Will a new VELUX fit my old VELUX flashing?

A VELUX size code, a letter and two digits stamped on the frame, fixes the width and height of the opening it sits in. VELUX has kept these codes constant across generations for exactly this reason: a house built with an MK06 or a CK04 opening thirty years ago still takes a current window of the same code, in the same rafter opening, without cutting the roof again. That is why the question is rarely "will the window fit the hole" and almost always "will the flashing that lines that hole still do its job".

If the code on the old window has worn away or was never recorded, it can usually still be read from the data plate inside the frame. Our guide to finding the size code shows where to look before anyone orders a replacement.

Flashing design has moved on more than the window has. Older installations often used a flashing shaped for a shallower frame and a simpler collar, fitted to the insulation standards of their day. Current VELUX flashings are built around a deeper, better insulated frame and a specific collar and lining kit, so the metal laps, the upstand height and the way it meets the collar are not always the same shape as the one already on the roof, even when the size code matches exactly.

The collar is the part most easily missed. It is what carries the vapour barrier and insulation from the loft up to the window frame, and it is designed to pair with a particular flashing generation. Fit a current window into an older flashing without checking the collar, and the join between plaster, insulation and frame can be left incomplete even though the roof outside looks tidy. Our page on installation collars and linings sets out how the two parts work together.

Reuse is worth considering when all of the following hold true:

• The new window carries the same size code as the one it replaces.
• The roof covering is the same as when the flashing was first fitted: the same tile profile, the same slate, or the same sheet material.
• The flashing metal is sound, with no rust through the joints, no split laps and no flattened or lifted sections.
• The flashing is a recessed or profiled type that matches the collar a current window expects, not an older lead only apron with no collar interface at all.

Meeting all four is common on a straight swap in a Fletton brick terrace or a new-town semi where the roof has not been re-covered since the window went in. It is less common on an older cottage roof around Stamford, where re-roofing in a different slate or a change from plain tile to interlocking tile has happened at some point without the window changing with it.

A table is the quickest way to see how the checks stack up on site.

Where the roof covering has changed since the original install, even a flashing in good condition was shaped for a different tile or slate thickness, so it will not sit flat against what is on the roof now. Our guide to which flashing suits your roof covering shows how the range is matched to plain tile, profiled tile, slate and flat coverings.

The survey covers the same four points every time: read the size code from the frame, record the roof covering, examine the flashing metal close up rather than from the ground, and note whether the window sits recessed into the roof or proud of it. Only once all four are confirmed do we say whether the flashing stays or is renewed alongside the window.

The same visit is when we plan the swap into the existing opening, and where relevant, check whether a new window will match the size and cladding colour of others already on the roof. Every quote also comes with a sightline sketch, so you can see how much sky the finished window will give you from the room before any decision is made on the flashing underneath it.

Can the flashing be reused if only the glazing is being changed, not the whole window?

Yes, more often than on a full window swap. Changing the glazing alone leaves the frame and the flashing untouched, so as long as the flashing was sound to begin with, there is nothing in that job to disturb it.

Does a different roof covering always mean a new flashing is needed?

Close to always. VELUX flashings are shaped to sit against a particular tile or slate profile and thickness, so a change of covering since the original fit almost always calls for a flashing matched to what is on the roof now, even when the window's size code has not changed.

Will reusing an old flashing affect the guarantee?

Our 10-year workmanship guarantee covers the work we carry out, so where we agree an existing flashing can be reused, that decision and the reasoning behind it are part of the same job and the same guarantee.

What if the size code has worn off the old frame completely?

The data plate inside the frame usually survives even when a painted or weathered code on the outside has not. See our guide on where the size code sits on an existing window for where to check, and how a size can be worked out on site if the plate is missing.

For a broader look at how a VELUX replacement is planned and priced, our VELUX window installation page and our guide to how many years a VELUX window should give cover the wider decision, and our cost guide sets out what affects the price of the work.