When a homeowner asks us to replace a VELUX window, the old one may have been in the roof for decades. The seals have gone soft, the glass has misted between the panes, the sash is stiff or the flashing has started to let water past. This guide follows a full replacement from the first look at the roof to the finished reveal inside, so you know what to expect. It belongs to our VELUX window service for Coventry, Kenilworth and the towns within 30 road miles.
How to tell when a window needs replacing
A roof window gives plenty of warning before it fails. The signs fall into two groups, those you see from inside the room and those that show on the roof.
- Droplets or a grey film trapped between the panes, which means the sealed glass unit has failed.
- Damp patches or staining on the lining and the ceiling around the frame.
- A sash that sticks, drops or no longer closes squarely against its seals.
- Condensation that returns every winter on the glass and runs down onto the sill.
- Moss, cracked mortar or lifted tiles around the flashing outside.
Not every one of these means the whole window has to go. A failed sealed unit in a sound frame is a glass-only job, and fitting new glass into a good frame is not a controlled fitting. A frame that has rotted or a flashing that has corroded points to a full replacement. We tell you which applies at the survey, and we write it on the quote.
Replacing the whole unit versus changing only the glass
This distinction matters because it decides the paperwork as well as the price drivers. Changing only the glass leaves the frame, the flashing and the lining in place, so the job is smaller and quicker. Replacing the whole unit takes the frame out of the roof, and the new unit has to meet the current energy standard.
| Question | New glass only | Whole unit |
|---|---|---|
| What comes out | The sealed glass unit | Sash, frame and flashing |
| Building Control | Not a controlled fitting | Notifiable building work |
| Suits | A sound frame with failed glazing | A worn frame, leaking flashing or an ageing unit |
Where does the leaking actually start
People often blame the glass for a leak that begins at the flashing. Water follows gravity and the line of least resistance, so a drip inside the room can enter the roof several courses of tile away from the window. The frame sits in the middle of a chain of parts: the tiles, the flashing kit, the underfelt, the insulation collar and the lining. A fault in any link looks, from inside, like a leaking window.
For that reason we look at the whole opening before we recommend a replacement. Where the flashing has failed and the frame is otherwise sound, a new flashing kit may be all that is needed. Where the frame has absorbed water for a long time, the timber behind the lining can be wet, and that changes the plan. We open the lining at the survey, where you are happy for us to do so, and look at the timber before we set the specification.
The replacement day, step by step
A like-for-like replacement of a roof window normally runs as a single visit, subject to weather and to what the old opening reveals once it is open. The order of work is predictable.
- Protect the room, lifting furniture away and covering the floor beneath the opening.
- Strip the old flashing and lift the tiles or slates that surround the frame.
- Release the old frame from the rafters and lower it out.
- Check the rafters and trimmers, and renew the underfelt collar around the opening.
- Fit the new frame to the same size, plumb and square, and fix it to the rafters.
- Install the new flashing kit, working from the lower edge upwards, and relay the tiles on top.
- Hang the sash, adjust the seals and test the opening and closing.
- Seal the junction between lining and frame inside, ready for the finish.
Because the opening is already formed, a replacement in the same size leaves the rafters alone. That keeps the job short and avoids new structure. Fitting a different size is a bigger task, and our guide to VELUX window sizes explains how that choice plays out.
Done properly, the best replacement is the one you stop noticing: the flashing is dry, the sash closes quietly and the reveal looks as though it was plastered with the room.
Building Regulations and the new unit
Replacing a whole rooflight or roof window unit is building work. It is notifiable, and Part L applies to the new unit. In an existing dwelling, Part L 2021 sets one limit for roof windows and a different one for flat-roof rooflights and lanterns; the Part L guide gives both. A unit in a pitched roof counts as a roof window, so the figure for a slope is the roof window figure.
Where the work is notifiable, we make the Building Control notification ourselves. The job is then signed off either by the local authority or by a registered building control approver, and you receive a completion certificate to keep with the house papers. Our page on building regulations for a rooflight covers what Building Control looks at.
Choosing the specification of the new window
A replacement is a chance to improve on what the old window did. The frame is the same size, but the glazing, the opening action and the finish can all be different, and each choice changes how the room feels.
- Glazing. A modern sealed unit with low-emissivity coatings holds heat better than the older double glazing it replaces, and some specifications add laminated glass on the inner pane for extra strength.
- Opening action. A centre-pivot sash turns on a central axis so you can clean the outside from within, while a top-hung sash opens outward from the base and gives a view out when raised.
- Frame finish. White-painted pine suits most bedrooms and lofts, while a polyurethane-coated frame stands up better to damp in a bathroom or kitchen.
- Operation. Manual handles are the standard, and an electric or solar-powered opening can be added where the window is out of reach.
We discuss these choices at the survey and set the final specification out on the written quote, so the unit that arrives is the unit you chose.
Finishing inside the room
A roof window is only as good as the reveal around it. When a window has been in place for decades, the lining often has cracks, stains or a poor junction with the plasterboard. Replacing the window gives the chance to improve all of that.
Where the roof depth allows, we splay the reveal so that the opening widens toward the room. A splayed reveal catches more daylight and lets it fall further across the ceiling and walls. We then plaster the splay, make good the surrounding plasterboard and leave the surface ready for your decorator or for paint. Where a splay cannot be formed because the roof build-up is shallow, we tell you at the survey and finish the lining square and neat.
Blinds, controls and fittings
Old windows often carry old blinds. When the frame goes, the brackets and cassette fixings usually go with it, so it helps to decide at the quote stage whether the blind will be reused or replaced. Our answer on fitting blinds to a VELUX window covers the compatibility questions. If you want the new window to open electrically, say so early, because a powered unit needs a cable route and a control point planned in advance.
What shapes the cost of a replacement
We do not quote figures in a guide, because each roof is different and a fixed written quote is the only honest number. What we can describe is what moves it.
- Whether it is glass only or a whole unit.
- The size of the frame and the specification of the glazing.
- The tile or slate type, and how much of the covering must be lifted and relaid around the frame.
- How high the roof is and how the scaffold or access is arranged.
- The finish inside, from a simple lining to a splayed and plastered reveal.
- Any extras, such as electric operation or a new blind.
The quote lists each of these separately, so you can see where the money goes and decide what to change.
Planning, conservation areas and old houses
Replacing a roof window in the same opening does not usually alter the look of the house, but older streets raise some questions. Being in a conservation area does not by itself end Class C permitted development, so a rooflight that stands proud of the roof plane by 150 mm or less and stays below the ridge is still allowed there. Some streets have those rights withdrawn by the council, and listed buildings need listed building consent regardless. A flat or maisonette carries no householder permitted development at all. Our planning guide for Coventry rooflights sets the full picture out, and we check the address before quoting.
Looking after the new window
A new window stays dry and quiet for longer when it is cared for lightly. Wipe condensation from the sash seals in the colder months, and keep the sill and lining clear of anything that holds moisture against the frame. Open the window for a short spell each day to let the room air out, particularly after bathing or drying clothes in the room.
Look at the roof side once a year if you can see it safely from a window or from the ground with binoculars. Moss, lifted tiles or debris collecting in the upper flashing are easy to spot early, and a quick word with us then saves a bigger conversation later. Keep the maker's paperwork and our completion certificate together, because both help when you sell the house.
What the quote shows before any work starts
Every replacement starts with a free survey. We read the data plate, check the frame and the flashing, and look at the lining from inside. You then receive a fixed written quote that lists the unit, the specification, the flashing kit, the internal finish and the Building Control notification. Nothing is ordered until you agree it. The finished work is covered by our 10-year workmanship guarantee, and the unit by the maker's own terms.
If you have an old window you are unsure about, book the survey and let us look at it. A clear picture of what is wrong, and a straight statement about whether it needs new glass or a whole new unit, is worth more than a guess. Our guide on how long VELUX windows last helps you judge what age to expect from a window before it is due for replacement.
