Dry rot, and the number that makes the name a lie
Dry rot needs wet wood. A spore cannot germinate until the timber reaches 30% moisture content, and decay stops below about 20%. Here is how to tell it from wet rot, what the conservation bodies and the treatment trade actually disagree about, and what the published costs say.
By The Damp GuyLast updated
The short version
Dry rot is the common name for one fungus, Serpula lacrymans, and the name describes the wreckage it leaves rather than the conditions it needs. It needs water. A spore cannot germinate until the timber it lands on reaches 30% moisture content, and once established it keeps going down to about 20%, below which decay ceases. That 10-point gap is the whole practical story. Something is wetting your timber, and the fungus is the consequence rather than the cause. So the first job is never the chemical. It is finding the water, stopping it, and getting the wood under 20%. What happens after that is genuinely argued about by the professional bodies, and this page publishes the argument rather than picking a side.
The name is doing real damage
Nearly everything frightening about dry rot follows from people believing it can start in dry wood. It cannot.
Here is the sentence the rest of this page rests on. Jagjit Singh, writing in The Building Conservation Directory gives the moisture content needed for a dry rot spore to germinate as 30%, the minimum for the fungus at about 20%, and its optimum growth range at 30 to 40%. Separately, Janice Carey and Colin Grant of the BRE Centre for Timber Technology and Construction write that decay ceases if the moisture content of the wood is reduced to below about 20 per cent, and that timber already below that presents little risk of further decay.
Read those two together and you have something almost nobody publishes. It takes 30% to start and about 20% to carry on. Those are different numbers, and the difference is where the entire subject lives.
Dry rot is not a wood disease that finds your house. It is what happens to timber that has been wet for a while, by a route somebody has yet to find.
| The threshold | Moisture content | What that number does not tell you |
|---|---|---|
| Dry rot spores germinate | 30% | that timber below this is safe forever. An established outbreak carries on well below it. |
| An established outbreak keeps growing down to | 20% | how long the timber takes to get there, which nothing published states. |
| It grows fastest between | 30 to 40% | anything about the condition of a particular piece of timber in your house. |
| Decay ceases below | 20% | that the fungus is dead. It can stay viable in a dormant state at low moisture. |
| Held preventively below | 16 to 18% | a target any existing building is obliged to meet. |
| Wet rots grow best between | 50 to 60% | which wet rot species you have, and there are several. |
Every figure above is a moisture content measured in timber, and that qualifier matters more than it looks. On plaster and masonry a damp meter reports something different, which is set out on what a damp meter reading actually measures. A 20% shown on a wall is not the same 20% as a 20% shown in a joist. Sources are Jagjit Singh, writing in The Building Conservation Directory and Janice Carey and Colin Grant of the BRE Centre for Timber Technology and Construction, both read 11 September 2026.
Two further sets of numbers rarely make it in front of a homeowner, and both make the organism sound a good deal less supernatural than the trade describes it.
The first is humidity. John Palfreyman of the Dry Rot Research Group at the University of Abertay Dundee reports laboratory work on wood blocks in which active decay was inhibited once relative humidity fell below 86%, and viability was not lost until humidity fell to 76%, below which the fungus rapidly lost viability and became visibly shrivelled and discoloured. Those are wood blocks in a laboratory rather than a wall in a house, and the distance between the two is real. They are still the only published figures of their kind on this subject.
The second is temperature. It grows best around 23°C, stops growing above about 25°C, and is rapidly killed above 40°C. A fungus with an upper limit below the temperature of a hot bath is a finite problem, not a curse.
Telling it from wet rot, which is what decides the quote
The two are priced differently and confused constantly. These are the tells that separate them, and none of them needs a laboratory.
| What you are comparing | Dry rot | Wet rot |
|---|---|---|
| Timber moisture it needs | 30% for a spore to germinate, then it carries on down to about 20% | Grows best at 50 to 60%, so it needs genuinely wet timber throughout |
| What it actually is | One fungus, Serpula lacrymans, a brown rot | Several. Most often Coniophora puteana, also Fibroporia vaillantii, Poria placenta, Donkioporia expansa. White or brown |
| What the damaged wood looks like | Darkens, then cracks into cubes as it dries | Brown species look similar. White species leave it bleached, softened and fibrous rather than cubed |
| The strands, dried out | Rhizomorphs up to 6mm thick, and they go brittle when dry | Hyphae stay flexible when dried |
| How far it gets from the water | Strands travel across and through masonry, so it reaches timber the water never touched | Usually limited to timber actually in contact with the water |
| The fruiting body | Fleshy, rust-red, and it can reach a metre across | Rarely seen on Coniophora puteana. More common on the other species |
Moisture figures from Jagjit Singh, writing in The Building Conservation Directory. Species, cracking and strand behaviour from the SPAB. Reach from the Property Care Association, which is a trade association whose members carry out the treatment. Fruiting body from a University of Dundee glossary. All read 11 September 2026.
The row that changes the price is the last but one. Wet rot is usually limited to timber actually in contact with the water, so the repair has the same boundary as the wetting. Dry rot does not respect that boundary, which is why the quote is bigger and why the argument about how much bigger is worth having.
One more separator nobody prints, and it costs nothing to use. Take a dried-out strand and bend it. the SPAB notes that brown wet rot is easily confused with dry rot, but that its hyphae stay flexible when dried where dry rot's become brittle. Dry rot strands reach 6mm thick and its fruiting body can reach 1 metre across, so when you do get a good look at a mature outbreak there is not much doubt about it.
The smell arrives before any of it. A damp, mushroomy smell in a room that looks fine is the earliest signal there is. It is also the one shared with wet rot, which is far more common and usually cheaper to put right, and it is worth reading that page before accepting the more expensive diagnosis. If the timber in question is in a roof space, the wetting mechanism is more often condensation forming on the underside of the felt than anything coming in through the tiles.
The masonry claim, and which half of it is a myth
This is the single most-repeated line about dry rot, and the body best placed to judge it splits it in two.
The trade position is stated plainly by the Property Care Association: dry rot will grow through structural types such as masonry, will grow away from the timber it is feeding from, and will move through very small cracks in search of new timber to colonise. Nothing in the research contradicts that, and it is the reason dry rot is treated as a more serious finding than wet rot.
What Janice Carey and Colin Grant of the BRE Centre for Timber Technology and Construction add is the part that never gets repeated. In their words, much of the mythology surrounding dry rot is founded on the ability of its strands to penetrate through non-wood building materials, to transport water to otherwise dry areas and for the fungus to manufacture its own water. In reality, they write, the delicate hyphae are the primary colonisers, and the ability to conduct water is limited and can be negated by good ventilation. They also note that although strands can grow through and across masonry, the fungus derives no nourishment from it.
So the crossing is real and the self-supply is not. That distinction decides what you are buying. A fungus that can manufacture its own water has to be chased through the building and killed. A fungus that crosses masonry looking for wet timber, and that loses its water supply when the building dries and ventilates, has to be starved.
There is one genuinely uncomfortable detail on the other side of the ledger, from John Palfreyman of the Dry Rot Research Group at the University of Abertay Dundee: alkaline minerals in masonry assist the fungus by helping neutralise the acid metabolites it produces, and in full-scale building models thick rubble walls buffered the decay potential for long periods with no overt growth. That is a real argument for taking an old wall seriously, and it is not an argument for irrigating it.
How much timber comes out, and the disagreement nobody shows a homeowner
Start with the three published distances, then read who is saying what, because the neat version of this story turns out to be wrong in two places.
| Who says it | What they say | What they have at stake |
|---|---|---|
| the SPAB | routinely cutting back 0.3 to 1.0m beyond the last signs of decay is rarely justified | a conservation charity. It sells nothing and campaigns against unnecessary treatment. |
| Permagard | cut back affected timber at least 500mm beyond visible growth | sells the preservers, biocides and fungicides named on the same page. |
| AXA | strip masonry one metre beyond visible fungal growth | an insurer, describing the practice rather than performing it. |
Notice what the two numeric figures have in common: both are quoted in order to be rejected. Nobody currently publishes a positive fixed distance. Historic Scotland's Technical Advice Note 24, which Historic Environment Scotland still publishes, says decayed beam and truss ends should be cut back to the last signs of structural decay, and adds pointedly that many practitioners still insist on cutting back one metre beyond them.
The same document is the bluntest thing published on the masonry question. Drilling infected masonry and injecting chemicals, it says, is not necessary; such procedures introduce more moisture into the masonry and there may be a potential health hazard. Irrigating a rubble wall with preservative is of little value, because the preservative will not reach much of the fungal material in the wall. It is a 2002 document and the trade it describes has moved on, and it is a public body saying it.
Historic England puts the same thought gently: treatment may not be necessary, and replacing timber or applying a chemical without solving the source of damp treats the symptom rather than the cause. Its website blocks automated reading, so that wording comes from an archived copy of Historic England's own page rather than from the live site.
Now the part that makes this a real disagreement rather than a morality tale, because the neat version of this story is wrong in two places.
First, the loudest critic has a business too. The article in the RICS Built Environment Journal calling the common practice untenable, and saying the author would like to see it viewed as potentially negligent, is written by the managing director of a firm that sells the environmental-control alternative. That does not make the argument wrong. It does mean both sides of this are being made by people with something to sell, which is worth knowing before you weigh them.
Second, the trade body does not say what it is usually accused of saying. The Property Care Association's own treatment page states that controlling dry rot is less about treatment and more about eliminating the moisture, and that chemical preservatives should not be relied upon as the primary, long term control measure. On the substance of the first step, the charity, the public bodies and the trade association are in agreement.
So the argument is narrower than it looks, and sharper for it. Everybody agrees the water goes first. What is genuinely contested is how far past the visible damage you keep cutting, and whether masonry needs chemically sterilising once the building is dry.
They have been converging on the process, too. RICS, Historic England and the Property Care Association published a joint position statement on investigating moisture in traditional buildings in September 2022, since adopted by Cadw, Historic Environment Scotland, the Historic Environment Division in Northern Ireland, the IHBC and the SPAB. It treats damp as a staged process, with a period for monitoring and natural evaporation before moving on to other treatments. It also contains the sentence most worth quoting back at anybody: that a remedy specified by the surveyor or contractor should not contain a product, treatment or process that benefits them financially without that being declared to the client.
None of this makes treatment wrong. Decayed timber has lost strength and has to go, and where it was carrying load it has to be replaced by somebody who knows what it was carrying. But the reader holding the quote has never been told the argument is happening, and it is their money settling it.
What it costs, and why the published figures disagree by ten times
Three UK publishers, three ranges, one job description. Nobody reconciles them, so here they are side by side.
| Publisher | Published range | Dated | What they are |
|---|---|---|---|
| Confused.com | £200 to £750 | published 5 August 2025 | a price-comparison site paid by insurers |
| BestBuilders | £1,000 to £4,000 and above | page states updated 29 August 2026 | a builder lead-generation site paid for enquiries |
| Homebuilding and Renovating | £1,500 to £5,000, and can be much higher | published 15 March 2024 | a consumer magazine carrying display advertising |
All three read 11 September 2026. There is deliberately no average here. Averaging three commercially interested ranges would invent a precision none of them supports. Checkatrade, MyBuilder and Rated People all block automated reading and carry no figures on this page.
An order of magnitude between the low end and the high end is not sloppiness on anybody's part. It is the same word covering two different jobs. Treating a rotted window sill is one thing. Lifting a floor, replacing joist ends into a wall and making good afterwards is another, and both get called dry rot treatment.
Which is why the useful question is never the headline figure. Homebuilding and Renovating names the lines that actually move a dry rot bill: structural timber replacement or strengthening, necessary pipework repairs, damp proofing work, replastering and decorating, and potentially removing kitchen or bathroom fittings to get at the affected area. Almost none of that is the fungicide. It is the building work that follows the fungicide, and it is the part quotes are least clear about, in the same way that the replastering is the half a damp proofing quote leaves out.
One figure almost nobody quotes. BestBuilders puts an insurance-backed guarantee at roughly 3 to 5% of contract value. Set that against the RICS journal's record of a 30-year guarantee failing after ten and you can price what the reassurance is actually worth to you.
Your insurance almost certainly will not pay
Everybody says this. What nobody notices is that the insurers never say the words.
AXA's wear and tear guidance, Direct Line's, and the Association of British Insurers' home insurance guidance have one thing in common: not one of them uses the phrase dry rot anywhere. It is not a named exclusion. It is excluded by category, as gradual deterioration and as lack of maintenance, which is a different thing and a more useful thing to know.
AXA's wording is that home insurance does not usually cover damage from wear and tear, and excludes general wear and tear from normal everyday use along with damage that happened because your home is not in good repair. Direct Line states that home insurance covers damage by specific listed incidents and generally excludes damage caused by gradual deterioration, ageing or everyday use. The ABI puts the principle in one line: home insurance is not a maintenance contract.
The exception is worth holding on to, because it is the one route to a payment. Confused.com notes that where dry rot follows a covered event, such as a burst pipe, a storm-damaged roof or a flood, it may be picked up as a secondary issue under that claim. Which puts a premium on establishing what wetted the timber and when, and is another reason the first job is finding the water rather than treating the fungus.
By contrast, AXA does name a neighbouring timber problem outright, stating that its home insurance policy does not cover damage caused by woodworm. Insurers name what they mean to name, which makes the silence on rot worth noticing rather than assuming.
What to ask the person quoting
You are not going to out-argue anybody on mycology, and you do not need to. Ask where the water came in, and how far back they are cutting.
- Where is the water coming from, and have you found it? The fungus needed 30% moisture content to start. Something put it there. A quote that describes the fungus in detail and the water source vaguely has diagnosed the symptom.
- What is the moisture content of the timber now, and where did you measure it? This is the one number that decides whether the decay is active. Ask what it was and which piece of timber it came from.
- Is this dry rot or wet rot, and what makes you say so? Cube-like cracking, brittle strands and reach beyond the wet area point one way. Bleached fibrous timber confined to the wet zone points the other. The answer changes the price.
- How far beyond the visible decay are you cutting, and why that far? The SPAB says a routine 0.3 to 1.0m margin is rarely justified. You are asking for the reasoning behind this particular distance on this particular timber.
- Is any of this timber structural, and who is confirming that? A joist end, a wall plate or a bearing is a different conversation from a skirting board, and it is the one place on this page where getting it wrong is dangerous rather than expensive.
- What is in the price, and what lands separately? Replastering, decorating, flooring and putting the kitchen back are where a dry rot bill actually grows.
- Are you quoting for the work you diagnosed? Not an accusation, just the shape of the trade. The firm identifying the problem is usually the firm selling the cure, and on a job at this price a second opinion from somebody with nothing to sell is cheap.
A last word on where this sits. The document most readers are holding is a damp and timber report, and the timber half of it usually carries two findings: rot and woodworm, where most of the flight holes turn out to be historic. The damp half will name a mechanism, and the three candidates are set out on penetrating damp and rising damp. Getting the mechanism right is what stops you paying twice, because a timber treated beautifully in a wall that is still wet is a timber you will be looking at again.
About this resource
The moisture and temperature figures come from Jagjit Singh, writing in The Building Conservation Directory, 1996, and from Janice Carey and Colin Grant of the BRE Centre for Timber Technology and Construction writing in the same publication. The humidity figures come from a University of Abertay Dundee conference paper, which attributes them to laboratory work on wood blocks rather than to measurements of a building. The control target, the differential detail and the cutback position come from the SPAB, a charity that sells no treatment. The trade position comes from the Property Care Association, a trade association whose members carry out the work, and is quoted as theirs. Every source is named on the page beside what it says, and all were read on 11 September 2026.
Several things are deliberately absent, and the absences are the honest part. There is no distance for how far dry rot travels through masonry, because the one study that would settle it is paywalled and I have not read it. Nothing here is attributed to BRE Digest 299 or 345: both are out of print and I could not reach either, so the BRE material comes from a named article by BRE authors instead. Historic England's wording comes from an archived copy of its own page, because its live site blocks automated reading, and the page says so where it is quoted. Technical Advice Note 24 is dated 2002 and that is stated too. Nothing is attributed to BS 5250 or BS 8417, which are paywalled, and nothing to the Property Care Association's own Code of Practice, which sits in a members' library I could not open. And there is no drying timescale, because nothing published states one.
No product is recommended on this page and there is no affiliate link on it. That is not a small thing here: the money terms on this subject are all treatment terms, and the page reports a live professional disagreement about that treatment. I do not survey, treat or quote for work, which is what lets me set the conservation bodies and the trade side by side and leave the argument standing. What I cannot do is tell you what is happening in your house. If the timber is soft, if it was holding something up, or if you cannot tell what you are looking at, that is the point to get somebody in front of it who has no interest in selling you the treatment afterwards. And if the timber turns out to be soaking rather than merely damp, you are probably reading the wrong page: wet rot needs 50 to 60% moisture content and stays where the water is.
Common questions
What causes dry rot?
Water, and nothing else will do it. A spore lands on timber and needs that timber at 30% moisture content before it can germinate, according to Jagjit Singh writing in the Building Conservation Directory. Ordinary indoor joinery is nowhere near that. So something has been wetting the wood: a leak, a blocked gutter, a failed flashing, a bridged damp proof course, a subfloor with its air bricks blocked. The fungus is the consequence. The wet is the cause, and it is the only thing on this list you can actually remove.
How do I get rid of dry rot?
Find the water and stop it, then get the timber below 20% moisture content, and take advice on the timber and the chemicals only after those two are done. The SPAB gives reducing moisture content below 20% as the first control measure. BRE's own authors put it plainly: decay ceases if the moisture content of the wood is reduced to below about 20 per cent. Everything else in the standard treatment sequence is argued about by the professional bodies, as set out above. That first step is not.
How serious is dry rot?
It depends entirely on what the timber was holding up, and that is the question to get answered first. Skirting, a door lining or a bit of architrave is a repair. A joist end, a wall plate, a bearing or a lintel is structural, and the honest answer there is somebody who has stood in front of it, preferably somebody with nothing to sell you afterwards. This website cannot tell you which you have and neither can a photograph.
What happens if dry rot is left untreated?
It carries on as long as the timber stays above about 20% moisture content, and it reaches timber the water never touched, because its strands travel across and through masonry. Homebuilding and Renovating puts untreated growth at 2 to 3m in a year, and that figure is theirs alone rather than a settled number. What the fungus will not do is carry on in dry wood. Take the moisture away and it stops, which is the whole reason the first step is the water and not the chemical.
Will dry rot spread?
Through masonry, yes, and that is the genuinely frightening part of it. Strands up to 6mm thick travel across and through non-woody materials, so an outbreak that started under a bathroom floor can turn up in a skirting two rooms away. What is disputed is the next bit. BRE's own authors write that much of the mythology rests on the idea that strands transport water to otherwise dry areas and that the fungus manufactures its own water, and that in reality the ability to conduct water is limited and can be negated by good ventilation. Both halves of that are worth holding on to.
Is dry rot expensive to fix?
The three published UK figures disagree by roughly ten times, which tells you something on its own. Confused.com gives £200 to £750 for more minor cases. BestBuilders gives £1,000 to £4,000 and above. Homebuilding and Renovating gives £1,500 to £5,000 and says it can be much higher. None of them reconciles the gap. What actually moves the number is not the fungicide: it is structural timber replacement, pipework repairs, damp proofing, replastering and decorating, and taking out kitchen or bathroom fittings to reach the affected area.
Will house insurance cover dry rot?
Almost certainly not, and the reason is more specific than people expect. Of the insurer and industry-body pages I read, AXA's wear and tear guide, Direct Line's and the ABI's, not one uses the phrase dry rot at all. It is excluded by category rather than by name, as gradual deterioration and lack of maintenance. The ABI puts it in one line: home insurance is not a maintenance contract. Confused.com notes the exception worth knowing, that where the rot follows a covered event such as a burst pipe or a storm-damaged roof it may be picked up as a secondary issue.
Can I sell a house with dry rot?
Yes. The risk sits in what you say about it rather than in having it, and that is a separate subject with its own rules. The Property Information Form and where the liability actually falls are covered on the page about selling a house with damp.