What if the timber around the opening has gone soft?

Why it matters

The timber around the opening does two jobs. It carries structural load, and it is what the new unit fixes to. Soft timber does neither reliably.

Fixing a new unit to decayed timber means the fixings have nothing to hold, the frame will not stay square, and within a couple of years the seal line moves and it leaks again.

What we do

Cut back to sound timber and splice in new. On a modest area that is straightforward carpentry: the affected trimmer or a section of it is replaced and the structure made good.

Where the decay is extensive, the opening is reframed, and where it has spread into a rafter or joist beyond the opening, that is structural and may need an engineer’s input.

How we price it

We flag it as a possibility at quotation with an indication of what it would add, so that if it happens it is a known risk rather than a surprise invoice.

We cannot see inside the structure before opening it up, and anybody who quotes a fixed price that ignores the possibility is either optimistic or intending to raise it later.

Warning signs beforehand

Staining or tide marks on the reveal. Peeling paint at the head. A frame that feels soft when pressed. Any history of a leak that was patched rather than fixed.

Where those are present we say at survey that the risk is higher than usual.

The argument for not delaying

Decay does not stop. A small area of soft timber this year is a larger one in two years, and the difference in cost between the two is considerably more than the cost of dealing with it now.

It is cut out and replaced

Decayed timber around a rooflight opening is not a reason to stop the job, and it is not something to fit a new window over. The affected timber is cut back to sound material, new timber is spliced or sistered in, and the opening is made good before the frame goes in.

It is normal work on an older installation and it is the reason our quotations name a contingency rather than pretending it cannot happen.

Why it happens

Almost always water that has been getting in slowly for years, from one of four causes: the wrong flashing kit for the covering, an underlay collar lapped incorrectly, the covering reinstated under the flashing rather than over it, or a failed seal at the frame.

Occasionally it is condensation rather than rain, where a missing insulation collar has kept the reveal permanently damp.

In every case it is a symptom, and the important part of the repair is fixing the cause rather than only the timber.

What we do when we find it

Stop, show you, and explain what it means before doing anything else. The scale ranges from a soft corner of a trimmer, which is an hour, to decayed rafter ends, which is a structural repair and a different conversation.

Where it is structural, it may need Building Control involvement and occasionally an engineer, and we will say so rather than quietly building over it.

Why an honest quotation names it in advance

Because the timber is behind the lining and under the covering, and nobody can see it until the old unit is out. A fixed price that assumes everything is sound is either carrying a hidden margin to cover the risk or it is going to change on the day.

We name the contingency with an indication of cost so the possibility is stated before you commit, not presented as an invoice afterwards.

How likely is it

On a unit that has been misted or leaking for years, a realistic possibility rather than a remote one. On a sound unit being replaced for age or performance, less likely.

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

What it means for the 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.

Why it is common on a replacement

The trimmers and rafter ends around a twenty five year old rooflight have had a long time to be exposed to any slow leak, and slow leaks around rooflights are extremely common because they produce no visible symptom until the timber is already gone.

The unit comes out, and that timber is visible for the only time in its life. It is the one chance to deal with it.

What happens

Anything soft is cut out and replaced. The extent is established by probing beyond the visibly affected area, because rot travels further along the grain than it appears to.

New timber is spliced or scarfed to sound material, treated, and the opening reformed to the correct dimensions.

Where the damage extends into a rafter beyond the opening, a repair is designed rather than improvised, and on a trussed rafter roof it needs an engineer.

How it should have been priced

As a provisional allowance in the quotation, so that finding it is a variation at a known rate rather than a stoppage and an argument on the day.

A quotation for a replacement of an old unit that makes no mention of timber condition is either optimistic or leaving you exposed. Ask before the work rather than during it.

Why it must not be skipped

The new unit is being fixed into that timber, and a frame fixed into soft wood will not stay square. Once it moves, the sash stops meeting its gasket evenly and the new installation starts leaking.

Covering rotten timber and fitting a new window onto it is the worst outcome, because it looks finished, costs the same, and fails inside a few years with the covering and the plaster back in place.