What the sealed unit is and why it fails
The glass in a modern roof window is not a single pane. It is a sealed unit: two or more sheets held apart by a spacer bar, with the edges bonded so that the air or gas between them stays dry. The unit sits in the sash and is held by beads or a pressure strip.
When the edge seal breaks, moisture gets between the panes. You see it as misting or droplets that you cannot wipe away, because they are inside the unit. The glass is not at fault, the seal is. A cracked outer pane is a different failure and often comes from a knock, a thermal stress or debris.
Because the unit is a separate part, it is sometimes possible to replace it without touching the frame. The roofer or glazier removes the bead, lifts out the old unit, and fits a new one made to the same dimensions and specification. The measurements, the thickness and the type of glass have to match what the sash was designed for, so the unit is ordered against the window's data label or the maker's parts information.
When the glass can be replaced alone
A new unit is a fair choice when the rest of the window is sound. A quick checklist helps decide:
- The frame and sash are straight, with no softness, flaking or rot.
- The gaskets and seals still close tight.
- The flashing is sound and the covering round it is in good order.
- The window still opens and closes smoothly.
- Replacement units can be made to the size and glass specification the sash needs.
If all five hold, replacing the unit is the smaller job. It can often be done from the inside or from the roof without opening the covering, which keeps disturbance low. If the data label has gone, the unit's thickness and the sash's dimensions can still be measured, and the maker may be able to identify the model from them.
A glazier may also be able to fit a different specification, for example a unit with a different coating, as long as the sash is built to take it. Ask which specification is being fitted and why, and keep the paperwork with the house records.
When the whole window is the better answer
Fitting new glass into a worn window is a poor trade. If the frame is soft, the seals are flat, the hinges are worn or the flashing is leaking, a new unit sits in the same weak surround and the problem returns. In that case, a replacement window in the existing opening deals with all of it at once.
Look at the whole window before you order glass. A leak at the frame's edge, rust on the flashing or a sash that will not close flat point at the surround, not the glass. Our guide to how long roof windows last sets out the signs of a tired window.
Where the window is fitted into an opening with an old flashing kit, the kit is worth renewing at the same time, because the covering has to be opened to reach it. It helps to compare the two routes on paper: a unit-only job and a full replacement, each priced after a survey, so you can weigh what each leaves behind. A unit that cures the mist but leaves a worn frame is a short-term answer.
What Part L asks of a replacement window
Replacing only the glazed unit leaves the window as it was, so the question is about the unit you choose. Replacing the whole window brings Table 4.2 of Approved Document L into play. The window row for new elements in an existing dwelling sets a figure of 1.4 or Window Energy Rating Band B, and the table notes that it takes in roof windows. The rooflight row is 2.2, which belongs to a different product.
Approved Document L (2021 edition with 2023 amendments) applies now. The 2026 edition takes effect on 24 March 2027 for use in England, so work carried out before that date is checked against the current edition. Building control at your own council can confirm which route applies before you order.
Ask the supplier for the energy label or U-value data for the window you choose, and keep it. For a fuller explanation, see roof window U-values and the main roof windows and skylights page. If the window is on a listed or conservation area house, a change of glazing may also be a matter for the council, so ask before ordering.
