Both need moisture; only one travels
Wet rot and dry rot are both fungal decay in timber, and both require timber that has become and stayed wet. The critical difference is behaviour. Wet rot stays in the wet timber. Dry rot, Serpula lacrymans, can grow through and across masonry and plaster in search of fresh timber, carrying moisture with it, which is why its extent can be much greater than the visible damage suggests.
What dry rot looks like
Deep cuboidal cracking across and along the grain; decayed timber that is dry, light and crumbles readily; white to grey mycelium with a cotton-wool appearance; fine grey strands that may cross masonry; fruiting bodies with a rust-coloured spore surface; rust-red spore dust on nearby surfaces; and a distinctive damp, mushroom-like smell.
What wet rot looks like
Timber that is darker than sound timber, soft, spongy or stringy, often with localised cracking along the grain; decay confined to the area the water reaches; sometimes a thin brown or black fungal growth; and no extensive strands through masonry, no rust-red spore dust. Coniophora puteana is the species most often responsible and it generally needs wetter timber than dry rot does.
Why the distinction changes the job
With wet rot, stopping the water and repairing or replacing the affected timber is usually proportionate work in a defined area. With dry rot, the outbreak has to be traced — including through masonry where relevant — before repairs can be priced honestly, and the scope of exposure is often the largest variable in the cost.
What both remedies have in common
Find the water and stop it. Fungal decay needs sustained moisture; timber in a normally heated, ventilated building sits comfortably below the moisture content decay requires. Correct the defect, allow the structure to dry, repair or replace timber that has lost strength, isolate new timber from damp masonry, and restore airflow to voids that have lost it.
Where chemical treatment fits
Treatment is appropriate in specific circumstances: timber that must remain in a position of residual risk, masonry implicated in the spread of dry rot, or new timber being built into a location that cannot be fully dried. It is not a substitute for stopping the water, and a blanket spray of a whole roof or floor is rarely justified by the evidence.
Limitations of general guidance
This article is general guidance. It cannot confirm the cause of dampness in a specific property. Recommendations should follow an inspection of the building, its external environment and its internal conditions.
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