Soft, cracked or discoloured timber is a reason to investigate moisture and structural condition, not a reason to buy the first fungicide on a shelf. “Dry rot” is a misleading name: the fungus still needs damp conditions to start. Wet rot is another group of wood-decaying fungi. The two can look similar, and an image alone is not a reliable species diagnosis. If a floor, joist, stair or roof member may have lost strength, keep people off it and ask a qualified surveyor or engineer to assess it.
The Society for the Protection of Ancient Buildings explains that the first step for either type of decay is to eliminate dampness and help timber dry. UK government guidance for older buildings warns that wet and dry rot can reduce timber’s load-bearing capacity.
Hi there, fellow DIYer,
A cellar smell and a soft skirting board are clues, but they are not a laboratory identification. Start with the water: a leaking gutter, roof, pipe, blocked air brick or wet masonry can keep hidden timber damp for a long time. Here is what you can record without breaking open sound parts of the building.
What the names mean
Dry rot, commonly associated with Serpula lacrymans, is a form of timber decay that can spread across masonry or other non-wood surfaces to reach more timber. It does not grow without a moisture source. The SPAB notes that affected timber may become darker and show cube-like cracking as it dries.
Wet rot covers other decay fungi that generally remain associated with persistently damp wood. One example is Coniophora puteana, sometimes called cellar fungus. Some wet rot makes timber soft and fibrous; brown wet rot can also crack and resemble dry rot. That overlap is why a surface photograph or a single colour description should not settle the diagnosis.
Mould on plaster is not the same as fungal decay of structural timber. A damp wall can have both. Look at the wood itself and trace where the water has come from.
Check for warning signs without making the damage worse
| Observation | What it can tell you | Safe next step |
|---|---|---|
| Timber feels soft, crumbles or has cube-like cracks | Decay is possible; strength may be reduced | Do not load it; arrange a timber and structural assessment |
| Wall, joist end or floor stays wet | A moisture source is still active | Find and repair leak or ventilation fault |
| Fungal strands or fruiting body on wood or masonry | Fungal activity may be present | Photograph it and seek specialist identification |
| Mould or staining only on the surface | Moisture needs investigating; rot is not proved | Check the adjacent timber and moisture source |
| Floor sags or stair feels unstable | Potential structural risk | Restrict access and get a qualified professional promptly |
Do not scrape off a fruiting body before a survey merely to make the area look cleaner. Record the location, photograph visible signs and note when a leak or overflow occurs. Avoid drilling into or removing a load-bearing member as a diagnostic experiment.
Fix the water before buying treatment
The SPAB says timber kept dry is not vulnerable to fungal attack and recommends eliminating sources of dampness and promoting drying. Examples include repairing a leaking downpipe and restoring ventilation. The aim is to bring timber moisture below 20%, but a handheld surface reading alone is not a complete survey of hidden joist ends or masonry.
Government housing guidance for England lists leaking pipes, blocked gutters and defective roofs as water-entry problems to repair. It also calls attention to roof and underfloor ventilation. Closing air bricks or coating a damp wall with an impermeable finish can make hidden timber wetter.
A chemical treatment does not replace a sound gutter, a repaired pipe or proper drying. Equally, “just improve ventilation” is not enough where rainwater or plumbing continues to enter.
When to bring in a specialist
Get a qualified timber survey when the decay extends behind finishes, returns after repairs or affects a floor, stair, roof, beam or joist. Ask the surveyor to identify the moisture source, map the extent of decay, explain how the fungus was identified and distinguish timber that can remain from timber that needs repair. If strength may be affected, ask for a structural engineer’s input before anyone removes supporting wood.
The SPAB cautions against automatic wholesale replacement, especially in older buildings. The least invasive effective repair depends on actual damage and whether the source of dampness can be removed. A quote should separate moisture repairs, drying, timber repairs and any proposed chemical treatment. There is no responsible fixed UK price per square metre for an unseen problem.
If the property is rented, report the damp and suspected timber damage promptly to the landlord or managing agent and keep photographs and dates. The legal route varies across the UK and by tenancy; do not rely on a generic promise that one party must pay every cost.
A short inspection record
| Location | Visible sign | Possible moisture source | Action and date |
|---|---|---|---|
| Example: joist end by outside wall | Soft, dark timber | Leaking downpipe above | Isolate area; request survey |
| Your finding |
The crucial distinction is between spotting a warning sign and proving a particular fungus. Find the moisture, protect anyone who might stand on weakened timber, and let a competent inspection guide the repair.
Best wishes from Fred Schouten