Wet rot, and the cut-back distance nobody can show you in writing
Wet rot is the commoner of the two and the less serious, and it gets quoted like the other one. What the published guidance actually says about how much timber comes out, and why almost every figure you will be quoted was written for dry rot.
By The Damp GuyLast updated
The short version
Wet rot is fungal decay in timber that has got wet and stayed wet. It is not one organism. It is a group of different fungi, named after how the wood looks rather than after any one of them, which is why the advice you will read about it is so inconsistent. The important part is what follows from it. Wet rot stays in the wet timber, so the honest fix is a repair to whatever is letting the water in, then carpentry. Below about 20% moisture content the decay stops, and every source I read agrees on that number. Dry rot is the one that justifies a programme of work, and wet rot is routinely quoted as though it were dry rot. Knowing which you have is worth more than any product.
Wet rot or dry rot: the check you can run before anybody arrives
You can get a long way on what you can see, smell and press, and nobody on the first page of results sets it out as a check. Here is what each side's own sources say the tells are.
| What to check | Wet rot | Dry rot |
|---|---|---|
| How far it has travelled | Confined to the timber that is wet. The mycelium does not spread into walls, though it may grow over masonry | Said to cross plaster and masonry to reach dry timber, though Ridout says that ability is not unique to it |
| The cracking | Mostly along the grain, with minor cracks across it. Cube-like cracking does occur, at a smaller size | Deep cracking into large cubes, across and along the grain |
| The colour of the timber | Darkened where a brown rot has it, bleached and stringy where a white rot has it | Darkened and brittle, a brown rot in every case |
| What it feels like | Soft and spongy while wet. A thin screwdriver goes in with no effort | Dry, brittle and light. It crumbles rather than compresses |
| What is on the surface | Often nothing. Cellar rot leaves dark brown strands on nearby wood or plaster | Strands that stay rigid when dry, and a fruiting body that sheds rust-red spore dust |
| What it costs to put right | Priced as a carpentry repair to the timber that has actually failed | Priced as a programme, because the published margins and the masonry treatment come with it |
The top row does most of the work. Wet rot is confined to the timber that is actually wet, because the mycelium does not spread into walls, though the Property Care Association notes it may grow over masonry and that in rare cases some wet rot fungi produce strands. Dry rot is the one said to travel through plaster and brickwork to reach dry timber somewhere else. Worth knowing that even that distinction is softer than the trade presents it: Brian Ridout, writing in a book Historic Scotland and English Heritage published together, says other fungi will also grow through walls where the alkalinity of old mortar has dropped enough, and that the ability is not, as frequently stated, unique to dry rot.
The bottom row is the money, and it is the reason to be sure before you sign anything. The two diagnoses buy completely different jobs.
None of this is a verdict on your timber, and it is not meant to be. A meter will not settle it either, for the same reasons set out on what a damp meter reading actually measures, and the SPAB adds a specific warning for timber: do not over-rely on surface readings from an electrical moisture meter, because sound oak roof timbers can show surprisingly elevated surface moisture at high humidity. The sibling page on dry rot and what has to be proved before that diagnosis stands takes the other half of this table.
One safety point before anything else, because it is the bit that gets skipped. Decayed floor joists and floorboards are a falling-through risk, and decayed roof timbers put weight over your head. BRE's own authors put it plainly: retaining affected timber presents problems for the structural integrity of the building, and falling debris can be a hazard to occupants and others if the decay continues. If a floor feels bouncy or a board gives underfoot, keep off it and get somebody who has stood in front of it to look. That is true before you have any idea which fungus you have.
Wet rot is a category, not an organism
This is the first thing to understand, and it explains why every page you read on the subject says something slightly different.
Dry rot is one fungus. Wet rot is several. The SPAB puts the classification cleanly: a fungus is described by the colour of the damaged timber, white or brown, and dry rot is a brown rot caused by the single fungus Serpula lacrymans, while wet rot is white or brown rots caused by various fungi, commonly Coniophora puteana.
The Property Care Association's own homeowner page goes further, and it is the single best sentence published anywhere on this subject: the term refers more to the appearance of the rotting wood, meaning that it looks wet, than it does to the amount of moisture needed to support the growth of the fungi. Ridout is blunter still. He says Coniophora puteana is the name usually applied to any fungus not obviously dry rot.
So "wet rot" on a report is a description of what the timber looks like, not an identification. And Ridout adds the line that takes the pressure off: although wet rot may be produced by any one of a number of fungi, the treatment is the same in all cases. You do not need to know which one you have. You need to know it is not the other one.
- Coniophora puteana, cellar fungus (brown rot). The commonest. Darkens the timber, cracking mostly along the grain. Cellars, solid floors and roofs.
- Fibroporia vaillantii, pore or mine fungus (brown rot). Similar decay to dry rot but less widespread. Its strands stay white and flexible when dried.
- Phellinus contiguus (white rot). Bleaches the wood to a stringy fibrous state with no cubical cracking. A major cause of decay in external joinery.
- Donkioporia expansa, oak rot (white rot). Damage can be very severe once fully investigated, and considerable structural repairs may follow.
- Asterostroma cervicolor (white rot). Ridout says it is almost inevitably identified as dry rot, with damaging and expensive consequences. It produces a white rot; dry rot produces a brown one.
Two of those are white rots, and that matters because the white rots do not produce the cube-shaped cracking everybody uses as the dry rot tell. Phellinus contiguus bleaches wood to a stringy fibrous state with no cubical cracking at all, and the PCA calls it a major cause of decay in external joinery, which is to say in your window frames. Asterostroma cervicolor is the one to know about, because Ridout says it is almost inevitably identified as dry rot, with damaging and expensive consequences, and that it can be told apart readily: it produces a white rot, and dry rot produces a brown one.
Here is how hard the industry makes this on a homeowner. The Property Care Association's technical guidance note has cellar fungus as Coniophora puteana and pore or mine fungus as Fibroporia vaillantii. Its own homeowner page has the two common names the other way round. Both documents are on the same website. You cannot resolve this by reading the trade, which is a decent argument for not trying to and for asking better questions instead.
The moisture number, and the two answers published by the same organisation
Wet rot needs wetter timber than dry rot does. How much wetter is genuinely disputed, and one public body has published both sides of the dispute.
You will see 50 to 60% quoted everywhere for wet rot. It is a real figure from real sources and it is almost always quoted wrongly, because it is an optimum rather than a minimum. It is the moisture content at which these fungi are happiest, not the level below which they cannot work.
| Source | What it says wet rot needs | What it says dry rot needs | Stated as |
|---|---|---|---|
| Historic Environment Scotland | Between 50 and 60%, and higher than dry rot needs | Thrives above 90% humidity at around 23C; no timber figure given | An optimum |
| Brian Ridout | Coniophora puteana: minimum 15 to 25%, optimum 30 to 70% | Serpula lacrymans: minimum 17 to 25%, optimum 20 to 55% | A minimum, or no figure |
| the Property Care Association | Decay always arises above 20% moisture content | The same 20% figure, for fungal decay generally | A minimum, or no figure |
| the SPAB | No figure. Dry rot is said to thrive at lower levels than wet rot | No figure. Below 20% moisture content, dry timber is immune | A minimum, or no figure |
| Janice Carey and Colin Grant of the Building Research Establishment | No wet rot figure | Decay ceases below about 20% | A minimum, or no figure |
Read the first two rows again. Historic Environment Scotland's guide on rot in timber says wet rot requires a higher moisture content than dry rot, between 50 and 60%. The book Historic Scotland published with English Heritage gives Coniophora puteana a laboratory minimum of 15 to 25% and an optimum of 30 to 70%, and it says in terms that the data provide no support for the commonly held belief that buildings dry through a stage unsuited to wet rot but suitable for dry rot attack.
Same body. Two publications. Opposite readings of the gap between the two rots. I have not found anybody else who has put them side by side, and I am not going to resolve it for you, because resolving it would mean picking one and printing it as settled, which is what everybody else on this subject already does.
What you can take from it is the practical part, and every single source agrees on this one. Below about 20% moisture content, decay stops. Historic Environment Scotland: if the moisture level drops below 20%, decay will cease. The SPAB: dry timber is immune to attack, so reduce the moisture content below 20%. The Property Care Association: fungal decay always arises because the wood has become wet, in excess of 20% moisture content. BRE's own authors: decay ceases below about 20%, and many extinct outbreaks are found in buildings where the fungus has died out because maintenance removed a water source.
Ridout gives that number its history too, which is worth having. It comes from Cartwright and Findlay, 1946, and it already contains a safety margin, because those authors believed the real minimum in most practical situations was probably 22 to 24%. So 20% is a conservative target rather than a cliff edge, and it is the number the whole of this page's advice hangs on.
The same threshold is what governs timber in a loft that is wet from condensation rather than from a leak, which is one of the routes into this that people miss entirely. The PCA makes the same point about voids: where there is a lot of water held in the air, in roofs and sub-floor spaces, wood takes up that atmospheric moisture, and wet rots can become widespread and do extensive damage. There is no leak in that story at all.
The fix is a repair to the water, then carpentry
Nine of the ten pages ranking for this term prescribe a fungicide. Four bodies that sell none of it question whether one is needed. Here is what each side actually published.
Start with the sequence, because everybody agrees on it and it is the part that decides whether the work lasts. Historic England: whichever type of rot you have, first stop the water ingress which has caused the damp conditions the fungus needs to survive, allow the environment to dry out and the moisture levels in the affected timber to reduce before assessing whether further work is needed, because replacing the timber or applying a chemical without solving the source of damp would treat the symptom and not the cause, and the problem would be likely to recur.
That is the same argument this website makes about every other kind of damp, arriving in a new room. Find the source or it comes back. The water is usually a building defect with a location, which is the subject of the page on water crossing the wall from outside and the defect letting it in, and sometimes it is ground moisture crossing a bridged or failed damp-proof course into a joist end or a wall plate. The PCA's homeowner page says as much: wet rots are often associated with joist or beam ends and wooden posts that rot where they meet wet walls or the ground.
Now the disputed part. What happens after the timber is dry.
What the bodies with nothing to sell say
Historic England asks the question nobody selling the treatment asks: the first question to consider is whether the timber needs treating at all, and treatment may not be necessary. It also notes that identifying the source of moisture avoids inappropriate, costly and damaging treatment being carried out.
Historic Environment Scotland says the most effective way of ridding a building of fungal rot is unquestionably to remove the source of moisture, and that chemical treatment will be of little value if that is not achieved. On precautionary treatment it is sharper: applying timber preservatives to sound timber unaffected by rot is unlikely to be of much value, and if a building is kept dry there will be no need for that often expensive process.
Ridout, on wet rot specifically, is the most direct statement I found anywhere. The primary remedial measure is the control of water penetration, and once that has been achieved the fungus will die. Decayed timber is usually removed and structural repairs undertaken. Spray treatment with a fungicide is frequently employed, but this is of little value because it will not reach fungus within the timber. In most cases, he writes, preservative treatment is probably not necessary.
The SPAB allows for targeted chemical treatment where an attack is serious and dampness will be hard to eliminate quickly, and it draws a line under it: not as a substitute for promoting drying, and not as a general precaution merely to obtain a guarantee.
What the trade says, including the parts against its own interest
The Property Care Association is the trade body for the firms that do this work, so the lines from it worth your attention are the ones that cost its members money. There are several, and they are more useful than anything on the first page of search results.
On brickwork: if the decay has been caused by Coniophora or other wet rot fungi, it is not necessary to sterilise brickwork. On applying the wrong treatment: a dry rot treatment carried out against wet rot will be effective but will be unnecessary, costly, and could be in breach of the Control of Substances Hazardous to Health Regulations 2002 as amended. On soaking walls, even for dry rot: use of wide-spread irrigation can rarely be justified, it extends the drying time for the wall, which is the primary control measure, and it can therefore be seen as counter-productive.
Its own instruction for wet rot names drying as the remedy: where wet rot has been caused by direct leakage it can often be entirely checked by removing the source of dampness and by thorough and rapid drying, and extra heating and ventilation will do much towards preventing further outbreaks.
Set that against what the market publishes. One manufacturer's page states that a biocidal treatment must be applied to the entire timber object. One contractor's page exists to argue that treating wet rot is never as simple as letting the wood dry out. And three of the same commercial pages concede in passing that stopping the water is what settles it, then carry on to the fungicide anyway.
I am not telling you a fungicide is never appropriate, and neither is any source above. Timber going back in should be pre-treated, and a targeted paste while a structure dries has defenders. What the sources will not support is a chemical programme sold as the cure, on a page that has already told you the cure is drying the timber out.
How much timber actually has to come out
This is the question that decides the bill, and here is the finding: every published cut-back distance in existence was written for dry rot.
When somebody quotes to cut back sound timber a set distance beyond the last visible decay, that convention has a paper trail. Ridout traced it, and these are the numbers.
| Published by | Distance beyond visible decay | Written for | Where it comes from |
|---|---|---|---|
| Norman Hickin | 3 ft, about 1 metre | dry rot | "The Dry Rot Problem", 1963, recorded by Ridout |
| Forest Products Research Laboratory | 12 to 18 in, 300 to 450mm | dry rot | Leaflet No. 6, 1947, recorded by Ridout |
| The British Wood Preserving Association | At least 600mm | dry rot | Code of Practice 1983, quoted by Ridout, with a note recognising special cases |
| Building Research Digest No. 299 | 300mm around built-in timbers, to check them | dry rot | 1993, quoted by Ridout. The digest itself is paywalled and was not read |
| The Property Care Association | No margin. Cut out structurally unsound timber | wet rot | Guidance Note "Fungal Decay in Buildings", May 2016 |
| Historic Environment Scotland | No margin. Adjacent boards stay unless they too show decay | wet rot | INFORM Guide "Rot in Timber", October 2008 |
Read the third column down. Every distance is a dry rot rule. The two rows that address wet rot give no distance at all, and one of them is the trade body's own guidance note, which says to open up the affected area and cut out and discard structurally unsound timbers. That is a condition, not a measurement.
Historic Environment Scotland is more specific about the ordinary case. Where the rot was in a relatively small area, the wholesale removal of large areas of timber will not be necessary. Its worked example is a floorboard: once the cause of the moisture is rectified, there should not be any need to remove the boards adjacent unless they too show signs of rot. The Property Care Association concedes the same thing even about dry rot, saying the safety margin can be excessive in the case of a floorboard, where it should normally be necessary only to cut away to the next joist.
And the conservation view of the whole convention is harder than either. The SPAB says routinely cutting timber back to a safety margin of 0.3 to 1.0 metres beyond the last signs of decay is rarely justified, and that the general removal of old plasterwork to expose timbers for inspection has little to recommend it. Ridout says rigid adherence to the one metre rule can be disastrously destructive, particularly to ceilings, and is rarely if ever necessary in historic buildings, and that there was never any generally accepted distance for cutting back in the first place.
So if a wet rot quote carries a 600mm or a one metre margin, ask where the figure comes from. It is a fair question with a real answer behind it, and the answer is that no document I could find applies one to wet rot.
One distinction worth keeping straight, because it cuts the other way. Ridout does give a radius, and it is an inspection radius rather than a removal one: all timbers within about 2 metres of the last signs of decay. Looking at timber costs you access and making good. Cutting it out costs you the timber, the structural work and everything attached to it. The two are not the same request and should not be priced as though they were.
Repairing rather than replacing, and where Building Regulations come in
Replacement is not the only option and the trade body says so: decayed timber should be cut out and replaced with pre-treated timber, or restored with resin or other reinforcement systems. Robin Russell, a conservation contractor writing on structural timber repairs, sets the default the other way about, on the principle of retaining the maximum amount of original timber. The usual answer, he writes, is to repair rather than replace, commonly by scarfing in a new piece of timber to the old, like for like, which suits exactly the cases wet rot produces: the foot of a post or jamb, or the end of a rafter, truss or beam affected by damp.
He is honest about resin too, which is rarer. Resins offer minimal loss of fabric, versatility, gap filling and repairs impossible by other methods. They are also, in his words, not particularly reversible and can be prone to failure in humid environments. That second half is worth holding on to, because a humid environment is the one you have.
On the regulations, be careful with anybody who sounds certain either way. Replacing structural timber is not on the list of work in Schedule 4 of the Building Regulations 2010 that needs no building notice, and those Regulations define building work to include a material alteration, meaning work that would leave the building less compliant with a relevant requirement than before. Part A, structure, is expressly a relevant requirement, and Approved Document A requires that dead, imposed and wind loads are sustained and transmitted to the ground safely and without deflection that impairs stability. Approved Document A says nothing about replacing a decayed joist specifically. LABC, the body behind local authority building control, adds that replacing timber floorboards triggers the thermal element rules where more than half the floor is affected, and says plainly that its own guidance is not definitive and the decision rests with the building control authority. Ridout's answer is the practical one: consult a structural engineer.
The short version is that nobody publishes a bright line, so treat a contractor who volunteers one with the same interest as any other confident claim.
What the work costs, and why the window sill is the clearest case
There is no single answer, because you are buying a carpentry repair to a specific piece of timber. The published ranges disagree, and the disagreement tracks who profits.
| The job | Published range | Who published it, and what they sell |
|---|---|---|
| Rotten window sill, replaced | £80 to £195, average £155 | MyJobQuote, as above. |
| Rotten window sill, spliced by a joiner | £300 to £600 depending on how far the rot has gone | Prime Sash Windows, a timber sash window maker that sells the replacement window, and its published tipping point is the figure above which it says to buy one. |
| Floorboards and a floor joist repaired | £300 to £500 | MyJobQuote, a quote-matching marketplace that earns by connecting homeowners to trades, so it has an interest in a figure that reads as approachable. |
| Floor joist, localised repair | £400 to £900, and £1,000 to £2,000+ for a larger subfloor job | MyJobQuote, a quote-matching marketplace that earns by connecting homeowners to trades, so it has an interest in a figure that reads as approachable. |
| Roof joist repaired | £350 to £600 per beam, average £575 | MyJobQuote, as above. |
| Wet rot, minor or local patch | £200 to £700 | INSITU Building Preservation, a timber and damp preservation contractor that sells the work it is pricing. |
| Wet rot, structural joist or wall plate replacement | £1,800 to £3,500 | INSITU Building Preservation, a timber and damp preservation contractor that sells the work it is pricing. |
| Wet rot, extensive structural work | £3,000 to £10,000 or more | Surveyor Local, sells RICS surveys, so it has a direct interest in a reader booking an inspection. |
The first two rows are the same job. A rotten timber window sill is £80 to £195 on a quote-matching marketplace, and £300 to £600 from a joinery firm that makes replacement windows. Roughly three to four times the price, for rotten timber in a window, and the higher figure belongs to the party that benefits if repairing starts to look uneconomic. The same firm publishes its tipping point openly: past about £500 per window in repairs, it says to buy a new window at £1,500 to £2,500.
I have not averaged any of these and I am not going to, for the same reason this website never averages two cost guides. Averaging invents a number nobody published and then lends it the authority of both sources. What the spread is genuinely useful for is reading a quote: if yours sits well outside every published range for that job, that is a question to ask rather than a verdict on the firm.
One warning about the ranges the contractors publish. Three of the wet rot cost pages I read share a near-identical structure, tone and band format, and were published within months of each other. I have carried their figures because they are the only wet rot figures published at all, and they are pricing positions rather than market data.
The full picture on what damp work costs, what a quote usually leaves out and which lines to challenge sits on the page about what damp work actually costs. Wet rot has one feature that page's subject does not: because the job is carpentry, a joiner or a builder can quote it, and it does not have to be bought as damp proofing at damp proofing rates.
The survey, and the cheapest thing on the bill
| What it is | Published price | Who published it, and what they sell |
|---|---|---|
| An independent damp and timber survey | £150 to £400 | HomeBuyerCheck, sells property due-diligence reports, so it has an interest in paid inspection over a free one. |
| A specialist wet rot survey | £150 to £500 | Surveyor Local, sells RICS surveys, so it has a direct interest in a reader booking an inspection. |
| A contractor survey, refundable against the work | £95 to £350 | INSITU Building Preservation, a timber and damp preservation contractor that sells the work it is pricing. |
| A treatment firm survey, with no price published for the work | Free | HomeBuyerCheck, sells property due-diligence reports, so it has an interest in paid inspection over a free one. |
That table is the whole argument of this website in four rows. The inspections that cost money are the ones sold by people with nothing else to sell you. The inspection that costs nothing is sold by the firm that will quote for the work, and the one in between is refundable against the work, which is the same incentive wearing a price tag.
Ridout wrote the plainest version of this in 2000, and nothing has aged about it: free surveys have ultimately to be paid for by work done. The SPAB says where advice is sought it should come from an independent chartered surveyor or consultant, not a remedial treatment contractor. Neither of them is accusing anybody of anything. They are describing how the money moves, and the money moves the same way whoever is holding the torch. Which kind of inspection answers which question is set out on the page on damp surveys and who is paying for them.
Worth knowing that the same trade body and the same conservation bodies do agree on something, and it is not nothing. RICS, Historic England and the Property Care Association jointly wrote a position statement on investigating moisture in traditional buildings in September 2022, adopted by Cadw, Historic Environment Scotland, the Historic Environment Division in Northern Ireland, the IHBC and the SPAB. It is a competency framework rather than a technical manual, so it carries no thresholds and no distances. What it establishes is that all of them signed up to remedial work treating the cause rather than the symptoms.
Insurance, and the exclusion that is not the end of the answer
Rot is excluded from a standard buildings policy as gradual damage. That is true, and on its own it is a worse answer than the one available.
Here is one insurer's current wording, from its own published booklet, under the heading for changes that happen over time. It does not cover: "Wear and tear. Lowering in value. Any damage caused gradually. Damage caused by rot, fungus, woodworm, beetles, moths, insects or vermin."
Every commercial page on this subject stops there, in a sentence, and most of them do not quote anything. Two things sit underneath it that are more use to you.
The first is that the same booklet covers water escaping from fixed water and heating systems, underground drains and pipes, domestic appliances and storage tanks. It also covers, separately, removing and replacing part of the building to find the source of that escaping water, with limits of £2,500, £5,000 or £10,000 depending on the tier. That matters because lifting a floor to find a leak is often the expensive half of the job, and it is not the same line as the rot repair. The same booklet excludes the tanks, pipes and appliances themselves, and damage caused by failure of, wear and tear to, or lack of, grouting or sealant. So a leaking shower seal and a failed pipe are treated differently, which is worth knowing before anybody says the word claim.
The second is that the exclusion has been tested and it does not always hold. The Financial Ombudsman Service has published its position on this kind of clause. Its view is that "this service's view is that a strict application of the type of gradual causes exclusion included in this policy may often mean that customers aren't treated fairly by their insurers". And it has published the test it applies: "We think it's unfair to decline a claim on the basis that the damage occurred gradually if the customer wasn't aware that the damage was occurring - and it couldn't reasonably be said that they ought to have been aware."
In the decision that wording comes from, rot under a concrete floor was traced to a leaking cold water feed pipe and a cracked soil pipe. The insurer declined under its gradual causes clause, which named wet and dry rot in terms. The ombudsman upheld the homeowner's complaint, accepted that the leak more likely than not created the conditions that let the rot develop, and directed the insurer to reconsider without relying on that exclusion, with interest and £250 for distress and inconvenience.
One decision is not a rule and I am not telling you a claim will succeed. What I am telling you is that "rot is excluded" is the beginning of the conversation rather than the end of it, and that the route through is usually the escape of water rather than the rot.
What to ask the person quoting
Four questions, and every one of them is about the water rather than the fungus. A quote that cannot answer them is pricing the wrong job.
- Which is it, and what makes you say so? Wet rot and dry rot buy different jobs at very different prices. The answer should point at something on the timber, not at the room.
- Where is the water coming from? Every source worth reading puts this first. If the quote does not name a defect and a repair to it, it is pricing the symptom.
- How far back are you cutting, and where does that figure come from? Every published margin was written for dry rot. Nothing found in the research applies one to wet rot, so this is a fair question with nowhere for a vague answer to hide.
- Is the chemical treatment doing something drying will not? Historic England asks whether the timber needs treating at all, and the PCA says a dry rot treatment applied to wet rot is unnecessary and costly. Ask which one you are being sold.
- Are you sterilising masonry, and why? The PCA's own guidance says brickwork sterilising is not necessary where the decay is wet rot. If it is on the quote, there should be a reason on the quote.
- Can this be spliced instead of replaced? Scarfing in new timber is the conservation default and the trade body accepts resin and reinforcement systems. Replacement is sometimes right; it should be a decision rather than a reflex.
- Is this structural, and does it need an engineer or building control? Joists, beams, wall plates and rafters carry load. Nobody publishes a clean rule on when the Regulations bite, so the honest answer involves a structural engineer or the local authority rather than certainty on a doorstep.
- Are you quoting for the work you diagnosed? Not an accusation, just the shape of the trade. The firm finding the problem is usually the firm selling the cure, and a second opinion from someone with nothing to sell is a reasonable thing to want. The same question is worth asking about the woodworm treatment that arrives on the same report, because a damp and timber survey sells both halves off one visit.
About this resource
The weight on this page is carried by four sources with nothing to sell in this market: the SPAB, Historic England, Historic Environment Scotland, and the book on timber decay that Historic Scotland and English Heritage published together in 2000. The Property Care Association's 2016 guidance note is quoted throughout, and the reason it is trustworthy here is narrow and worth stating: its members sell the treatment, so the lines I have used are the ones that cost them money.
The limits, plainly. BRE Digest 345 on wet rots is paywalled and I have not read it, and the 300mm figure in the table above reaches this page through Ridout quoting BRE Digest 299 rather than from the digest itself. BS EN 335, BS 8417, BS EN 350 and BS 5250 are all paywalled and none was read, so no clause number from any of them appears here. Historic England's website blocks automated access and was read through a text proxy. The Historic Environment Scotland guide is a scanned document with no text layer. Everything on this page was read on 11 September 2026, and if a figure matters to a decision you are making, go to the source rather than take it from me.
This site does not survey, treat or quote for work. It takes no fee for sending anyone anywhere. So it cannot tell you which fungus is in your floor or how much of that joist has gone. That needs somebody who has stood in front of it, and preferably somebody who is not also quoting to fix it. Where the timber is structural and the floor feels unsafe, that stops being advice and becomes the only sensible next step.
Common questions
Is wet rot worse than dry rot?
No, and the trade body that represents the firms treating both says so. The Property Care Association calls dry rot the most serious form of fungal decay of timber, and says wet rot occurs more frequently but is less serious, because the decay is typically confined to the area where the timber has become and stays wet. That is the whole difference, and it is why the two are quoted so differently. Dry rot is priced as a programme. Wet rot is a carpentry repair to the timber that has actually failed, plus a repair to whatever is letting the water in.
What are the first signs of wet rot?
Timber that has gone soft and spongy, usually somewhere that gets wet and stays wet: a window sill, a skirting against a damp wall, the end of a joist where it enters the masonry, the boards under a bath or a shower. A thin screwdriver goes into sound timber with real effort and into decayed timber with almost none. The colour changes too, darker where a brown rot has it and bleached and stringy where a white rot has it. There is often nothing at all to see on the surface, which is why it gets found late.
Can you fix wet rot?
Yes, and the order matters more than the products. Find and stop the water, dry the timber out, then cut out what has actually failed and repair or replace it. Historic England says to stop the water first and let the moisture content come down before assessing whether any further work is needed, because replacing timber or applying a chemical without solving the damp treats the symptom and the problem recurs. Below about 20% moisture content decay stops. Fix the water and leave it wet and you have bought yourself the same job again.
Will insurance cover wet rot?
Usually not as rot, and that is not always the end of it. One insurer's current booklet excludes wear and tear, anything that happens gradually, and damage caused by rot, fungus or woodworm outright. But the same booklet covers water escaping from fixed systems, drains, appliances and tanks, and covers the cost of opening up the building to find that leak under a separate trace and access limit. The Financial Ombudsman Service has also published its view that applying a gradual-causes exclusion strictly often means customers are not treated fairly, and that declining a claim because damage happened gradually is unfair where the customer did not know and could not reasonably have known it was happening.
How much does it cost to fix wet rot?
It depends entirely on which timber, and the published ranges disagree with each other in a way worth understanding before you read a quote. A window sill is priced at £80 to £195 by a quote-matching marketplace and at £300 to £600 by a joinery firm that also sells replacement windows. A localised floor joist repair runs £400 to £900 on one guide. A contractor puts structural joist or wall plate replacement at £1,800 to £3,500. None of those is a quote for your house, and the spread between them tracks who profits from the answer.
What not to say to home insurance?
Do not guess. The honest risk on a rot claim is not saying too little, it is inventing a timeline you cannot support, because the gradual-damage exclusion turns on when the damage started and whether you knew. Say what you actually observed and when you observed it. The Ombudsman has published the test it applies: it thinks it unfair to decline a claim because damage happened gradually where the customer was unaware it was happening and could not reasonably be expected to have been aware. That test rewards an accurate account, and a guess that turns out wrong undermines one.