How much does it cost to swap a misted rooflight unit?

Two different jobs sit behind the words "misted rooflight swap". In one, the sealed glass unit comes out and a new one goes into the frame that is already there. In the other, the whole rooflight is lifted out and a new one is set into the opening. The mist looks identical from the floor, so the price can only follow the job once someone has looked closely at what surrounds the glass.

A frame that sits square, shuts with an even gap and has sound joints is a good home for new glass. A frame that has moved is a poor one, because fresh glass in a tired frame spends new money on an old problem. Our guide to whether to swap the glass or the whole unit walks through that decision.

A replacement sealed unit can be made to the measurements of the one it replaces, so the size is read off the old unit before anything else. On a VELUX roof window the size code is stamped on a sign near the top of the opened sash. The size code on that sign tells you the width and height, so a clear photo of the plate saves a ladder trip.

The glass is the second lever. Double or triple glazing is the first choice, and the third pane adds to the unit. VELUX also lists extras on some models: an inner pane of laminated glass, a dirt-shedding coating, a rain-noise build. What a given window offers differs from one variant to the next, so the shortlist is drawn up at survey. Pilkington names rooflights as one use for its self-cleaning Activ coating, which suits a unit that is awkward to reach with a cloth.

Last comes the opening. A manual unit and an electric one are different pieces of kit, and VELUX says a rain sensor is built into its electric and solar roof windows, shutting them as rain begins.

New glass meets the same weather the old glass did, so the survey looks at where that weather goes. On a flat roof, watch what rainwater does after a shower has passed. A puddle that stays against the kerb is a question about the fall and the outlet, and a new unit in the same kerb meets the same puddle. The kerb or upstand itself has to be sound and square to take a new unit.

On a pitched roof, a roof window sits in the plane of the slope and is dressed in with flashing that laps the tiles or slates around it. If the covering around an old window has slipped, or the flashing no longer suits a new unit, that is extra roof work beside the glass.

Standing water and a slipped covering are easier to find before a quote than after it, and that is the reason the survey comes first. Our misted rooflights page covers the whole replacement service.

The paperwork depends on the job. The Building Regulations treat windows, roof windows and rooflights as controlled fittings, so putting one in counts as building work. Swapping a whole roof window for an equivalent one is covered too. A glass-only swap that leaves the existing frame alone is outside the energy rules. Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver.

A whole replacement unit has an energy target. In Wales, Approved Document L says a replacement roof window should reach 1.4 W/m²K or Window Energy Rating band B, and a replacement rooflight 2.2 W/m²K. Ventilation carries over as well: an old window with trickle vents should be followed by one with vents of at least the same size.

A bigger opening, or a rooflight where there was none, becomes new-opening work. For a new rooflight, gov.wales expects Building Regulations approval to be needed in general, and a planning application can come into it too. Our guides to skylight Building Regulations in Wales and skylight planning permission in Wales set out the rules.

Everything above depends on something you can only see up close: the frame, the kerb, the flashing and the fall of the roof. A number worked out from a photo would be a guess at every one. With us the route is a free survey of the room and roof, then a fixed written quote, and our workmanship carries a written 10-year guarantee.

Before you ring, it helps to know which room sits under the glass, whether the roof is flat or pitched, and whether the window opens. A photo of the type sign on a VELUX sash is handy too. If you are still wondering whether mist matters, read is a misted rooflight a problem, and for what to expect afterwards, see how long sealed units last. To arrange the survey, ring 01792 929062 or use the booking form.