Open the sash of an old VELUX window and look at the frame. Somewhere on it sits a label with a short code, and that code does most of the work when you want the same window back in the same hole. Everything else about a swap follows from what the opening holds: how the rafters sit, how the battens run, how deep the lining is and whether anything behind the plaster has gone soft.
For the wider range, from blinds to motors, see our VELUX windows page, and for a rough idea of what moves the figure on a quote, our VELUX window cost guide lists the lines.
The code on the old frame
Lift the sash on a pivot window, or open a top-hung one fully, and the label is usually fixed to the frame, on a side or along the top. It carries a size code such as MK04. The letters give the width and the digits give the height, so a swap starts by finding the code and looking for a current window in the same pair.
A photograph of that label, taken with the sash open, tells us a great deal before anyone visits. It is better still alongside two more pictures: the window seen from inside with the lining in view, and the roof around it from the garden or the road. From those three we can usually say which size we expect to fit, and which parts of the surround we want to look at closely.
When the label has gone
Paint, damp and years of cleaning take labels off. If yours has gone, we measure the frame at survey and match the nearest code from the opening, and we also measure the gap between the rafters either side. Where an old window was installed in a hurry, the opening may be a slightly odd size, and the measurements tell us whether a standard code fits or whether the surround needs a little timber to close the difference.
A code is a starting point
Two roofs that carry the same code can ask for different work. One may have clay plain tiles that stand proud of the frame, the other a thin slate. One may have a lining that is deep because the roof was insulated later. Reading the code tells us which window to order; the roof tells us how it will be fixed.
Three photographs that save a visit
The label with the sash fully open, the window and lining from the room, and the slope around it from outside. Send them on WhatsApp to 07880 284133 and we can name the likely size before the survey.
What same size means in the roof
People say a swap is the same size, and what they mean is that nothing in the surround has to move. That depends on four things sitting right: the gap between the rafters, the height of the battens above and below, the thickness of the covering around the frame, and the shape of the lining inside.
Rafters are the first check. A roof window usually sits between two of them, with a small margin each side for the flashing. If the old window sat there happily, the new one of the same code will too. If the old one was squeezed in with a cut rafter, we look at how that was trimmed, because the next window will inherit it.
Battens come next. The old frame was fixed to battens at a certain height, and a new frame of a slightly different depth or a different fixing system may need those battens moved. Moving a batten or two is a small job that belongs on the quote.
The flashing is never reused
The old flashing is stripped with the old frame. A new kit is fitted for the new window, chosen to suit the tile or slate around it. A plain clay tile roof, an interlocking concrete tile and a slate each need a different kit, and our guide to roof window flashings for Kent roofs goes through them one by one.
The lining, from the room side
Inside, the lining is the splay of plasterboard or timber that carries the opening from the roof slope to the ceiling or the wall. Old linings are often thin, and the roof insulation around them may be thinner than it would be today. When the new frame goes in, the lining is taken back and refitted with insulation behind it, so the inside of the window stops being the coldest surface in the room.
| Part of the opening | Usually | Reason |
|---|---|---|
| Rafters either side | Kept | They set the width, so a matching code drops between them |
| Battens | Checked, sometimes moved | A new frame may fix at a different height |
| Flashing kit | Replaced | It is formed to the old frame and the old tiles |
| Frame and sash | Replaced | Seals, hinges and glass age together |
| Lining | Replaced | Stains and soft board mean damp has reached it |
| Blind and controls | Fresh ones ordered with it | Brackets are cut to a particular frame |
The new window has an energy limit to meet
A replacement roof window falls under Part L of the Building Regulations, which for an existing dwelling sets a limit of 1.4 W/m²K, or a Window Energy Rating of at least band B. The old unit may well have been fitted before that standard applied, so the new glazing is likely to be a clear step up for the room beneath it.
The glass is only part of the gain. The gap between frame and rafter is where heat leaks on an older job, and it is filled with insulation as the frame goes in. In a loft bedroom, the difference tends to show on a cold morning at the sill and in the lining, both of which should stay drier. Our guide to Part L and rooflights explains the limits for each type of unit, and the page on rooflight U-values shows what the figures mean on the label.
Why a roof window is not a rooflight
The two limits are different. A roof window sits in the plane of the roof and takes the window limit; a rooflight stands on a kerb and takes 2.2 W/m²K. A VELUX window of the kind this guide covers is a roof window, which is why the tighter figure applies to it.
A loft lining that was never insulated behind is the coldest surface in the room
Planning, escape and Building Control on a swap
Swapping like for like usually keeps the window where it was, so the planning question is about the house and the frame depth. On a house, a roof window is usually permitted development under Class C when the frame, measured with its casing, sits no more than 150 mm proud of the original roof slope. Flats and maisonettes carry no such rights. A condition on an old planning permission can have removed them from a house, and a listed house will almost always need listed building consent for a changed roof window. Sitting in a conservation area makes no difference to Class C on its own; an Article 4 direction would. We check these at survey, and the skylight planning permission guide gives the full list, including the rule for side slopes.
Building Regulations enter in two ways. The new glazing has to meet the energy limit above, and if the work counts as notifiable, we handle the Building Control notification. Skylights and Building Regulations goes through the steps.
When the old window was an escape window
In a chalet bungalow or a bungalow with rooms in the roof, a roof window can be the escape route from a bedroom. For that to work in a replacement, the opening has to be at least 0.33 m², with a clear width and height of at least 450 mm each, the bottom of the opening no more than 1,100 mm above the floor, and a sash that stays open without being held. A new window of a different height changes where the sill falls, and the sill is the figure to watch on a swap in a bedroom. If yours is a bedroom with a window like that, we raise it at the survey.
A loft room over a two-storey house sits higher than 4.5 m above the ground. Its escape normally runs down a protected stair, and the roof window in that room is not accepted as the escape route.
When the new window is a different size
Some owners use a swap as the moment to change a window. A taller one for more sky, a narrower one to clear a rafter, or a lower one that you can see out of from a bed. Each moves the job towards new building work.
A narrower window leaves a gap that is closed with timber and flashing, which is simple. A wider window means cutting at least one rafter and carrying the ends on a trimmer, with new flashing around a larger opening. A taller or lower window shifts the sill, and the sill decides what you see from the room and, in a bedroom in the roof, whether the escape figures still work.
Two windows where there was one
A pair of windows side by side in place of a single one is a new opening with a rafter between. It is structure first, then flashing, then linings. If that is your plan, see fitting a skylight into a finished roof, which covers the structure from the rafters up.
A roof that has moved on
On houses with a reroofed slope, the covering around the window may no longer be the same thickness as when the old unit went in. A thicker covering needs a deeper kit; we read this on the roof.
Questions that decide the route
- Is the code readable? If so, the nearest current match is the starting point.
- Does the surround hold up? Rafters, battens and lining are inspected once the old unit is out.
- Does the sill need to move? A different height changes the view and, in some bedrooms, the escape figures.
- Is the new size wider? Wider means a cut rafter and a trimmer, so the job is priced as a new opening.
Blinds, a motor and the other things ordered with it
A swap is the cheapest moment to add what the old window never had. The frame is out, the lining is open, and a cable or a bracket goes in with very little extra work. Blinds made for the window clip into brackets on the new frame, so they sit close to the glass. A motor, a rain sensor or a solar-powered sash can also be specified when the window is ordered.
Electric operation brings an electrician into the job, and a cable route behind the lining. Our electric opening rooflights page explains the controls, and the guide on powering an electric rooflight covers the three ways a motor can be fed.
If the old window is otherwise sound but the sash glass alone has misted, a full swap is not the only route. Our notes on glass only or whole unit set out when a glazed unit can be changed alone and when the frame has aged along with it.
Terms on a swap quote
- Size code
- The letter and digit pair on the frame label; the letters give the width and the digits the height.
- Flashing kit
- Formed metal pieces that seal the joint between the frame and the roof covering.
- Trimmer
- A timber fixed across the rafters to carry the ends of one that has been cut.
- Lining
- The splayed sides of the opening inside the room, from the window to the ceiling.
From the survey to the new frame
The visit is free, and it covers the label, the surround, the covering and the room beneath. We measure the opening, note the batten heights and look at the lining from below. The fixed written quote follows, with the window, flashing kit, lining and any blind or controls named as separate lines.
On the day, the floor and furniture are sheeted before the old unit is lifted out. The opening is inspected as soon as it is bare, since a rotten batten or a wet rafter is easier to put right with the roof open. The new frame, flashing and lining then go in, and the sash is hung and adjusted until it closes evenly. Anything the open roof reveals is explained before we carry on.
Our 10-year workmanship guarantee comes in writing with the job. To start, send the three photographs described above, or book a survey. If you are not sure the swap is the right route, the guide on common VELUX window problems shows how to tell a worn window from a leaking surround.
