The short answer
Often, yes. The glazed pane or the whole sash of a roof window can usually be replaced without taking the frame out of the roof, so the flashing and the lining stay undisturbed. Which route suits depends on the model, its age, whether the seal has failed and whether parts are still made. A misted, cracked or leaking pane is the common reason to ask.
When a pane or sash can be swapped, how to spot a failed sealed unit, and when a whole new window is the better buy.
Pane, sash or the whole window
A roof window has three parts that matter here: the frame fixed into the roof, the sash that opens, and the sealed glass unit held in the sash. Each can be replaced on its own in principle. Swapping the glass unit is the smallest job. Swapping the whole sash is more involved, but it renews the glass, the seals and the hinges together. Replacing the frame is a full window replacement, with the flashing taken apart and rebuilt.
What is possible depends on the model, because parts are made for particular frames and sizes. Read the data plate on the sash or frame and note the size code and the date, then ask the supplier whether a matching pane or sash is still made. Our answer on Velux window sizes shows how to read that plate. An old window may have a frame that is sound but a sash no longer available, in which case the maker or a stockist may offer an alternative that fits the existing frame.
Most of the time the frame stays and the sash is swapped. That avoids opening up the roof covering, so it is quicker, causes less mess and costs less than a new window.
Signs the sealed unit has failed
A double-glazed unit has two panes sealed at the edge, with a gap between them. When the seal fails, moist air gets in and condenses on the inner faces of the glass, where you cannot wipe it. Misting that appears between the panes and clears and returns with the weather is the classic sign. Wiping the room side and the outside will tell you which faces are wet. If it is on the room side only, you are looking at ordinary condensation, which our answer on condensation on a Velux window covers.
A crack is simpler. Impact from a branch or a fall of tile can break the outer pane, and a cracked outer pane lets water into the gap even if the inner one is intact. Hairline cracks in toughened glass can spread, so a cracked pane should be covered from outside and replaced soon. Taped film is not a fix.
Other signs are a drip at the bottom of the pane, a gap in the rubber seal, or a sash that will not shut snugly. Those point at seals and hinges as well as glass. Photos of the whole window, taken inside and out, let us tell which it is before a visit.
Upgrading the glazing while you are there
Replacing glass is the moment to improve it. Newer units are built with better coatings and gas fills than windows of a few decades back, and the gain is felt as a warmer room and less misting. Laminated or toughened panes add safety, and a pane built to deaden rain drumming is available for some models. Ask what the supplier offers for your size and whether it fits your existing sash or frame.
The energy rules also matter. Under Approved Document L 2021, Table 4.2, a new or replacement roof window should reach a U-value of 1.4, or Window Energy Rating Band B. Whether a pane or sash swap counts the way a whole new window does is a question for building control, so ask your building control body how the rules apply before ordering. In Crawley that is Crawley Building Control.
If your window has a mechanism, a lining or an inner pane that has aged poorly, an upgrade can be the point at which a full replacement becomes sensible. Our answer on replacing a Velux window sets out how that job runs, and one written total price follows the survey.
What it costs, and when to replace instead
As a national guide, Checkatrade's July 2026 Velux repair table gives £165 to £1,050 for a repair, and £1,000 to £2,500 for a complete replacement of the same kind. A simple pane swap sits near the bottom of that scale and a sash or frame swap with scaffold sits higher. Treat the guide as a yardstick only: your own figure is set by the model, how the window is reached and the survey. Make sure any quote you get says whether VAT and scaffolding are included.
When is a new window the better buy? If the frame has rotted, the flashing has failed, the sash is no longer made or the window has been repaired more than once, spending on glass alone delays the bigger job. A rule of thumb is to compare the repair with half the cost of a replacement, and to remember that a new unit comes with new seals, a fresh flashing and a fresh warranty. How long a window lasts is a separate question.
We photograph the window inside and out, give you the findings and a single price, and say plainly when glass alone will do. The hub page for roof windows and skylights has the rest.
More on this in Velux Roof Windows & Skylights in Crawley
Two more glass questions
Can misted glass be cleared without replacing it?
Not reliably. Moisture sits inside the sealed gap, so wiping cannot reach it. Some firms drill and treat the unit, but the seal has already failed and the mist tends to come back. A new sealed unit or sash is the lasting answer, and the cost is usually well below that of a whole new window.
Is it safe to leave a cracked pane for a few weeks?
Only with care. Water can enter the gap and drip into the room, and cracks can spread in cold or sun. Cover it from outside only if a safe access exists, or ask a roofer to secure it, then book the replacement. Never climb onto a roof yourself to tape or cover glass.