Can You Replace Just the Glass in a VELUX Window?

A VELUX window is a frame fixed to the roof, a sash that holds the glass, and a sealed double glazed unit within the sash. The unit is a component. When its seal fails, or the outer pane cracks, the unit can be taken out and a new one fitted, without disturbing the frame or the roof covering around it.

A cracked pane is the other common reason. Stones, a fallen branch or a hard frost can crack the outer glass while the rest of the window carries on working. The cracked pane lets moisture reach the gap, so the sealed unit tends to cloud soon after, and replacing the unit deals with both at once. Because the damage is to the glass alone, the frame, flashings and tiles around it stay exactly as they were. In a Victorian terrace with a slate roof, that matters, since disturbing old slates around a window adds work you did not need.

That route works well when:

• the timber or polyurethane frame is sound, with no soft spots or flaking at the sill;
• the sash opens and closes properly, and the hinges are in order;
• the window is a pattern that a matching replacement unit can still be made for.

The job is mostly done from outside, and the room below stays almost as it was. It is the lighter option, and the page on rooflight sealed unit replacement shows each stage of the swap.

The sealed unit is made to the rebate of the sash, so it needs to be the right thickness, the right build and the right shape. The glass for a top-hung window is not the same as the glass for a centre-pivot, and a laminated inner pane has a different thickness from toughened glass. Getting this wrong means a pane that will not seat or will not seal, and that is why measuring beats guessing from a photograph. We also note whether the glass is toughened outside with a laminated inner pane, so the replacement matches the original in its safety build.

We take the measurements and the specification from the window itself, and quote the unit by name. The meaning of the codes is in our guide to VELUX size codes, and bear in mind that VELUX is a brand written in capitals, with INTEGRA as its electric and solar range. If your window is part of that range, the electrics sit in the frame and are left alone.

New glass does not cure a worn frame. If the sill is soft, the paint has failed, the seals around the sash have flattened or the flashing no longer sits well, then new glass is going into a window with other problems, and it is a poor use of money.

A whole new window also lets you change type altogether. The choice often comes down to what the room needs. A landing that stays warm and close could do with a sash that tilts out; a stairwell that only needs light can keep a fixed pane. A fixed pane in a bathroom could become an opening roof window with a vent bar, so that damp air has somewhere to go. How the two routes compare on price is shown as broad market ranges on the costs page, and our fuller look at how long VELUX windows last helps judge whether the frame has years left.

Fitting just a new sealed unit to the frame you have is generally outside notification, whereas replacing the whole window, frame and glass, is, and the Building Control notification is ours to make when it applies. A new roof window is then held to the 1.4 W/m²K window limit under Part L. The rest of the rules are covered in our Building Regulations answer on rooflight glass.

On the day, one team arrives with the unit already made. Rain cover stays on hand, the opening is closed up before we leave, and dust sheets protect the floor below. Our 10-year workmanship guarantee applies, and the survey beforehand costs nothing.

The wider set of replacements lives in the misted and failed rooflights. If you are weighing value for money, the page on how long a sealed unit lasts shows what to expect from a new pane.