How Is Wet Rot and Dry Rot Diagnosed and Treated?

Summary

Timber decay in buildings is caused by wood-rotting fungi, which require moisture, oxygen, and a food source (the timber) to grow. There are two broad categories in UK buildings: dry rot (Serpula lacrymans) and wet rot (a group of species, most commonly Coniophora puteana — cellar fungus). Despite its misleading name, dry rot requires moisture to grow — the name refers to the dry, crumbly appearance of severely affected timber. The distinction matters because dry rot requires far more extensive remediation.

Misidentification is common and costly. A surveyor who identifies wet rot but misses dry rot, or who treats dry rot inadequately, will be called back to a failed remediation. The key identification criteria are described in detail below. Where doubt exists, a sample should be sent to a specialist laboratory for mycological identification.

In older UK housing stock — Victorian and Edwardian terraces, pre-war semi-detached, and wartime prefab — roof structures, floor joists, and timber window and door frames are the most commonly affected elements. The causes are almost always poor drainage maintenance, failed DPCs, penetrating damp, or condensation over prolonged periods.

Key Facts

• Serpula lacrymans — dry rot fungus; grey/white mycelium and rust-coloured fruiting body (sporophore); strands (hyphae) can travel through masonry, plaster, and non-timber materials in search of new food source; can be active at moisture contents as low as 20–30% in the wood
• Coniophora puteana — the commonest wet rot species; dark brown/olive mycelium; thin, flat strands; confined to areas of continuous dampness (timber moisture content typically above 30–35%)
• Poria vaillantii (mine fungus) — another common wet rot; white mycelium; can be confused with dry rot mycelium by colour but strands are distinctively white and cottony
• Moisture threshold — timber at below 20% moisture content will not support fungal growth; eliminating the moisture source will arrest active decay
• Dry rot spread — mycelial strands can penetrate masonry, brickwork, plaster, and even some metals; can transport moisture from a damp source to dry timber some distance away
• Removal margin (dry rot) — all visibly affected timber plus a minimum 600mm margin into apparently sound timber must be removed; this margin may be extended if there is any doubt
• Removal margin (wet rot) — affected timber should be removed back to sound wood; wet rot does not spread beyond the zone of active dampness so margins are smaller, but all wood with a moisture content above 20% should still be examined
• Boron treatment — sodium borate (Boron 10, Tim-Bor) is the preferred preservative for in-situ timber preservation; low toxicity; applied by diffusion paste, solution flood, or Boron rod
• Boron rod — fused borate glass rod inserted into drilled holes in structural timber; releases borate slowly over years; used for long-term protection in areas of moisture risk
• Epoxy consolidant repair — for wet rot in timber where full replacement is not possible (e.g. window frames); rotten wood removed, dried, consolidated with low-viscosity epoxy resin (Ronseal, Repair Care), and filled with two-part epoxy filler; structural replacement required if more than 50% of section is lost
• BS 8417:2011 — British Standard for preservation of wood; specifies preservative types, application methods, and retention levels for different use classes
• CSRT — Certificated Surveyor in Remedial Treatment; the PCA qualification for surveyors specifying remedial timber and damp treatment

Quick Reference Table

Detailed Guidance

Identification

Correct identification is the foundation of effective treatment. On site, the key diagnostic criteria are:

Mycelium — examine the surface of affected timber and adjacent masonry for fungal growth. Dry rot mycelium is initially white and cotton-wool-like, developing into flat grey sheets with lilac tinges. It produces silky strands (rhizomorphs) that can be several millimetres thick and extend across masonry. Wet rot mycelium (Coniophora) is dark brown to olive and forms thin flat growths on wet surfaces. Poria vaillantii produces white, cottony growth that can be confused with dry rot.

Fruiting bodies — if a sporophore is present, dry rot is unmistakable: a large, flat, rust-orange bracket with an off-white margin, producing vast quantities of rust-coloured spore dust. The presence of red spore dust on horizontal surfaces near affected timber is diagnostic. Wet rot fruiting bodies are rare in buildings.

Timber condition — dry rot causes cuboidal cracking of the wood (cracks across and along the grain, dividing the timber into roughly cuboid chunks). The timber is red-brown and dry. Wet rot typically causes longitudinal cracking, dark staining, and the timber may feel wet and spongy. In advanced cases the outer surface may look sound while the interior is fully decayed (shell rot).

Strand test — if you find white strands in masonry or plaster away from obvious timber, this indicates dry rot. Wet rot strands are confined to the immediate vicinity of wet timber and do not penetrate masonry.

If identification is uncertain, collect a sample (approximately 50mm of mycelium or affected timber) and send to a mycological laboratory for species identification. Several laboratories offer this service; the PCA or CSRT surveyor can advise on the appropriate laboratory.

Moisture Source Elimination

No treatment — chemical or physical — will provide a lasting solution unless the moisture source is identified and eliminated first. This is the single most important step.

Carry out a full moisture investigation:

• Check all DPCs — are they present, continuous, and above external ground level?
• Check drainage — are gutters, downpipes, and surface drainage functioning? Is there ponding against the wall?
• Check roof coverings and flashings — are they intact?
• Check internal plumbing — any leaks from pipes, sanitary fittings, or heating systems?
• Measure moisture content with a calibrated resistance meter at multiple points in and around the affected area
• Map the pattern of high moisture readings to identify the moisture pathway

It is not sufficient to simply dry out the building without fixing the source. The moisture will return.

Dry Rot Treatment Procedure

Dry rot treatment must be systematic and thorough. Inadequate treatment — particularly failing to remove sufficient timber or to treat masonry — will result in re-growth.

Stage 1 — Containment Before and during removal, take steps to prevent spore dispersal. Close off air conditioning and ventilation systems in adjacent spaces. Consider erecting polythene sheeting barriers to contain dust and spores. Spores of Serpula lacrymans can remain viable for years.

Stage 2 — Structural assessment and removal Survey the full extent of infection. Probe all accessible timber within 2 metres of visible growth with a bradawl or spike — rotted timber will accept the probe without resistance. Mark the boundary of apparently sound timber. Remove all affected timber plus a minimum 600mm margin beyond that boundary. All removed timber must be double-bagged and removed from site; do not burn it on site as this spreads spores.

Stage 3 — Masonry treatment Using a wire brush or disc grinder, remove all mycelium from masonry surfaces to a minimum 300mm beyond the visible growth. Apply a fungicidal masonry treatment (Safeguard Biocide Spray, Wykamol Tri-Ethylene Glycol solution, or similar PCA-approved product) to all masonry within the affected zone. Two coats, ensuring full penetration of mortar joints. Allow to dry between coats.

Drill and inject fungicidal fluid into mortar joints in the affected wall area if strands have penetrated the masonry. Use an injection system (e.g. Wykamol Basiment Injection or similar) at approximately 200mm centres in a grid pattern.

Stage 4 — New timber installation All replacement timber must be pre-treated to BS 8417 Use Class 2 or 3 (depending on the application). End grain of replacement timbers must be treated with preservative before installation, as end grain is most vulnerable to moisture and fungal attack. Apply boron paste or flood application to new timbers and adjacent existing timber.

Stage 5 — Ventilation improvement Dry rot in subfloor zones is almost always associated with inadequate ventilation. Inspect air bricks — are they clear, sufficient in number, and correctly positioned to create a through-draught? The minimum required is 1500mm² of free ventilation per metre run of external wall (Approved Document C), and cross-ventilation must be achieved. Add airbricks where needed.

Wet Rot Treatment Procedure

Wet rot treatment is less extensive than dry rot but the same principles apply.

Stage 1 — Source identification and elimination As above. Wet rot will not recur in dry timber.

Stage 2 — Removal of affected timber Remove all decayed timber. Probe surrounding timber and remove anything with a moisture content above 20% or that accepts a bradawl probe. Wet rot is confined to the damp zone, so removal margins can be smaller than for dry rot — but be thorough in the damp zone itself.

Stage 3 — Treatment of remaining in-situ timber Flood-apply sodium borate solution (Boron 10, Tim-Bor at 15% concentration) to all in-situ timber within and adjacent to the affected area. For structural timber in areas of ongoing moisture risk, install Boron rods into drilled holes at 100–150mm centres. The rods diffuse borate into the timber over several years providing long-term protection.

Stage 4 — Replacement timber Pre-treated timber to BS 8417 Use Class 2 (internal, risk of exposure to moisture). End-treat cut ends with preservative before installation.

Stage 5 — Epoxy repair for joinery Where wet rot in window frames, door frames, or decorative joinery does not warrant full replacement, epoxy consolidant repair is appropriate provided no more than approximately 30–50% of the timber section is lost. Probe the extent of decay, remove all soft material, dry thoroughly (moisture content must be below 20% before repair), apply Ronseal High Performance Wood Hardener or Repair Care Dry Flex to consolidate the remaining fibre, then fill with compatible two-part epoxy filler, shape to profile, and finish with appropriate paint system.

Long-Term Timber Preservation and Monitoring

Following remediation, structural timber in at-risk zones (subfloor, flat roof, ground floor joists in solid-floor perimeter areas) should be monitored. Calibrated resistance moisture meters with remote probes installed in key joists can provide early warning of rising moisture before decay restarts.

Annual maintenance inspection should check all exposed timber, airbricks, gutters, and drainage around the building. The cost of preventive maintenance is a small fraction of the cost of remediation.

Frequently Asked Questions

Can I treat dry rot myself without a specialist?

Small, isolated patches of wet rot in window frames or joinery can be treated by a competent DIYer using epoxy consolidants. However, dry rot in structural timber requires a specialist treatment by a CSRT-qualified contractor. The consequences of inadequate dry rot remediation are severe — re-growth is common, and legal liability for re-infestation following inadequate treatment is a significant issue if the property is sold.

How long does dry rot treatment take?

A typical dry rot treatment in a subfloor (affecting 2–4 floor joists and adjacent masonry) takes 3–5 days for a two-person team: one day for investigation and marking, one day for removal, one day for treatment and reinstatement begins. More extensive infestations affecting multiple floors or party walls take correspondingly longer. Allow 4–6 weeks for the masonry to dry and the replacement timber to acclimatise before applying floor finishes.

Will my property insurer pay for dry rot treatment?

Typically no. Most buildings insurance policies exclude gradual deterioration and damage caused by failure of maintenance, which is the category under which dry rot usually falls. Some policies cover sudden damage from water leaks (e.g. burst pipe) that leads to rot, but these cases are contested. Check your policy wording carefully.

What is the difference between Boron treatment and traditional Lindane/pentachlorophenol treatments?

Boron (sodium borate) has largely replaced organo-chlorine and organo-phosphate preservatives in the UK. Lindane (gamma-HCH) was banned for use in timber treatment in the UK in 2002 due to its persistence and toxicity to humans and the environment. Boron is significantly less toxic, has low environmental persistence, and is effective against both fungi and wood-boring insects. It is water-soluble, so it requires a dry substrate for application and cannot be used where the wood will be permanently wet or exposed to rain.

Should I replace all the timber or just the rotten parts?

For dry rot: remove all affected timber plus 600mm margin regardless of apparent visual condition. The cost of re-treating is far greater than the cost of additional replacement timber. For wet rot: remove all timber with moisture content above 20% or showing any decay. Do not patch where more than 50% of the section is affected; replace the full structural member.

Regulations & Standards

• BS 8417:2011 — Preservation of wood; code of practice covering preservative types, use classes, and application methods

• BS EN 335:2013 — Durability of wood and wood-based products; use classes; defines Use Class 1–5 for timber in different exposure conditions

• Building Regulations Approved Document C — requires adequate subfloor ventilation to prevent timber decay

• HSE COSHH regulations — apply to handling preservative chemicals on site; data sheets for all products must be available and read before use

• PCA Code of Practice for Remedial Timber Treatment — professional body guidance for CSRT-qualified surveyors and contractors

• Property Care Association — Timber Decay Technical Guidance — CSRT qualification, member directory, technical bulletins

• BRE Good Repair Guide 9 — Repairing and replacing timber window boards — guidance on epoxy repair of joinery

• Historic England — Timber Decay in Historic Buildings — guidance specific to traditional construction and heritage buildings

• Wykamol Technical Data Sheets — fungicidal treatment product data

• Repair Care International — Dry Flex System — epoxy consolidant system for joinery repair

• penetrating damp — moisture sources leading to timber decay

• damp proof membrane — ground floor moisture control preventing subfloor decay

• breathable membranes — vapour-open construction reducing condensation risk in roof timbers

• interstitial condensation — condensation within wall and roof build-ups causing concealed decay