A roof window that has done its time usually shows it in the frame first. The timber is soft at the bottom rail, the seals have set hard, the glass has misted between the panes, or the flashing around it has been patched again and again. At that point the question is no longer whether to repair it but what to put in its place.
There are two routes. A like-for-like swap fits a new unit into the opening the old one occupied, with a fresh flashing kit sized to match. An upgrade changes the size, the type or the position, and that brings the rafters, the planning rules and the escape rules into play. This guide covers both: how the old unit comes out, what the new one has to achieve under the energy standard, when a bigger opening needs structural work, and how the new window is flashed into the roof. If only the glazing has failed, read our answer on replacing roof window glass first, because that can be a smaller job. The wider picture is on our roof windows and skylights page.
Deciding between a swap and a bigger opening
Start with what the old window is telling you. If the opening is the right size, the position suits the room and the rafters either side are sound, a straight swap is the simplest path. The new unit is chosen to fit the existing gap between the rafters, so the structural timber stays untouched and only the covering around the frame is lifted and re-laid.
A different case arises when the old unit is smaller than the room now deserves, or when a loft that was once storage has become a bedroom. Enlarging means cutting at least one rafter and framing round the new opening, which is a structural job covered in our guide to trimming rafters for a roof window.
What the survey checks
Before anyone orders a window we look at four things. First, the measurements of the existing frame and the clear gap between the rafters. Second, the timber at the head and cill of the opening, since a long-standing leak often rots the trimmer that the frame sits on. Third, how the old flashing was formed and what it was made from. Fourth, the room below: ceiling lining, insulation and any water staining. We photograph each of these on your own roof so the quote lists what is actually there.
When a straight swap is not straight
Old windows were often made in sizes that no longer exist, or were fitted slightly off-square by a previous owner. A modern unit is made to a fixed manufacturer size code, so a gap that is a few centimetres out has to be closed with timber packing or opened with a small cut. Neither is difficult, but the factory order has to be right first time, which is why we measure the opening itself and not the old frame.
A replacement window is ordered against the measured opening, not the old unit. Frames shrink, swell and get repaired; the gap in the roof is the only dimension that stays put.
Taking the old unit out without wrecking the roof around it
The sequence matters because the roof is open to the weather once the covering is lifted. We lift the slates or tiles for a width either side of the frame and a few courses above and below, and set them aside in order. On a Stroud hillside, with narrow lanes and little room for stacking, that tidy storage is part of the access planning that goes into the quote.
Next the old flashing comes away. This is the sheet metal or lead that dresses the covering up to the frame, and in older installations it is often a mix of materials added over the years. The frame is then unscrewed from the rafters. Units that were sealed with mastic or fixed with nails need patient work with a bar so the timber that stays is not split.
The opening, once empty
With the unit out, the opening shows what the old window was hiding. Underlay that has perished at the edges is cut back and renewed. Battens gone soft where water ran down the side of the frame are replaced. Insulation that has been wet is lifted, because it no longer insulates and keeps feeding damp into the lining. If a rafter or trimmer has rotted, that is a repair on top, and we tell you before the new window is fixed, not after.
Weather protection during the swap
While the gap is open it is covered with a temporary sheet, and on a normal swap the new unit is fixed the same day so the opening is never left overnight. A bigger opening that needs framing is planned across a longer period, with the roof made watertight at each close of work.
What the new window has to achieve under Part L
A replacement window is a new building element, and the energy standard applies to it. Approved Document L (2021 edition) expects a replacement element to be no worse than the one it replaces and to meet the limits in its Table 4.2. The roof window figure is a U-value of 1.4, or alternatively an energy-rating label of Band B. A rooflight, the different product for flat or low-pitch roofs, is held to 2.2. The two sit in separate rows because the rules treat them as different things.
Timber windows were allowed a weaker figure of 1.6, or Band C, until 14 June 2023. The full standard has applied from 15 June 2023, so an old catalogue or quote that mentions the higher figure is out of date.
What the figure means in practice
A U-value measures how much heat passes through a material, and a lower figure is better. Reaching 1.4 means the glazing, frame and edge spacer work as a set, which is why current roof windows are double or triple glazed with coated glass and a sealed gas-filled gap. When you compare quotes, ask for the rating of the exact unit listed, because a product name alone does not tell you.
The frame is only half of it
How well the gap around the frame is insulated matters too. A purpose-made insulation collar, supplied with many window ranges, fills that gap. Without it the cold bridge around the frame can cause condensation on the lining even when the glass is perfectly good. Our guide to roof window U-values goes into the numbers in more depth.
Going bigger, going different: size, type and position
Once the old window is out, it is natural to ask whether the new one should be larger. A bigger opening gives more daylight, but it needs framing, a larger flashing kit and sometimes a rearranged run of battens. Position matters as well: a unit set too low in the slope gives sky but no garden, and one set too high leaves the lower part of the room dark. We cover this in our guide to where to put a roof window.
Swapping the type
A fixed unit can become an opening one, and a top-hung unit a centre-pivot, as long as the new product fits the opening. Electric opening is available on many ranges. MyBuilder's roof window guide (updated 21 August 2026) puts a standard centre-pivot window at about £305, electric ones at £420 to £590, and fitting at £350 to £600 for a standard job, with scaffold about £500 on top where the roof needs it. Those are national figures; your own written quote covers the work after the survey.
Planning rules for a larger unit
Roof windows fall under Class C of the General Permitted Development Order, with limits. Two limits matter for a larger unit. It may stand no more than 0.15 metres proud of the slope the roof already had, and it may not poke above the existing ridge line. A slope that forms a side elevation also needs obscured glass in every case, and the window must be fixed shut unless the opening parts sit more than 1.7 metres above the floor. A listed house is different, because the council treats every listed building as protected whatever its grade; ask the planning team before altering an opening on one. Our roof window planning answer explains the permission side.
Escape rules when the room is a bedroom
If the room below is a bedroom or loft room, the replacement has to be thought of as a possible way out. Approved Document B treats upper storeys up to 4.5 metres above the ground differently from higher ones. Where a habitable room on such a storey depends on a single stair, it needs an escape window or external door, or a direct route to a protected stairway. A loft room above that height follows a different set of rules.
What counts as an escape window
The document asks for a clear openable area of at least 0.33 m², with a height and width each of at least 450 mm, and the bottom of the openable part no more than 1,100 mm above the floor. The window must also stay open without being held. A roof window with a high cill can miss that last test even when the glass is large, so its position in the slope matters again.
A roof window is not always the escape route
In a typical two-storey house the loft room sits higher than that, so the roof window is not usually counted as its escape. It still has to be safe to use. For a replacement we check the existing arrangement and tell you if the new unit changes it. Whether a change of use needs a protected stair is a question for your registered building control approver or the council's building control team.
If a change of window also changes how the room is used, tell the building control body before work starts, not after.
New flashings: the part that keeps the roof dry
A flashing is the weatherproof metal strip that stops rain reaching the gap between frame and covering. On a roof window it is normally a kit made for that model and for the type of covering. A kit for plain tiles is not the same as one for a profiled tile, a slate or stone slates, because each covering sits at a different height above the battens.
The kit is a set of pieces: a bottom apron that sheds water onto the covering below, side gutters that carry water past the frame, and a top piece that covers the head. Where the covering is thick or uneven, as on stone slates, the kit may need adapting with lead work in place of the standard pieces. Our guide to roof window flashings gives the full account.
Why the old flashing is not reused
The old flashing was made to match the old frame. A new frame, even from the same maker, will not line up with the old pieces, and any gap between them is a route for water. Reusing a bent or weathered flashing to save a little removes the main protection the new window has.
Underlay details
Under the covering, the membrane around the opening is cut and lapped so that water getting past the tiles is led away from the frame and out at the bottom, not into the roof space. This is the second line of defence. Done properly it is invisible; done carelessly it is why a new window leaks in its first winter.
The kit is fixed and the covering returned around it, every slate and tile going back in the course it came from. On a Cotswold stone roof that means reusing the sorted stone in its original order.
Cost, order of work and what the quote lists
A replacement roof window is a modest job beside a new roof, but the cost depends on more than the unit. The MyBuilder figures above cover a standard job: the window, the labour to remove the old one and fit the new, and scaffold where needed. They leave out rotten timber, enlarged openings, extra insulation and making good plaster inside, so a survey is how the real number emerges.
What goes on a written quote
After the survey we specify the window by model and size, the flashing kit, any timber repairs, scaffold or access equipment, labour, waste and making good in the written quote. If the rafters need work, we tell you, and it is included in the quote rather than folded into a vague total. VAT appears on the quote too, as roofing work is normally standard-rated.
Doing several windows together
If a roof has several old windows, replacing them in one visit usually makes better use of scaffold, one of the larger fixed costs on a small job. We still assess each opening on its own, because one may be a straight swap and another may need timber repairs.
After the work
When the new window is in, we check that it opens freely, that the seals sit evenly and that the flashing is dressed neatly. Waste goes with us. Our installation carries a written 10-year workmanship guarantee, while the window has its own manufacturer warranty. If you are also weighing up leaks on an existing unit, our guide to why roof windows leak and our account of flat roof rooflights cover the neighbouring cases.