Can You Replace Just the Sash of a VELUX?

Every VELUX roof window is built from an outer frame and an inner sash, and they age very differently. The frame sits fixed to the roof structure, wrapped in flashing that sheds water around the tiles or slates, and lined into the plaster below. The sash is the part that moves: it carries the glazed unit, the hinges, the handle and the locking gear, and it takes the daily wear of being opened, closed and cleaned. On a window ten or fifteen years old, it is very often the sash that has tired first, through misted glass, a stiff hinge or hardware that no longer locks cleanly, while the frame itself, protected by the flashing and never handled directly, can still be perfectly sound.

That split is exactly why VELUX offers replacement sashes on their own for a wide range of older windows. It is not a universal fix, and it depends on what is already on the roof, but where it applies it means the part doing the wearing gets renewed without touching the part that is still doing its job.

A replacement sash is made to fit a specific frame generation and size, so three things need to line up before it is worth considering.

• The frame has to be the compatible generation. VELUX has changed frame designs over the decades, and a current replacement sash is built to match frames from a defined production period onward, not every frame VELUX has ever made.
• The size code has to match. Our guide to where the size code sits on an existing VELUX window covers the data plate that confirms this before anything is ordered.
• The frame itself has to be sound: square in its opening, free of rot or movement in the timber, and with flashing that is still doing its job.

Where all three hold, the sash is the only part that changes. Where any one of them does not, the frame goes too, because a new sash bolted onto a frame it was never designed for will not seal, hang or lock the way it should.

Because the frame, the flashing and the lining around it stay untouched, a sash swap is a lighter job than a full window replacement. The hinges are released from inside the room, the old sash comes away complete with its glass and hardware, and the new sash is set onto the same frame and hinged back into place. Most of that work happens from inside the room, since the frame and its weathering are not being disturbed. The roof outside is not opened up, the plaster reveal is not cut back, and there is no new flashing to bed in around the tiles or slates.

The glazing that comes with the new sash is current specification, so a tired unit that misted years ago is replaced with glass built to today's standards. Our page on VELUX glazing options sets out what those choices are, and they apply to a sash swap in the same way they would to a full new window.

A full window replacement, frame and sash together, is the right call in several situations rather than one single test.

Timber decay is usually found at the sill, where condensation or a slow flashing fault has sat against the wood longest, and once the frame has softened or twisted there is nothing for a new sash's hinges to seat squarely into. Whether an existing flashing can be kept for a new window is a separate question again, but if the frame is coming out, the flashing is coming out with it in almost every case, since the two are fitted as one weathering system.

Changing the opening type, say from a centre-pivot window to a top-hung one, or moving from a fixed light to an opening window, also rules out a sash-only swap, because the hinge positions and the frame itself are built differently for each type. Our comparison of centre-pivot and top-hung VELUX windows sets out how differently the two are built into the frame, not just the sash.

A sash-only swap is the quicker, lighter job of the two, but it is worth being clear about what it does and does not change. The frame, and everything about its position, depth and reveal, stays exactly as built. If the sightline from the bed or the sofa already gave a good, clean run of sky with nothing crossing it, that is preserved without a second thought. If the existing frame sits a little too far into the room, or the reveal was cut deeper or shallower than it should have been, a sash swap carries that forward unchanged, because it only ever touches the moving part.

A full replacement is the point where that can be reconsidered. A new frame can be set with a different reveal splay, and how much frame shows from inside a VELUX becomes a choice again rather than something inherited from the original fit. We look at this at survey stage on every job, sash swap or full replacement, and set out what the view will actually be on the sightline sketch that comes with the quote, so it is never left to be discovered once the work is done.

Before ordering anything, the size code and frame condition settle the question, not a guess based on the window's age alone. Two windows fitted in the same year, on the same street, can need different treatment: one with a sound frame and a tired sash, the other with a frame that has already started to move. Replacing an older VELUX in the same opening covers the full replacement route in more detail, and how many years a VELUX window should give sets out realistic lifespans for the sash, the glass and the frame separately, since they rarely wear out on the same schedule. Where the work is notifiable, we handle the Building Control notification as part of either job, and both are installed to current Building Regulations. The VELUX Window Installation in Peterborough page covers how the survey decides between the two routes before any glass or hardware is ordered.

Can any old VELUX take a modern replacement sash?

No. A replacement sash is built to fit frames from a defined production period onward, matched by size code and frame generation, so it depends on what is already on the roof.

Does a sash-only swap need scaffolding or work on the roof outside?

Usually not. Because the frame and its flashing stay in place, a sash swap is mostly carried out from inside the room, unlike a full replacement where the frame and flashing are renewed together.

Will a sash-only replacement change how much sky I see?

No. The frame position, the reveal and the sightline all stay exactly as they were, since only the moving part of the window is changed.

How do I know if my frame is still sound enough for a new sash?

A survey checks the frame for square, movement and timber condition, alongside the size code, before confirming whether a sash-only swap or a full replacement is the right route.