Timber decay in UK homes falls into two broad categories — dry rot and wet rot — and confusing them is a costly mistake. Dry rot is a serious, aggressive fungal infection that can spread through masonry and destroy structural timber far beyond the original wet zone. Wet rot, while destructive at the location of moisture, is self-limiting and straightforward to treat once the water source is fixed. The distinction matters enormously for treatment scope and cost.
The Fundamental Difference
Both dry rot and wet rot are caused by fungi that break down the cellulose and/or lignin in timber, but they behave very differently.
Dry rot (Serpula lacrymans) is so-called because it can appear in timber that looks and feels dry by the time visible damage appears. The fungus draws moisture from the wood itself, drying it out and leaving it cracked in a characteristic brick-like pattern. More critically, Serpula lacrymans can transmit moisture and nutrients through specialised root-like structures called rhizomorphs, which can penetrate brick, plaster, and mortar. This allows dry rot to spread well beyond the initial wet zone into apparently dry areas of the building.
Wet rot is a broad term for several fungal species (Coniophora puteana — cellar fungus — being the most common) that require continuously wet timber to survive. Wet rot cannot spread through masonry and dies when the moisture source is removed. It is confined to the zone of dampness.
Identifying Dry Rot
Dry rot has a distinctive set of signs that, once seen, are difficult to confuse:
- Cuboidal or brick-like cracking of the timber — the wood splits in both directions, breaking into roughly square or rectangular blocks 10–50 mm across. This is the most diagnostic sign.
- Silky grey mycelium on or around the affected timber, sometimes with lilac or yellow tinges
- Orange-brown sporophore (fruiting body) — a flat or bracket-shaped pancake-like fruiting body with a rusty orange centre and white fleshy edges, often producing clouds of reddish-brown spores
- Rhizomorphs — grey or black root-like strands 2–5 mm diameter running across masonry, plaster, and through mortar joints
- Musty, mushroom-like smell — distinctive and often noticed before visible signs are present
- The timber sounds hollow when tapped and crumbles easily
Identifying Wet Rot
Wet rot signs are distinct from dry rot:
- Timber appears dark, soft, or spongy rather than cracked in a cuboidal pattern
- Longitudinal cracking along the grain (rather than the brick-like pattern of dry rot), if present
- Mycelium is fine, thread-like, and white or olive-brown — less visible than dry rot mycelium and does not form rhizomorphs
- No spreading through masonry — wet rot is confined to visibly damp timber
- The decay is directly correlated with the moisture source — follow the wet rot to find the leak, failed flashing, or blocked gutter
- Timber may spring back when pressed if early-stage, or feel spongy and break apart if advanced
Comparison Table
| Feature | Dry Rot (Serpula lacrymans) | Wet Rot (e.g. Coniophora puteana) |
|---|---|---|
| Moisture required | 20–30% (can self-sustain) | >30% (requires continuous moisture) |
| Crack pattern | Cuboidal (brick-like, across and along grain) | Longitudinal (along grain) or none |
| Mycelium colour | Silky grey, white, lilac or yellow tinges | White, olive, or brown; fine threads |
| Fruiting body | Fleshy, rusty-orange, flat pancake | Rarely visible; resupinate if present |
| Rhizomorphs | Yes — spreads through masonry | No |
| Spread potential | Can spread far beyond wet zone | Contained to wet zone |
| Smell | Musty, mushroom-like | Earthy or musty |
| Treatment complexity | High — masonry treatment required | Lower — fix moisture source, replace timber |
Why Dry Rot Treatment Is More Extensive
Because Serpula lacrymans can penetrate masonry via rhizomorphs, effective treatment must go beyond removing and replacing the affected timber. The standard treatment protocol involves:
- Remove all visibly affected timber with a margin of at least 300–500 mm of apparently sound timber beyond the last visible sign of infection
- Treat all masonry in the affected area and for 300 mm beyond the visible infection boundary with a fungicidal masonry biocide — typically sodium octaborate or a proprietary dry rot treatment solution applied by brush, spray, or injection into drill holes
- Treat all replacement and retained timber with a preservative, usually boron-based or copper-based
- Address the moisture source — without eliminating the dampness that originally triggered the outbreak, recurrence is likely
- Ventilate the treated area adequately during and after remediation
Any masonry treatment and replacement structural timber should be specified by a surveyor or structural engineer with rot remediation experience, and carried out by a PCA-registered contractor.
Treating Wet Rot
Wet rot treatment is more straightforward:
- Identify and fix the moisture source — this is the critical first step; all else is futile without it. Check gutters, flashings, downpipes, roofing, plumbing, and ground drainage.
- Remove affected timber — wet rot that has advanced beyond surface-deep softening requires the removal and replacement of the timber member
- Allow to dry out — wet rot dies once the moisture content of the timber falls below approximately 20%. Allow adequate drying time before closing up the area.
- Apply wood hardener and filler for minor decay — proprietary two-part epoxy consolidant and filler systems can be used for minor wet rot in non-structural joinery (window sills, fascia boards, cill timbers). Not appropriate for structural members.
- Replace structural timber where significantly weakened, and treat new timber with a preservative
Cost Guide
| Work | Typical Cost (2026) |
|---|---|
| Independent survey and report | £250–£500 |
| Wet rot treatment — minor joinery (epoxy repair) | £100–£500 per window or door |
| Wet rot treatment — structural timber replacement | £300–£3,000 depending on extent |
| Dry rot treatment — small isolated outbreak | £1,500–£3,500 |
| Dry rot treatment — moderate infestation (e.g. one room) | £3,000–£7,000 |
| Dry rot treatment — severe or multi-room | £7,000–£20,000+ |
| Masonry biocide treatment (per m²) | £15–£40 |
These ranges are wide because the extent of masonry involvement, the accessibility of the affected area, and the structural replacement required vary enormously.
When to Call a Specialist
Any suspected dry rot must be assessed by a specialist — ideally from a company that is PCA-registered and that employs a CSRT (Certificated Surveyor in Remedial Treatment). Because treatment inevitably involves opening up wall and floor structures, it is advisable to have an independent structural engineer verify the structural integrity of retained timbers and specify any replacement members.
Wet rot in non-structural joinery (window frames, sills, fascia) can be assessed and treated by a competent general contractor. Wet rot in floor joists, roof rafters, or other structural members warrants a specialist assessment to confirm the extent of decay and structural adequacy.
Building Regulations
Dry rot and wet rot repairs do not routinely require Building Regulations approval when carried out as like-for-like repairs. However, if structural timber is replaced or significantly altered, Building Control notification may be required. Specialist treatments involving chemical injection into masonry should comply with the requirements of BS 1282:2009 (wood preservation) and BS 8102:2022 (protection against water from the ground) where basement or below-ground work is involved.
As with all specialist remediation work, retain the contractor’s guarantee documentation — most specialist contractors offer 20–30 year guarantees backed by specialist insurance, and this documentation has value at the point of future sale.
