Rot in a timber VELUX frame almost always starts in one place: the bottom rail of the sash, the horizontal timber along the lower edge of the opening part. That is where water sits longest and where the internal finish takes the most punishment from condensation running down the glass. Once the timber there has gone soft, the rot is usually already into the frame behind it, and at that point the window is a replacement, not a rescue. You cannot fill or paint your way back from wet, spongy timber in a roof window. This page shows you where to look, how to test it safely from inside, and why the finish you choose next matters more than most people realise.
Apex Skylights replaces roof windows across Ipswich, Suffolk and north Essex. Our installers have 25 years fitting VELUX, Keylite and Roto units, and we do not patch or resurface rotten frames, because on a roof window the rot is a symptom of age that the whole unit shares.
How an older roof window is built, and why it rots
Older roof windows are made from laminated pine, finished on the inside with a clear lacquer or a paint, and capped on the outside with aluminium so the weather never touches bare timber. That construction is sound when it is new and maintained. The problem is what happens over fifteen or twenty years of a loft or bathroom environment.
Timber rot needs moisture and time, and a roof window offers both. Warm, damp air inside the room rises to the highest, coldest surface, which is the glass, condenses there, and runs down onto the bottom rail of the sash. If the room is a bathroom, an ensuite or a poorly ventilated loft, that happens every day. The lacquer or paint is meant to keep that water off the wood. It does, right up until the finish develops its first hairline crack or the seal at a corner joint opens slightly. After that, water gets behind the finish and stays there, held against the timber by the very coating that was supposed to protect it.
That is the trap with lacquer and paint maintenance on a roof window. A coating in good order sheds water. A coating with one flaw traps it. And because the failure hides behind an intact-looking finish, the timber can be soft long before the surface tells you anything is wrong. By the time the lacquer is visibly lifting and staining dark, the wood underneath has usually been wet through several winters.
Testing your own frame, safely, from inside only
You can check a roof window frame yourself in a couple of minutes, and you never need to go near the roof to do it. Do not go onto the roof. Everything worth knowing is reachable from inside the room with the sash rotated to the cleaning position.
- Rotate the sash and lock it. Swing the opening part round to the cleaning position and engage the catch so it stays put. Now the bottom rail of the sash and the lower frame member are both in front of you.
- Look before you touch. Sound timber under lacquer is even in colour. Rot shows as dark grey or black staining, a lifted or flaking finish, and sometimes a fine crazing in the lacquer along the bottom rail. Green or black mould on the surface is a damp warning even if the timber is still firm.
- Do the thumb-press test. Press the bottom rail of the sash firmly with your thumb. Healthy timber is hard and does not move. If it gives, dents, or feels spongy, the timber has gone. Work along the rail and into the corners, because rot travels along the grain and collects in the joints.
- Check the frame, not just the sash. Press the lower fixed frame timber the same way. If the sash bottom rail has failed, the frame below it has very often taken water too, because they share the same run-off.
- Look for the giveaways at the head. A black mould line across the top reveal is usually a cold-bridge condensation problem from a missing insulation collar rather than rot, but it belongs in the same survey because it tells us how the window has been performing.
If the thumb-press gives, stop maintaining and start planning. Soft timber cannot be filled, treated or over-painted back to strength in a roof window. Wood filler bridges a hole; it does not restore a rail that carries the sash and holds the weatherseal. Once the bottom rail is soft, the replacement conversation has started, and the only open question is timing.
Why we replace rather than repair a rotten frame
On a wall window a joiner can sometimes splice in new timber. On a roof window it does not work, for reasons specific to how the unit sits in the roof.
- The rot is rarely confined to what you can see. By the time the sash bottom rail is soft, moisture has usually tracked into the frame, the corner joints and sometimes the timber the fixings bite into. Repairing the visible rail leaves the hidden rot to carry on.
- The sash carries the glazing and the weatherseal. A roof window sash is a structural and weatherproofing component, not trim. Splicing it compromises the very geometry that pulls the sash evenly onto the gasket. A frame a few millimetres out will not seal, and that is a draught and a leak within a few years.
- The rest of the window is the same age. A frame that has rotted has usually also blown its sealed unit, worn its hinges and reached the end of its flashing life. That parallel ageing is covered on our misted VELUX glazing page. Renewing one soft rail on a twenty-year-old unit spends money on the frame while leaving four other components ready to fail.
So we do not resurface or splice rotten roof windows. The route is a full replacement, and it is the honest one, not the expensive one. On a like-for-like swap the old unit comes out and a new one goes in without touching the rafters, in a day. The wider case for renewal over patching is set out on why an old VELUX is replace, not repair.
Choosing the right finish next time
If your last window rotted in a bathroom or a humid loft, the finish is the decision to get right on the replacement. Natural lacquered pine is the traditional look and it is fine in a dry bedroom. In a wet room it is the finish most likely to repeat the failure you have just had. Modern white polyurethane-finished units are built for exactly the humid rooms where pine struggles, and the small extra cost is the right money to spend.
| Finish | What it is | Best suited to | Avoid in | Maintenance |
|---|---|---|---|---|
| Lacquered pine | Clear lacquer over laminated pine, the traditional finish | Dry bedrooms and living lofts, low-humidity rooms | Bathrooms, ensuites, kitchens, poorly ventilated lofts | Periodic re-lacquering; a single flaw traps water |
| Painted timber | A factory white paint over the pine core | A white interior look in a moderate-humidity room | Persistently wet rooms; still timber-cored beneath | Repaint when the coating chips or crazes |
| White polyurethane | A polyurethane finish moulded over the timber core, no exposed wood grain to wet | Bathrooms, ensuites, kitchens, utility rooms, humid or unventilated lofts | Nowhere on humidity grounds; a modest add on cost only | Wipe clean; nothing to re-coat |
The white polyurethane finish is a modest addition to the unit price and is the right choice in any bathroom or humid room. We flag it at survey wherever the room warrants it.
One practical point that catches people out: the finish is chosen at the same time as the size code and the blind fit, and the blinds you have will not necessarily carry across to the new frame. That is worth reading about before you order, on our blind compatibility page, so the finish and the blinds are budgeted together.
The coastal difference
Where you are changes how fast a frame rots. Salt-laden air off the North Sea holds moisture against timber because salt is hygroscopic, so it stays damp between showers rather than drying out. On an east or north-east facing slope within a mile of open sea at Felixstowe, Harwich, Frinton or Aldeburgh, we see frame rot at around fifteen years that would take twenty five inland. If your window sits on a seaward slope and was fitted around 2008, it is worth doing the thumb-press test now rather than next winter. The full coastal picture, including why we specify 316L stainless and upgraded flashing there, is on our coastal VELUX frame rot page. Estuarine spots like Woodbridge and Manningtree sit somewhere in between, moderate salt loading rather than open-sea exposure.
What replacement looks like
A rotten roof window is a replacement job, and on a like-for-like swap it is quick. We survey, read the data plate or measure the structural opening, and check the covering so we specify the right flashing kit. The old unit comes out, we inspect the trimmers, and if we find rot in the timber the window was fixed into we photograph it and price it before we go further, never quietly. Then the new frame goes in, packed and squared, both collars fitted, the flashing kit dressed into your covering, and the covering re-hung. The roof is watertight the same day. The full stage-by-stage sequence is on our VELUX window installation hub.
Common questions about rotten roof window frames
Can a rotten VELUX frame be repaired or treated?
No, not on a roof window. Once the bottom rail of the sash is soft, the rot has usually spread into the frame and joints, and the sash is a structural, weatherproofing part that cannot be spliced without compromising how it seals. Filler and treatment might tidy the surface, but they do not restore timber that carries the glass and the weatherseal. The route is a full replacement, and on a like-for-like swap it is a one-day job.
How do I check my roof window for rot without going on the roof?
You never need to. Rotate the sash to the cleaning position from inside and lock it, then press the bottom rail of the sash firmly with your thumb. Sound timber is hard and does not move. If it gives or feels spongy, the timber has gone. Check the frame timber below the sash the same way, and look for dark staining or a lifted, crazed lacquer. Do not climb onto the roof to inspect a window.
Why does the lacquer keep failing in my bathroom roof window?
Because a bathroom pushes warm, damp air onto the coldest surface, the glass, which runs condensation onto the bottom rail every day. Lacquer sheds water while it is perfect, but the first hairline crack lets water behind the finish, where it is then trapped against the timber. In a wet room that is a losing battle. The fix is not more lacquer but a white polyurethane-finished unit, which has no exposed wood grain to wet and only needs wiping clean.
Should I get a white finish instead of pine for a humid room?
Yes. In a bathroom, ensuite, kitchen or unventilated loft, a white polyurethane-finished unit is the right choice. The polyurethane is moulded over the timber core so there is no bare wood grain for condensation to soak into, and it wipes clean rather than needing re-lacquering. It is a modest addition to the unit cost and it is the difference between a window that lasts and a repeat of the rot you have just had. We recommend it at survey wherever the room is humid.
Get a fixed price for a rotten roof window
Send a photograph of the sash bottom rail from inside, one of the window from the room, and one of the roof from the garden showing the covering. That is usually enough for us to say whether a like-for-like replacement fits and roughly what it costs. Then we survey, confirm the opening and the flashing kit, advise on the right finish for the room, and give you one written price.
That price is fixed, and every installation carries a 10-year workmanship guarantee. Request a fixed quote for a rotten VELUX frame in Ipswich, Suffolk or north Essex, or see our published prices across every service first.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.