What is found when the timber is opened up

Decayed timber

The most common finding. A unit that has been failing for years has usually been letting a little water past for some of them, and the timber immediately around the opening takes it.

Soft timber is cut back to sound material and spliced. There is no version of this job where a new unit is fixed to decayed timber.

The original installation was wrong

Regularly. The commonest is a flat profile flashing kit on a profiled tile, which explains a leak that was blamed on the unit for a decade.

Others: an underlay collar missing entirely, battens left unsupported over the opening and dropped, or on a flat roof a kerb too low for the covering ever to have terminated properly.

Where we find one, we say so and fix it, because refitting to the same detail reproduces the fault.

No insulation collar

Very common on older installations, and the explanation for reveal condensation the owner has lived with for years. Adding one during replacement costs very little.

No vapour control

Also common, particularly on flat roofs. Sealing the layer to the new frame is straightforward while the opening is exposed.

Pleasant surprises

Sometimes a sound opening in good timber with the original detailing done properly, in which case the job is quick and cheap. That is more common than the horror stories suggest.

How we handle the uncertainty

Flag it at quotation with an indication of what remedial work would add. We cannot see inside a structure before opening it, and a fixed price that ignores the possibility is either optimistic or intends to raise it later.

Why nothing is certain until the unit is out

The timber around a rooflight opening is behind the internal lining and under the roof covering. Nobody can see it, and no survey from a ladder or a photograph from the ground reveals its condition.

That is the honest reason a good quotation carries a stated contingency rather than a single confident number. A fixed price that assumes everything is sound is either carrying a hidden margin to cover the risk, in which case you pay for it whether or not anything is wrong, or it is going to change on the day.

Four things we find, in order of likelihood

1. The old flashing kit was the wrong profile. The most common finding by a wide margin.

A flat kit intended for slate fitted to a profiled interlocking tile, a universal flashing improvised to fit, or a head flashing lapped the wrong way. Where a unit has been leaking or the timber is damp, this is usually why.

It costs nothing extra to correct, because the new installation is done properly regardless, and it explains a great deal about the history of the window.

2. There was never an insulation collar. Frequently absent on work from the 1980s and 1990s.

Its absence shows as condensation on the plaster reveal every winter, which the owner has usually been living with and blaming on the window. Again not extra work, because we fit one as standard.

3. No underlay at all. Normal on any pre war roof that has never been stripped: tiles or slates hung on riven battens with the rafters visible from inside.

Modern weathering assumes a membrane to lap into, and where there is none the detail is built around the opening instead. Entirely workable, more time, and on a period property it should be anticipated rather than treated as a surprise.

4. Decayed timber. The one that costs money.

Where decay shows up

Water gets in at the top and runs down, so decay concentrates at the bottom.

The sill trimmer and the lower corners of the opening, where water that has passed the flashing collects.

Rafter ends either side of the opening, where a leak has been running down the side of the frame for years.

Battens around the opening, which are thin and go first.

The frame itself, which on an old timber unit may be soft even where the structure around it is sound.

You can check the frame yourself before anybody quotes. Press it at the sill and the lower corners with a thumb. Timber that gives has gone, and that changes the conversation from a reglazing job to a replacement.

How likely it is

It depends entirely on the history of the window.

On a unit that has been misting or leaking for years, decay is a realistic possibility rather than a remote one. The water has been getting somewhere, and timber is where it goes.

On a sound unit being replaced for age or thermal performance, considerably less likely.

On a period property with no underlay, more likely again, because there has been nothing behind the covering to catch water that got past the flashing.

What happens when we find it

Work stops and we show you, before doing anything else.

The scale ranges enormously. A soft corner on a trimmer is an hour and a thirty second conversation. Decayed rafter ends are a structural repair, may need Building Control involvement and occasionally an engineer, and are a different conversation entirely.

What we do not do is build over it. Decay is caused by water getting in and it does not stop until the cause is corrected, so a new window fitted over soft timber is a job that will be done again.

The repair

Affected timber is cut back to sound material and new timber is spliced or sistered in, sized to match what it replaces and fixed to carry the same load.

Where a trimmer has gone, a new one goes in. Where rafter ends have decayed, they are cut back and repaired or the rafter is sistered alongside.

Preservative treatment on adjacent sound timber is worth doing while it is exposed, and it is not a substitute for cutting out what has failed.

On a listed building

Different, and slower, deliberately.

Where a historic timber turns out to be significant, work stops and the conservation officer is consulted before anything is cut. Historic rafters may be part of what is listed, and repairing rather than replacing them may be a condition.

That is the correct sequence rather than an inconvenience, and it is one reason surveys on listed buildings establish where a unit can physically go before anybody discusses what it should be.

The rarer findings

An opening that was never trimmed. A rafter cut and nothing put in to carry the load. It happens, and it is corrected as part of the work.

A size that does not match the data plate, because a previous replacement altered the opening and packed the unit to suit.

Asbestos, occasionally, in older flat roof build ups and around some mid century installations. Where we suspect it, work stops and it is tested rather than guessed at. Removal is a licensed activity and not something we do.

Something living in the roof. Bats are protected and work stops if there is any sign of them, which is a legal position rather than a preference.

How we price it

A stated contingency with an indication of cost, so the possibility is priced before you commit rather than presented as an invoice afterwards.

Where nothing is found, nothing is charged. That is the point of naming it rather than burying it in the headline number.

What it means for your decision

Any timber decay means replacing the window rather than just the glass. Putting new glazing into a frame surrounded by decayed structure is money spent on the wrong part of the problem.

It is also an argument for not deferring a failing rooflight indefinitely. A misted unit is not urgent and it is not doing harm. A unit that is letting water in is, and every winter it continues makes the eventual job larger.

Rot, and how to read it

The most common discovery, and it is almost always around the sill and the lower corners of the opening, because that is where water sits.

Two kinds behave differently.

Wet rot is confined to timber that is actually damp. It is soft, dark and crumbly, and it stops where the moisture stops. Cut back to sound timber and it does not return provided the water source is fixed.

Dry rot travels. It can grow through masonry to reach new timber and it does not need the wood to stay wet once established. It is far less common and far more serious, and finding it changes the job from a repair into an investigation.

The test is a screwdriver or a bradawl. Sound timber resists; affected timber takes the point easily. Probe well beyond the visibly damaged area, because rot travels further along the grain than it appears to on the surface.

The other findings

Underlay cut straight across the opening with no collar. Extremely common on installations from the eighties and nineties, and it explains a history of unexplained damp. Water reaching the membrane above the window arrives at the cut edge and goes into the opening.

No insulation collar and an uninsulated reveal. Invisible from inside, and the cause of the cold band and condensation at the frame edge that the owner has lived with for years.

Trimmers that were never adequate. Occasionally a rafter cut without proper trimming, carrying load through nothing much. That is an engineer conversation rather than a carpentry one.

Old repairs. Filler, expanding foam packed into gaps, plates screwed over splits. All indicating somebody dealt with a symptom.

The one that stops work

Asbestos containing material, on buildings from the relevant period. Older flat roof build ups, some mid century boards, certain bituminous products.

Where it is suspected, work stops and the material is tested rather than removed. Any competent contractor raises this at survey where the building age suggests it, rather than discovering it with a saw.

How the repair is done

Affected timber cut out well beyond the damage, new timber spliced or scarfed to sound material with proper joints rather than butted and plated, treated, and the opening reformed square to the correct dimensions.

Where damage extends into a rafter beyond the opening, the repair is designed. On a trussed rafter roof it needs an engineer, because a truss is an engineered component.

Why the quotation should anticipate it

None of this is visible until the unit is out, and by then the roof is open and the crew is on site. A quotation carrying a stated provisional allowance turns a discovery into a priced variation. One that says nothing turns it into a stoppage and an argument.

On any replacement of a unit more than fifteen years old, that allowance should be there.