Know what you are looking at
Timber Rot: Dry Rot, Wet Rot and How to Tell Them Apart
Dry rot and wet rot can both weaken structural timbers, floorboards, roof timbers and joinery. However, they are caused by different fungi, behave differently within a building and can require very different levels of investigation.
This guide explains what causes timber decay, the signs associated with common rot fungi and why the problem should be properly identified before decisions are made about repairs.
Where decay starts
Timber Decay Needs Wet Timber
Timber does not decay simply because it is old. Fungal decay requires a sustained source of moisture, which may come from leaking roofs or pipework, defective rainwater goods, poor sub-floor ventilation, penetrating damp or prolonged condensation.
Timber with a moisture content above approximately 20% may be vulnerable to fungal decay. However, a moisture reading alone cannot establish which fungus is present, how far the decay extends or what is causing the timber to remain wet.
A professional timber survey considers the moisture pattern, condition of the surrounding building fabric and visible characteristics of the affected wood. The aim is to identify both the type of decay and the moisture source allowing it to develop.
The fungi responsible
Four Timber Decay Fungi Commonly Found in Buildings
Dry rot and wet rot are broad descriptions rather than a diagnosis in themselves. Several different fungi can affect timber, and some produce very similar patterns of cracking, discolouration and surface growth.
Correct identification matters. It helps establish how the decay is behaving, where further investigation may be required and whether damage could extend beyond the immediately visible area.
True Dry Rot
Serpula lacrymans
- Type of decay
- Brown rot
- Effect on the wood
- The timber shrinks, darkens and develops deep cracks across and along the grain, creating a distinctive cuboidal appearance.
- Where it occurs
- Usually found in damp, poorly ventilated and concealed areas where timber has remained wet for an extended period.
- Strands
- Grey or off-white strands may develop and can become as thick as a pencil. They are typically brittle when completely dry.
- Surface growth
- White cotton wool-like growth may appear, sometimes developing grey areas with yellow or lilac tinges.
- Fruiting body
- A fleshy, pancake or bracket-shaped growth with an ochre or reddish-brown centre and a pale margin when young.
- First noticeable sign
- Fine rust-coloured spore dust may sometimes be noticed before the full extent of the outbreak becomes apparent.
True dry rot can grow across and through damp masonry while searching for further timber. This ability to extend beyond the original moisture source is why its full extent must be investigated carefully.
Cellar Fungus
Coniophora puteana
- Type of decay
- Brown rot, commonly classified as wet rot
- Effect on the wood
- Affected timber generally becomes darker and develops cracking predominantly along the grain.
- Where it occurs
- Commonly associated with very damp areas, including cellars, suspended floors, roof structures and timber affected by persistent leaks.
- Strands
- Fine strands may appear yellowish when young before becoming dark brown or almost black.
- Surface growth
- Visible surface growth may be limited to a thin skin and can be entirely absent.
- Fruiting body
- Rarely identified inside buildings. Where present, it may form an olive-brown, sheet-like growth.
Cellar fungus is one of the wet rot fungi most frequently encountered in buildings. Decay generally remains associated with the area of persistent moisture rather than travelling extensively through dry building materials.
Pore or Mine Fungus
Fibroporia vaillantii
- Type of decay
- Brown rot, commonly classified as wet rot
- Effect on the wood
- The resulting timber damage can resemble dry rot, although the decay is usually more closely associated with the wet area.
- Where it occurs
- Often found in persistently damp structural timber and concealed building voids.
- Strands
- White or cream-coloured strands may be present. These are generally finer than dry rot strands and remain flexible after drying.
- Surface growth
- Cream or white sheets of growth may form a distinctive fern-like pattern.
- Fruiting body
- White sheets or plates containing numerous very small pores.
Because the resulting timber damage can resemble dry rot, appearance alone may lead to the wrong conclusion. The surrounding moisture conditions and characteristics of the fungal growth must also be considered.
White Rot of Joinery
Phellinus contiguus
- Type of decay
- White rot
- Effect on the wood
- The timber becomes paler and develops a soft, fibrous or string-like texture rather than cuboidal cracking.
- Where it occurs
- Most frequently found in external joinery, including window frames, doors and other exposed timber components.
- Strands
- No significant strands are normally produced.
- Surface growth
- Tawny or brown tufted growth may occasionally appear inside voids or on the timber surface.
- Fruiting body
- Tough brown growths may form, sometimes with an elongated shape and a surface containing numerous small pores.
This fungus is an important cause of deterioration in external joinery. Its fibrous appearance makes it visually different from the brittle, cuboidal damage normally associated with brown rot.
At a glance
Dry Rot or Wet Rot?
Not every piece of rotten timber has dry rot. Wet rot is considerably more common and usually remains close to the moisture source, while true dry rot can extend into adjoining areas and reach further timbers.
Some warning signs overlap, and fungal growth may be concealed beneath floors, behind plaster or inside roof and sub-floor voids. A reliable diagnosis should therefore consider the condition of the timber, the surrounding moisture pattern and the construction of the property rather than relying on one visual feature.
| Dry rot | Wet rot | |
|---|---|---|
| Spread through masonry | Can grow across or through damp masonry via fungal strands | Normally remains within the persistently damp area |
| Surface growth | Can be extensive and cotton wool-like | Often limited and sometimes absent |
| Fruiting bodies | More commonly found | Relatively uncommon inside buildings |
| Timber cracking | Deep cuboidal cracking across and along the grain | Cracking may follow the grain or vary by fungus |
| Moisture conditions | Develops where timber has remained sufficiently damp | Usually associated with a continuing local source of significant moisture |
| Investigation | The full extent must be traced beyond the visibly affected timber | Inspection generally concentrates on the wet area and associated timbers |
| First priority | Identify the moisture source, fungus and extent of decay | Identify and resolve the source keeping the timber wet |
Warning signs
Signs of Timber Rot to Look Out For
Timber decay is not always immediately visible. Floorboards may feel unusually soft or springy, skirting boards can begin to crumble and painted joinery may conceal deterioration beneath the surface.
Other warning signs include:
- Darkened, bleached or cracked timber
- Soft, brittle or fibrous wood
- Cuboidal cracking across the surface
- White, grey or yellow fungal growth
- Rust-coloured spore dust
- A persistent damp or mushroom-like smell
- Floor movement or localised loss of strength
- Decay around leaks, external walls or poorly ventilated voids
These signs can indicate a problem, but they do not confirm the species of fungus or reveal the full extent of the damage. Where structural or concealed timber may be affected, further investigation is sensible.
Professional investigation
What Does a Timber Rot Survey Include?
A timber survey investigates the affected areas, accessible structural timbers and the building defects that may be contributing to the decay. This can include floorboards, joists, wall plates, roof timbers, skirting boards and accessible external joinery.
The surveyor will consider moisture readings, ventilation, fungal growth, timber condition and relevant defects within the surrounding building fabric. Where access and prior permission allow, further investigation may be recommended to establish whether decay continues beneath floors or behind finishes.
Following the inspection, you will receive a written report explaining what was found, the likely cause, the extent visible at the time of the survey and any further investigation or work considered necessary.
Data note
Questions
Timber Rot FAQs
If yours is not here, ring 0330 100 25 45 and ask. We will tell you which survey you need, or that you do not need one.
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Book a Timber Survey
If you have discovered cracked, softened or decaying timber, the first step is to establish what type of rot is present, what is causing it and how far the problem extends.
Tell us where the affected timber is located and what you have noticed. We will recommend the most appropriate survey, confirm what the inspection will cover and provide the cost before anything is booked.
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- info@dampsurveyorsuk.com
- Unit 24, Storforth Lane Trading Estate, Hasland
Chesterfield S41 0QL
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