Can you replace a VELUX without changing the size?

Every VELUX carries a size code, and it is the single most useful thing to know before you swap one out. It is a letter followed by two digits, printed on a small metal or plastic data plate. To read yours, open the window and look at the top bar of the moving sash, on the inside face. You are after something like MK04, CK02, SK06 or UK08. The letter tells you the width, the digits tell you the height.

The width letters run to a standard set of measurements that have not changed in decades:

• C is 55cm wide
• F is 66cm wide
• M is 78cm wide
• P is 94cm wide
• S is 114cm wide
• U is 134cm wide

The two digits climb with the height of the window, so a 04 is shorter than a 06 and a 06 is shorter than an 08. Once you have the full code you know the exact external frame size, which flashing to order and which model will sit in the same opening. Write it down. If the plate has faded to the point where you cannot read it, the width can be measured across the outer timber frame and matched back to the letter, though the plate is quicker and surer.

The size that matters for a straight swap is the rough opening in the roof, the timber-lined hole the old window sits in. VELUX has held the external frame dimensions steady for each size code across its model generations, which is the whole reason a same-code replacement works. An MK04 fitted twenty years ago and an MK04 bought today occupy the same 78cm-wide opening, so the batten spacing above and below stays put and the rafters are left alone.

What can differ between generations is the detail rather than the outline. Mounting brackets, the depth the frame sits into the roof and the trim around the glass have all moved on over the years, but none of that changes the opening. It means the fitter works from inside and outside the existing hole: lift the tiles or slates around the head and sides, release the old flashing, draw the old unit out, set the new one to the same depth line, then dress a new flashing kit of the matching code back into the covering. The roof structure is never opened up. That is what keeps a like-for-like far quicker and cleaner than a new installation cut into fresh roof.

One thing to match honestly is the flashing to your roof covering. Flashing kits come in versions for slate and thin coverings and for profiled or plain tiles, so the code alone is not the end of it. Tell whoever quotes you what the roof is finished in, because the wrong flashing is where a neat replacement turns into a wet ceiling later.

Here is the part most people miss. The size can stay identical while the glass inside the frame changes completely, and a replacement is the one moment when changing it costs you almost nothing extra in labour, because the opening is already there and the only real variable left is what goes into it.

This matters most on a south-facing roof, and in Southend that is a great many of them. The city faces south across the Thames Estuary, so the back of the typical 1930s semi, where the rear rooflight usually sits, catches sun from mid-morning until it swings west. A plain clear double-glazed VELUX from fifteen or twenty years ago might have a G-value near 0.6, meaning around sixty per cent of the sun’s heat comes straight through into the room below. Solar-control glazing brings that down to roughly 0.3 for the same size of window. You keep the code, you keep the opening, and you halve the heat load on the room without lifting a single tile more than the swap already requires.

So if a south-facing bedroom or a kitchen extension runs hot in July, do not replace like the exact glass for the exact glass out of habit. Same size, better spec is the sensible brief. We go into the numbers behind that on our energy-efficient skylight installation page, and it is the thinking we bring to every VELUX window installation we do. Facing north the calculation flips, and clear glass with its useful winter warmth is the right call, so orientation drives the answer rather than a blanket rule.

A same-size swap is not always possible or wise, and it is worth knowing the cases where the opening genuinely has to move.

The first is tired timber. If the surround the window sits in has gone soft, the opening has to be made good before a new frame goes in, and occasionally rebuilding it sensibly means a slightly different size. That is a job for a roofer to assess the structure first; sorting failed timber is not something to paper over with a new window on top.

The second is a very old or non-standard unit. A handful of early or obscure sizes were made that no current model matches exactly. If your code does not map to anything on the modern range, the nearest standard size is fitted and the opening is adjusted to suit, which is a bigger piece of work than a straight swap.

The third is simply wanting more light. If the room is deep and dim, going up a size, or from one window to two, is the moment to do it, since the roof is open anyway. Enlarging a rooflight is notifiable under Building Regulations, with Part L covering the thermal performance and Part K the safety glazing, and making that notification to the local authority is part of the job on our side. Bear in mind that a bigger south-facing opening also gathers more heat, so a larger window is exactly where the glass specification earns its keep.

Start with the code. Open the window, read the plate on the top of the sash, and write down the letter and the two digits. That one line tells you the window can be replaced at the same size and tells whoever quotes you precisely what to order.

Then decide on the glass rather than defaulting to it. Note which way the roof faces and whether the room overheats in summer. If it faces south or west and gets hot, ask for solar-control glazing at the same size code. If it faces north, clear glass is fine. Costs for a like-for-like swap against a full new installation are set out on our skylight installation costs page.

If you would rather hand the measuring over, read us the size code and tell us which way the roof faces, and we will come back with the matching unit, the right glass and a price. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the window coming out of it.