The short answer
Often, yes. When the frame, sill and flashing are sound, the glass can be replaced alone, as a new sealed glazing unit or a replacement sash made for your model. A reglaze avoids lifting the roof covering. It makes no sense if the timber is rotten or the surround leaks, and the new glass should meet the Part L value for roof windows.
Two ways to renew the glass
There are two routes. The first is a new sealed glazing unit, the double-glazed sandwich of glass, fitted into the existing sash. The old unit is released, lifted out and replaced by one made to the same size and build, with new seals and fixings. The sash, frame and flashing all stay.
The second route is a new sash, with glass already in it. The old sash is taken off its hinges and the replacement hung in its place. This is quicker on site and gives you new gaskets and a fresh opening action, though the part has to be matched to your model, and it costs more than the glass alone.
Which route suits depends on the age and make of the window. On some, the manufacturer sells replacement sashes and units for years after a model is retired. On others, spare parts have been discontinued and the choice narrows. The label stamped on the frame or sash gives the model reference, and that is what the supplier needs.
Our answer on a misted roof window covers one reason to replace the glass.
Checks before you commit to glass alone
A reglaze pays only when the rest of the window is sound, so the checks come first. Press the sill and the lower frame with a screwdriver tip: soft, spongy timber means rot, and a new unit in a rotten frame is money wasted. Look at the lining for stains from water that has got in from outside.
Look outside too, if it is safe to see from the garden. The flashing should lie flat, the tiles around it should be intact, and there should be no moss or debris holding water against the frame. If water has been getting in at the edges, the glass is not the problem and a new unit will not cure it.
Finally, think about how long you expect to stay and how old the window is. A reglaze extends the life of a good frame. A tired frame with a new unit will still need replacing sooner than a new window would have done, and our answer on how long roof windows last sets out what ages first.
Looking from inside the loft and from outside settles it, and photos show you why.
Part L and the glass you order
Windows in existing homes have a minimum performance when they are replaced. Approved Document L, Table 4.2, sets a U-value of 1.4 as the benchmark for replacement windows, with Band B on the Window Energy Rating scale as the other route, and its note says the row includes roof windows. Rooflights have their own row, at 2.2. A roof window takes the first figure, so name it when you order.
Roof windows are treated as windows, not as part of the roof, which is why they sit in the window row of that table. Whether a unit-only change counts as replacing the window is a question for your building control body, so put it to them before the glass is ordered.
Either way, ask the supplier for written details of the unit: its make, size, build and stated performance. Keep that with your records. It shows what went in if a buyer's surveyor asks, and it helps whoever services the window next. For the whole service, see our roof windows and skylights page.
When a full window earns its keep
There is a point where reglazing stops paying. A frame with soft timber, a sash that no longer closes square, a flashing that has failed or leaks that keep returning all point to a whole new window. The glass is a small part of what a roof window does, and the other parts need to be sound for it to earn its keep.
A new window also lets you change what the room gets. A larger opening, a different opening style or a motor can be chosen when the frame is replaced, and the flashing and covering around it are renewed at the same time. That is a bigger job and a bigger price, and for an old frame it often leaves you better off.
We lay out the options after the survey: glass only, sash and glass, or a full replacement, with photos that show why. The right repair is the one that fits what we find, and we say if a reglaze would be money spent in the wrong place.
If the old window was in the roof before a loft conversion, see roof windows for a loft conversion too.
