The short answer
Often, yes, but it is not automatic. A new rooflight or roof window made to the same size fits the old opening when the frame, kerb and structure around it are sound and square. Where the old unit was an odd size, the roof has moved, or you want something bigger, the opening has to be adjusted, and that changes the job.
Measure the opening, not the glass
The size that matters is the size of the opening in the roof and the unit that sits in it, not the visible glass. A roof window is named for its outer frame and sits between rafters at a set spacing. A flat rooflight sits on a kerb, and the rooflight is made to the outside or inside dimensions of that kerb. Glass seen from the room is smaller than the frame it sits in, so measuring the pane alone leads to an order that is wrong by the width of the frame.
Measuring is done at the survey, from the roof and from the room. We note the width and height of the existing frame or kerb, the pitch, the rafter spacing, and how the lining meets the ceiling. These go into the quote and the finish schedule, so that what is ordered is what is fitted.
Roof windows: standard sizes help
Roof windows come in a range of standard sizes, and many installed in the last few decades follow one of them. Where the old window is a known size, a new one of the same size usually drops in the same space between the rafters. The size code, such as MK04 or CK02, is a letter for width and two digits for height, and it tells you which window it is. If the old window is not a standard size, or the label has gone, the nearest size is chosen from measurements instead.
The flashing kit has to suit both the new window and the roof covering, and this is where a match is not just a matter of size. A kit for interlocking tiles is not the same as one for plain tiles or slate. If the covering has changed since the old window went in, the kit may change too. Our page on replacing a VELUX window like for like goes into this in more detail.
Flat rooflights: the kerb sets the size
A flat rooflight is fitted to a kerb, a short upstand that the roof covering is dressed up to. If the kerb is sound and the right height, a new rooflight can be made to sit on it. If the kerb is timber that has rotted, or low enough that water could pass over it, it is replaced and the roof covering is reworked around it. A new kerb can also be a chance to improve the insulation around the opening, which helps the new unit meet current standards.
Rooflights that were made to an imperial size, or fitted by a previous builder to a rough opening, can be out of square. A new unit is made accurately, and a kerb that is a few millimetres out in one direction is usually dealt with by adjusting the kerb, not by forcing the unit.
When the opening has to change
There are good reasons to change the size, and there are cases where the old opening cannot take what you want.
- Going bigger. A larger opening means cutting or trimming rafters, which is a structural alteration and needs Building Control approval. A trimmer may be needed on each side.
- Going smaller. The opening is reduced with new framing, and the roof covering is made good around it.
- Changing the type. Swapping a roof window for a flat rooflight, or a fixed unit for an opening one, can change the frame depth and the way it is weathered.
- A deeper build. If the roof has been insulated more since the old unit was fitted, the new unit may need a different frame depth to sit properly.
What it means inside the room
The size of the new frame sets how it meets the lining and the ceiling. If the new unit is slightly smaller than the old one, there is a gap in the lining to fill. If it is larger, the lining has to be cut back and made good, and the plaster around the opening is disturbed. Either way, the edge you see from the room is formed again, so the plaster and paint line need to be planned.
Where the ceiling is lath and plaster, as in many Georgian and Victorian houses in Exeter and Topsham, cutting back is done with care to avoid cracking the old plaster. We explain how in replacing a rooflight without damaging the ceiling. If you want a splayed lining, a change of unit size is a good moment to think about it, and our note on what a splayed rooflight lining is sets out what it does.
Building Regulations and the new unit
A complete replacement, frame and glass, has to be notified to Building Control: windows are controlled fittings and the new one must meet Part L. The new unit will usually perform better than the one it replaces, and we make the notification. If the opening is enlarged, the structural alteration is part of the same notification. A new sealed glass unit dropped into the frame you already have needs no notification and never raises the question of fit, because the frame stays.
How to check before you order
- Do not order from the glass size alone.
- Photograph the frame, the flashing and the lining from outside and inside.
- Note any label or size code on the frame, usually on the inside edge.
- Ask for the fit to be confirmed in writing in the quote and finish schedule.
If the glass alone has failed, you may not need a new frame at all, and replacing just the glass in a rooflight explains when that works. Otherwise, ask for a quote and we will measure the opening at the survey.