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12 Oct 2026
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A soft patch on a loft floorboard. A musty, mushroomy smell when you push open the hatch. Small round holes peppering a roof timber that were not there last year.
None of these look dramatic on their own, which is exactly why timber decay in a roof gets left alone until it is a much bigger job than it needed to be.
Wet rot, dry rot and woodworm are three different problems with three different causes. UK homeowners often lump them together as “the roof’s gone soft” and hope it sorts itself out.
It will not. Left alone, any of the three can weaken the structure that holds your roof covering up, not just the covering itself.
This guide explains what each one actually is, how to tell them apart, what causes them, and what proper treatment involves. It also covers what it costs in 2026, and where home insurance does and does not help.
“Timber decay” covers three distinct problems. Knowing which one you are dealing with matters, because the treatment, the cost and the urgency are different for each.
Wet rot (most commonly caused by the fungus Coniophora puteana) needs a constant, fairly high level of moisture to take hold. It stays close to its water source.
A leaking valley, a cracked tile, or a blocked gutter pushing water back under the roof covering for months are classic triggers.
Affected timber usually darkens, feels soft and spongy when you press it, and can crumble along the grain.
Because wet rot stays localised, it is often the cheaper and faster of the two fungal problems to put right, provided the moisture source gets fixed at the same time.
Despite the name, dry rot (Serpula lacrymans) also needs moisture to start. Once established, it can spread through masonry and behind plaster in search of new timber, well beyond the original wet patch.
That is what makes it the more serious of the two.
Look for timber that has shrunk and cracked into small cube-like blocks, a cotton-wool-like white or grey growth (the mycelium), and rust-red spore dust settling on nearby surfaces. A strong, musty mushroom smell in an enclosed loft space is another giveaway.
Dry rot that has been active for a while is rarely a localised fix.
Woodworm is not rot at all. It is the larvae of several UK wood-boring beetles, most often the common furniture beetle, tunnelling through structural timber.
They emerge as small round flight holes, typically 1 to 2mm across, usually in late spring. Fine powdery wood dust (frass) around the holes is a sign of recent, active infestation rather than old, treated damage.
A handful of old, discoloured holes in a Victorian roof void is not automatically an emergency. Fresh holes, pale new dust, and soft timber nearby usually is.
None of these three problems announce themselves from the ground. By the time staining shows through a ceiling or a roofline visibly sags, the timber behind it has usually been losing strength for a long time.
If you are not sure what you are looking at, that is reason enough to get it checked rather than guess. A proper inspection costs far less than treating a problem that has been allowed to spread for another two winters.
All three problems share one root cause: moisture getting into timber that was built to stay dry.
The usual routes in are a damaged or missing tile or slate, perished lead flashing around a chimney or valley, or a blocked gutter pushing water back under the eaves.
Poor roof ventilation plays a part too, trapping warm, moist air inside a cold loft space.
That last one catches a lot of homeowners out. Insulating a loft without maintaining adequate airflow is a well-documented way to create exactly the damp, still conditions that both wet rot and dry rot need to establish.
Our guide to roof insulation and EPC ratings covers how to insulate without creating a new ventilation problem.
Gutters and downpipes play a bigger role than most people expect, too. Water that cannot get away from the roofline finds its way back under tiles and into timber instead.
Our guide to downpipes and gutters covers the maintenance that keeps that water moving where it should.
Cosmetically, yes, it is unsightly. Structurally, the concern is real.
Rafters and purlins are load-bearing. Their job is to carry the weight of the roof covering, the battens, and whatever snow or wind load the roof takes on in a bad winter.
Decayed or beetle-riddled timber loses that strength gradually, not all at once, which is exactly why it tends to go unnoticed until something visibly moves.
A roof does not need every timber replaced to be safe again. It needs the active decay stopped and the moisture source fixed.
The genuinely weakened sections then get properly repaired or swapped out, confirmed by someone who knows the difference between old, inactive staining and a live problem.
That is also why a DIY spray-and-hope approach rarely solves anything on its own. Treating the surface of a timber that is still being fed by an active leak just delays the point at which the real cause gets found.
For load-bearing repairs on anything more than a minor, localised patch, get a structural engineer’s input on the replacement timber sizing. A roofer who tells you this themselves, unprompted, is one worth trusting with the rest of the job.
| Job | Typical UK cost (2026) |
| Standalone timber decay survey/inspection | £200 – £500 (around £300 average for a straightforward home) |
| Minor, localised wet rot treatment | £500 – £1,000 |
| Moderate wet rot, single area | £1,000 – £2,500 |
| Small, localised dry rot patch | £500 – £1,500 |
| Dry rot with structural repair, single room/area | £1,500 – £5,000 |
| Widespread dry rot across a property | £5,000 – £12,000+ |
| Individual timber/joist replacement | £250 – £350 per section |
| Fungicidal spray or injection treatment | £70 – £100 per linear metre |
Most homeowners should budget an extra 15 to 20% on top of an initial quote. The real extent of decay often only becomes clear once affected timber is exposed.
A moderate dry rot outbreak with timber replacement and redecoration typically lands around £6,000 in total. Severe, long-neglected cases with extensive structural damage can run to £12,000–£20,000 or more.
These figures track closely with national cost-guide data from tradesperson marketplace MyBuilder’s 2026 dry rot treatment cost guide, cross-checked against its separate guide to dry rot repair costs.
That gap between a few hundred pounds and five figures is the real financial argument for catching this early rather than late.
Usually not, and that catches people out. Standard buildings insurance is built to cover sudden, unforeseen damage, such as a storm ripping tiles off overnight.
Gradual deterioration from an undetected leak or years of poor ventilation tends to fall under maintenance, which most policies explicitly exclude.
Consumer body Which? puts it plainly: insurers offer buildings cover on the condition that a home is maintained, and claims are often turned down when that is not the case, roof-related claims included.
According to Which?’s guide to making a home insurance claim, keeping evidence of regular roof inspections genuinely helps if you need to claim for something that is covered, such as storm damage, since it shows the property was reasonably maintained beforehand.
The practical takeaway: a periodic roof check is not just about catching timber decay early. It is also the paper trail that can make the difference if you need to claim for something else entirely further down the line.
A handful of jobs are reasonable to tackle yourself: clearing a blocked gutter, replacing a cracked gutter outlet, or keeping an eye on a loft space after heavy rain.
Beyond that, timber decay is not a sensible DIY project, for reasons that go beyond simply wanting the work done properly.
Specialist contractors who carry out this kind of work are often assessed and audited by the Property Care Association, the UK’s trade body for timber decay, damp and related property care specialists.
Membership means a company’s procedures, training and health and safety have been independently checked. It is worth asking about before anyone starts cutting into your roof structure.
Once a timber decay problem has been properly treated, keeping it from returning comes down to the same few habits.
Clear gutters at least twice a year so water keeps moving away from the roofline, and check flashing and roof coverings annually for damage that lets water in.
Make sure any loft insulation upgrade is paired with proper ventilation, rather than sealing the space up.
Moss buildup on tiles can also hold moisture against the roof covering for longer than it should. Our guide to moss removal from roof tiles covers how to deal with it before it becomes a bigger problem.
A periodic professional inspection, covered in our guide to what a roof inspection report actually involves, is the single most reliable way to catch a new leak before it has had months to feed decay into the timber below.
Timber decay work sits in an awkward spot between roofing and specialist damp/timber treatment. That is exactly where corners get cut by whoever is cheapest rather than whoever actually fixes the cause.
A contractor who understands both the roof covering and what is happening to the structure underneath it is the one who fixes the problem once, not the one who treats the symptom and leaves the leak in place.
As Confederation of Roofing Contractors members, we carry out this kind of structural repair work backed by an insurance-backed guarantee, so the repair is protected even if something were to happen to us as a business down the line.
If you are weighing up contractors for a job like this, our guide on how to choose a roofing contractor in the UK covers the questions worth asking before anyone gets on your roof.
Wet rot generally stays close to its moisture source, so it is less likely to spread far from the roof. Dry rot is different: it can travel through masonry and behind plaster in search of new timber to feed on.
That is why a suspected dry rot outbreak needs a proper survey of the surrounding structure, not just the visible patch.
Common furniture beetle larvae can spend several years feeding inside timber before emerging as adult beetles through flight holes, so damage builds up slowly and often goes unnoticed.
A small number of old, discoloured holes is usually historic. Fresh, pale-coloured dust around new-looking holes points to an active infestation worth treating.
Walk carefully and stay on the boarded areas or joists if you need to look, and avoid putting weight on any timber that looks visibly soft, cracked or discoloured.
If you can see a sagging section, or are at all unsure about the loft floor’s condition, it is safer to view it from the hatch and arrange a professional inspection rather than risk a fall through the ceiling.
No, and this is the most common mistake. Treating or replacing decayed timber without finding and repairing whatever let moisture in just buys time before the same problem comes back.
Whether that is a cracked tile, failed flashing or a blocked gutter, a proper job always addresses the moisture source first.
If the roof is part of the building’s structure rather than your own demise, responsibility for repairs usually sits with the freeholder or management company, funded through the service charge.
It is still worth raising any signs you notice, such as staining or a musty smell in a top-floor flat, since decay left unreported tends to get more expensive for everyone paying into that service charge.
