, sash, hinges and flashing are all in good order and the window is not very old. The work then happens mostly at the sash, with less disturbance to the covering around it.
How do I find out if the glass is available?
Look for the label on the inside of the frame or sash, which carries the model and size. Photograph it and send it to us with pictures of the window taken indoors and out. We can then check whether a matching unit is on offer. Some makers sell replacement glazed units for current windows. For older models, a matching unit may no longer be made, or a made-to-measure unit may not suit the sash.
It also helps to know what you want from the new glass. Tell us if the room is too hot in summer, noisy in rain or cold in winter. Different glazing suits different problems, and that is a better conversation before the order than after it. If a unit is a close fit but not an exact match, we tell you before anything is ordered, since a wrong-sized pane is no use to anyone.
When a whole new window is the better buy
A pane is only part of a roof window. If the frame is rotting, the seals around the sash have perished, the hinges are loose or the flashing has failed, a new pane just puts new glass into an old problem.
Is it worth putting new glass in an old frame?
Not when the frame causes the leak. A window that lets rain in at its corners will go on doing it with brand new glass. In that case a whole new window, with new flashing, is the better buy, and the opening is a chance to look at the rafters, battens and lining. The labour to reach the window is much the same either way.
When the whole window is replaced, the energy standard for a new element applies. Approved Document L, Table 4.2, sets the bar for windows, roof windows included, at a U-value of 1.4 or a Window Energy Rating of Band B. A pane in an old sash is a different case, so ask your building control body if you are unsure. Our U-value answer explains the figure. MyJobQuote lists a mid-range standard Velux at about £1,100, with a plain swap around £100 under that (Sep 2026), which is the scale to weigh a new window against.
Questions to ask before you order
Before you agree to a pane replacement, a short list of questions saves disappointment later. They are worth putting to any firm, including us.
What should a quote for new glass tell me?
It should name the unit being fitted, by size and type, and say whether it matches the original glazing or changes it. It should say whether the sash is being removed and refitted, whether new seals are included, and what happens if the unit does not fit once the sash is opened up. It should also say whether any access equipment is needed, because a high window may call for a tower.
Ask what happens to the old unit and who takes it away. Ask for the price as one total in writing. Ours follows the survey and comes with photographs of your own window: the label, the glass and the frame. We recommend a new pane or a new window and give our reasons. If the better choice costs more, we say that plainly and you decide. For the other route, replacing a Velux in the same opening shows what a whole-window swap involves, and the roof windows and skylights page covers the wider options.
More on roof window glass
Can I replace a cracked pane myself?
We would advise against it. The sash is heavy, the unit is fragile, and working on a sloping roof is a fall risk. The unit also has to be the correct size and fitted with the right seals, or water gets in around it. A roofer can do it safely and check the frame while the sash is open.
Will new glass stop my window misting?
If the cause is a failed sealed unit, a new unit ends the misting inside the gap. If the mist is condensation on the room side, new glass will not cure it, because the cause is moisture in the room. Ventilation and heating usually matter more, and a look at the room tells us which applies.
The full picture: Velux roof window and skylight installation in Taunton · More short answers