Why Replace Rather Than Repair a Rotten Rooflight?
A rotten rooflight is replaced rather than repaired because rot is always further advanced than the soft patch you can see. Fungal decay travels along the grain ahead of any visible discolouration, so cutting back to sound timber means removing far more of the sash than expected. Splice it and the moisture path that caused the rot is still there, feeding the new timber.
This is the single most common argument we have with people, and it is a reasonable argument to have. A splice looks cheap. Filler and paint look cheaper still. So it is worth setting out exactly what is happening inside that timber and why the sums do not work.
What rot does before you can see it
Wet rot in joinery is a fungus. It needs timber above roughly 20 per cent moisture content, and once it is established it feeds along the length of the grain because that is the path of least resistance for both moisture and the fungal strands themselves. The visible symptoms, darkening, cracking across the grain, paint lifting, arrive well after the fibres have started to break down.
Push a screwdriver into the soft area on a rooflight sash and you find the give continues for another 100 to 200mm past where the paint looked bad. On a bottom rail, which is where it almost always starts, the decay usually runs into the mortice at each end, because the joint holds moisture and the end grain there drinks it. That is why the honest answer to “can you just cut the rotten bit out” is that the rotten bit is the whole rail plus the joints at both ends, which is most of what holds the sash square.
There is a second stage most people never see. Once the sash has softened, the glazing rebate loses its grip on the glass unit, the perimeter seal breaks and water starts running down the inside face of the frame instead of the outside. The decay then jumps to the fixed frame, and the fixed frame is built into the roof.
Why splicing timber fails here
Splicing is a legitimate technique on plenty of joinery. On a rooflight it fails for four reasons that stack.
- The moisture source is still live. Rot is a symptom. On a rooflight the cause is a failed glazing seal, a failed flashing lap or a missing insulation collar creating condensation on cold timber. New timber spliced into the same wet detail rots on the same timetable.
- The new joint is the weak point. A scarf or half-lap in a sash rail sits in a component that flexes every time the window is opened and every time the roof heats and cools. Adhesive lines in that position, exposed to full weather on a slope, do not hold like a factory joint.
- You cannot re-seal what you cannot dismantle. To make a spliced sash weathertight again you need to take the glazing unit out, and a sealed unit that has been in a rebate for twenty years usually does not survive removal. Now you are buying glass as well.
- The frame is next. Even a perfect sash repair leaves the fixed frame in the roof, the same age, in the same weather, with the same seals. Doing half the assembly means being back at the same window inside a few years.
People do reseal and splice these units, and it does hold for a while. That is exactly the problem. It holds long enough to feel like the right decision and not long enough to be one.
The whole life cost, honestly
Set aside our position for a second and just do the arithmetic. A timber sash repair with new glass, scaffold access and repainting is not a small number, because the scaffold and the labour are the bulk of the cost and they are the same whether the unit is being patched or swapped. The window itself is often the cheapest part of the job.
| Approach | Addresses the moisture source | Resets the seals and glazing | Lets you see the trimmers and underlay | Guarantee |
|---|---|---|---|---|
| Filler and paint | No | No | No | None |
| Spliced timber in the sash | No | Partly, if the glass survives removal | No | On the splice only |
| Full unit replacement | Yes | Yes | Yes | 10 years workmanship |
Scaffold and access cost broadly the same in all three rows. That is what makes the middle row poor value rather than a sensible compromise.
Two rows of that table cost real money and leave the roof in a condition where the next winter is a question rather than a certainty. On a like for like replacement of an existing pitched unit up to 780 x 1180mm we publish a range of £1,250 to £1,850. Set that against a repair, a repaint, a new sealed unit and the scaffold twice, and the gap closes fast. Full ranges are on our pitched rooflight installation cost page.
What we find when the old unit comes out
This is the part that justifies the decision more than any of the above. Taking the unit out is the only way anyone sees the condition of the structure around it.
Typically, on a rooflight that has been rotting for a few years, the trimmer under the sill has a tide line and the top 5 to 10mm of it is soft. The underlay was cut for the original opening and never formed into a collar, so anything getting past the tiles has been arriving directly on that trimmer. The insulation around the perimeter is compressed and damp, which means it has been performing as a cold bridge as well as a sponge, adding condensation to the rain water already there.
None of that is visible from inside the room and none of it is reachable with a repair. Replacement lets us dry and treat the timber, sister or replace anything that has lost section, form the underfelt collar properly so water is discharged back out over the drainage plane, and fit a new insulation collar with the vapour control layer taped to the frame. That last detail is what stops the new window from developing a condensation problem that looks exactly like the leak you just spent money on.
Timber, conservation and what replaces it
A timber rooflight often sits on a period roof, and in Suffolk and north Essex that frequently means a conservation area or a listed building. Replacement does not mean losing the look. A conservation pattern unit sits flush with the roof plane, has a slim dark frame and a central vertical glazing bar, and is flashed in dressed lead rather than a visible aluminium kit. On a listed building, consent is required for the work regardless of which slope it is on. Our pages on what a conservation rooflight is and conservation area rooflight rules cover the detail.
Modern timber rooflights are available with factory-applied coatings and engineered laminated sections that behave far better than a 1970s solid softwood sash ever did. If the appearance does not need to be timber, a thermally broken aluminium or composite frame removes the decay question entirely.
Related reading
- Why a rooflight leaks only in driving rain, which is often the cause of the rot in the first place.
- Water pouring through a loft rooflight, if it has reached that stage.
- Draughty loft rooflight, where perished seals show up as air movement before they show as water.
- Pitched roof skylight installation, the parent guide.
Get a fixed price
Tell us the rough age of the unit, whether the frame is timber, and where the soft area is. Nobody needs to go outside or up a ladder to answer that. We survey from inside the loft and from the roof with our own access, across Ipswich, Suffolk and north Essex. Request a fixed quote and you will get one written price, with the unit, the flashing kit and the collars itemised, backed by a 10-year workmanship guarantee.
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.