The difference between a same-opening swap and an enlargement, how the old unit is measured, why the flashing kit matters, how a leak differs from condensation, and a typical UK cost.
A roof window that sticks, steams up or lets in a drip eventually raises the question of replacement. People search for the brand name, Velux, because that is the name on the frame, but the work itself is the same whichever maker built the original. The best-case job is a swap in the same opening: the old sash and frame come out, a new frame goes into the same hole in the roof, and the surrounding covering is made good with a new flashing kit.
That best case holds only if the new unit really does fit the existing hole. A different size, a different position or a different pitch turns a swap into an alteration, with rafters to cut and trim, and a different set of rules. So the first job is always to look and measure, not to order.
The sections below cover the swap and the enlargement, the measuring, the flashing, the tell-tale signs of a leak or condensation, the MyJobQuote average, and the regulations that attach. If you are choosing between a roof window and another kind of glazing, our page on roof windows and skylights compares them.
Same opening, or something bigger?
A like-for-like swap keeps the rafters as they are. The new frame sits within the same gap between two rafters and the same pair of battens, and only the flashing and the lining around it change. An enlargement is a different job. Making the hole wider or taller means cutting a rafter or a batch of battens and framing the new edges with trimmers (the cross-timbers that carry the load around an opening), so the roof structure is altered.
| Point | Same opening | Larger opening |
|---|---|---|
| Rafters | Left alone | Cut and trimmed |
| Structure | Unchanged | Needs a check |
| Inside | Lining made good | Plaster and lining redone |
| Planning check | Short | Fuller |
A bigger window can appeal once the new unit is on the drawing board and the old one looks small. If so, say so at the survey, because the same visit can price both and show the difference. The planning position for either route is checked at the survey, since a replacement window must still respect the roof-slope conditions in Class C, and the roof window planning pages cover the detail.
For a loft room there is one more reason to be careful. If the window is the escape route, its openable area and height above the floor are set by Approved Document B, and a new unit of a different size can change that. Keep the size, or recheck the numbers.
Measuring the old unit so the new one fits
The sizing of a roof window usually comes from the old frame, not from a tape held across the hole, because frames are made in standard widths and lengths and the new one has to match them. Looking for the following saves a wrong order, which is a costly mistake once the frame is on site:
- the label or marking on the old frame or sash, which often gives the maker's size code;
- the outside width and height of the frame, measured to the outer edge;
- the roof pitch, since some products are offered for a limited range of pitches;
- the covering profile, which decides which flashing kit is needed;
- the distance between the rafters on each side.
A tape measure from inside only gives the lining, which is smaller than the frame. A good survey measures from the roof or from photographs taken square on, and cross-checks with the label. The head and sill of the new frame must also land on sound battens.
Where the old unit is badly worn, an exact match may no longer be made, and the nearest size forces a decision between a slightly smaller window with extra lining, or a slightly larger one that changes the opening. That is the point at which a swap can become an enlargement, so it is raised before the order goes in, not after the old frame is lifted out.
Photographs of the old unit, from inside and out, help a surveyor settle this before anyone climbs up. You can send them on WhatsApp.
Flashing kits: the part that keeps the rain out
The frame gets the attention, but the edges are where a roof window meets the weather. The kit that surrounds the window, called a flashing kit, is a set of pre-formed pieces that carry water from the roof covering over and round the frame. It is made for a particular kind of covering, because a plain tile, a pantile and a slate all shed water differently.
A flashing kit does three things:
- it directs water that reaches the window down and past the frame;
- it makes a weathertight junction between the frame and the covering;
- it allows for the thickness of the tiles or slates, which differs by product.
When the old unit comes out, the old flashing comes with it, and a new kit is fitted. Reusing old flashing pieces on a new frame is a false saving, since the new frame sets at a slightly different height. Where lead soakers are used with a slate or tile covering, Code 3 is the lead sheet recommended for soakers alone, and the code for any other lead detail is chosen in the specification.
Battens around the opening also need to be right, and so does the underlay that dresses into the frame. If water has been getting in for a while, they may have gone soft, and replacing them is part of a proper swap. Our page on the roof window flashing kit goes further into the kit itself.
Leak or condensation: telling them apart
A wet roof window can mean a leak or condensation, and the two are easy to confuse. The cause decides the cure, so it is worth working out which you have before anyone is called out.
| What you notice | Likely cause |
|---|---|
| Misting on the glass in cold weather | Condensation |
| Water along the bottom of the frame on dry mornings | Condensation |
| Drips only after rain | Leak at the flashing or seal |
| Stain that spreads from one corner | Leak at that corner |
The test is timing. Condensation appears when the air indoors is warm and moist and the glass is cold, whatever the weather outside. A leak depends on rain. Wipe the sash dry, watch it over a dry night, and compare it with what happens during a shower. Black mould on the lining and damp reveals fit condensation, whereas a rust-brown stain or a wet batten fits a leak.
If it proves to be condensation, a new window may not fix it. The usual remedies are better ventilation in the room and, where the design allows, a window with a more efficient glass unit. If it proves to be a leak, the fault is as likely to be the surround as the window itself, which is why a flashing check should come before a new unit is ordered. Our pages on roof window condensation and leaking roof windows take each in turn.
One published cost, and what the Regulations ask for
Only one cost figure belongs here. MyJobQuote's cost guide, updated in September 2026, puts the average for a mid-range standard Velux at £1,100. Treat that as a general UK average, and no one's quote for a specific house. The flashing kit, access and making good inside all depend on the roof, so your own price comes after a survey.
Part L of the Building Regulations sets the performance. For new elements in an existing house, Table 4.2 of Approved Document L sets these limits:
- a window, roof windows included: 1.4 W/(m²·K), or a Band B (or better) on the Window Energy Rating scale;
- a rooflight: 2.2 W/(m²·K).
Ask the supplier which figure the unit is rated against and keep the rating documents. The roof window figure is the stricter one, and a rooflight quoted at 2.2 is not a roof window.
Beyond energy, the rules to keep in mind are these:
- If the window is an escape route from a loft room, keep an openable area of at least 0.33 m² with the bottom of the opening no more than 1100 mm above the floor.
- Ask the building control team for your district whether the swap needs notifying. Thanet District Council's team is on 01843 577522.
- Check the planning position for your address if the new unit differs in size or position, or if the house is a listed building or lies within a conservation area.
What happens on the day of a swap
A same-opening swap is a short, tidy job when it goes to plan, though the weather has a say because the roof is open while the frame is out. A sensible order runs like this:
- The room below is cleared and the floor and furniture sheeted, because dust and old sealant fall inwards.
- The sash is lifted out and the opening checked from inside.
- From outside, the covering around the frame is lifted back and the old flashing is removed.
- The old frame comes out, and the rafters, battens and any lining are examined for damp or rot.
- The new frame is fixed to the rafters, set square and level, and the new flashing kit is built in with the tiles or slates, working from the bottom edge up.
- The covering is made good, the sash is hung, and the inside lining is sealed and finished.
- The window is opened, closed and tested before the tools go.
The forecast matters more than the clock, so a swap is planned for a settled spell, and the opening is never left uncovered once the crew has gone.
Access is the other variable. A window low on a single-storey extension can be reached from a ladder platform, while one high on a gable usually needs scaffold. Should the scaffold need to stand on the road or pavement, a Kent County Council licence is needed and the scaffolders apply for it.
