What if the timber around the opening has gone soft?
It gets cut out and new timber goes in. Soft wood around a rooflight opening is a common find, it is rarely as extensive as it first looks, and it adds work to a day rather than stopping it. What it does do is change a price, which is why a survey goes looking for it before a figure is agreed rather than discovering it with the roof open.
This page covers why the wood softens, how to tell wet timber from decayed timber, how far the damage usually travels, and what putting it right does to the programme. It sits within the wider skylight replacement work Sunspire carries out across Chelmsford and the thirty miles of Essex around it, where a large stock of nineteen seventies openings is now reaching the age at which this shows up.
Why timber around an opening softens
Fungal decay in structural softwood needs one thing above all others: moisture content held above roughly twenty per cent for a sustained period. Dry timber in a ventilated roof will sit there for a century and be perfectly sound.
Around a rooflight there are three ordinary ways that moisture arrives. Water getting past a perimeter detail, usually at a lap or a corner, and running along a trimmer before it ever shows on a ceiling. Condensation forming inside the roof build-up where warm moist air from the room below has found a route into a cold void. And a cold bridge at the kerb, where the frame conducts heat out and the timber immediately behind it runs several degrees colder than the rest of the structure, so it is the first surface in the roof to reach dew point on a January night.
Wet timber and decayed timber are not the same finding
This distinction decides how much work follows, and it is worth understanding before anybody quotes on it.
Timber that is simply wet has lost none of its strength. Dry it out, remove the source of the water, and it goes back to doing its job. Timber that has decayed has had its cellulose consumed by fungus, and that loss is permanent. The section may look intact and still be carrying almost nothing.
The field test is a bradawl or a screwdriver pushed into the face with light thumb pressure. Sound softwood resists and takes the point a millimetre or two. Decayed timber accepts it fifteen or twenty millimetres and the withdrawn point brings fibre with it. Brown rot shows as cuboidal cracking across the grain and a darkened, crumbly texture. Wet rot is softer, stringier and often darker at the surface than within.
Probing the trimmers before a price is fixed
On a pitched slope with loft access above, the trimmers around a roof window can be inspected directly, and that is the single most useful ten minutes of a survey on an older unit. The three places worth probing are the head trimmer, the underside of the sill trimmer where water collects, and the ends of the doubled rafters where they bear.
Where the ceiling is finished and there is no void to get into, the evidence is indirect: tide marks on the internal lining, a reveal that has moved, nail heads showing through paint, or a moisture meter reading taken through the plaster. None of those is proof, and all of them are enough to put a contingency in the conversation before the work starts.
How far the decay usually travels
Further than the stain suggests, and almost always along the grain rather than across it. Water runs down a timber face and wicks into end grain, so decay concentrates at cut ends, at bearings, and at the lowest point the water reached.
The working rule is to cut back at least three hundred millimetres beyond the last fibre that gives under a probe. That sounds generous and it is deliberate, because the fungal front runs ahead of the visible damage and leaving a short length of infected timber against new wood in a still-damp void is how the same conversation happens again in five years.
Splicing new timber into an existing structure
The work itself is ordinary carpentry done in an awkward position. The affected member is propped so the roof is supported independently, the decayed length is cut out square, and new treated softwood of the same section or heavier is spliced in.
How the splice is made depends on what the member is carrying. A trimmer taking load from cut rafters is usually sistered: a new full length timber bolted alongside the original so the joint is bypassed entirely rather than relied upon. A short length at a bearing can take a scarf with a bolted plate. Where the rafter ends themselves have gone at the wall plate, the honest answer is a new plate section and new rafter feet, which is a bigger day.
Chipboard decks and the flat roof version
On a flat roof the same problem presents differently, because the structure at risk is usually the deck rather than a trimmer. Nineteen seventies and eighties rear extensions across the county were decked in chipboard, and chipboard that has been wet delaminates rather than rots. It swells, loses its faces and turns to a fibrous mass that holds no screw at all.
It is replaced back to the nearest joists on either side, in whole boards, screwed down at proper centres. Where the roof is a cold deck and the void has been sealed rather than ventilated, the insulation between the joists usually comes out saturated as well, and it is worth getting that out while the deck is open.
What it adds to the day
Timber is inexpensive. Labour and access are what move the figure, and the honest range is half a day to a full day added to a straightforward unit swap, more where a trimmer has to be replaced full length or the deck comes back several boards.
The programme rule does not change. The roof is closed and watertight at the end of every working day, so a longer job means the covering is made good temporarily rather than the opening being left over a night. Where a structural member needs replacing rather than doubling, Building Control is told and the work is inspected as part of the notification that runs with the job anyway.
Where this turns up across Essex
Two patterns account for most of it. Flat roofed rear extensions and garages of the nineteen seventies, particularly across the Harlow and Basildon new town stock, where a domed unit sat on a shallow timber kerb with no insulation behind it and forty winters of condensation ran down the inside face.
The other is the Victorian rear addition, common in Chelmsford, Colchester and Maldon, where a rooflight was let into original boarding at some point in the nineteen eighties without the underlay being dressed properly. The boards themselves are usually excellent timber. It is the last two hundred millimetres either side of the opening that has gone.
Keeping the new opening dry
Cutting out decayed timber solves yesterday. What stops it recurring is the detailing around the new unit, and it is the same short list every time: upstand height, the fall of the roof approaching it, the laps in the flashing or membrane, and where the covering terminates.
Two additions matter specifically after a decay find. Insulate the kerb itself, so the timber behind the frame runs warm and stops being the coldest surface in the build-up. And make sure the void either breathes or is properly sealed against the room, because a half-ventilated cold roof is the condition that caused the problem in the first place. Those details are invisible once the job is finished, which is exactly why the workmanship carries a ten-year guarantee.
Related questions
- Do rooflights need planning permission?
- Do I need building regulations approval for a rooflight?
- Who notifies building control about a rooflight?
- How much does a rooflight cost?
- Do rooflights add value to a house?
- Is a rooflight cheaper than a dormer?
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