Dry rot
Also called: serpula lacrymans
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Dry rot is fungal decay in felled timber, driven mainly by dampness and poor air circulation rather than by dryness itself.
What allows it to start
Wood that stays damp and unventilated is at risk. Timber that has not been thoroughly dried before use is also vulnerable, and a deal, a sawn softwood board, can be especially liable where the pith, the tree's central core, remains in the middle. Dry rot can weaken wood, so treat persistent moisture as a structural problem rather than merely a surface defect.
Prevent the conditions
Use thoroughly seasoned timber, meaning wood dried before it is built in, and keep sapwood out where the construction demands that precaution. Provide free movement of air, especially around the ends of members and beneath a ground floor. Timber that stays dry can be durable, but damp enclosed positions change the risk.
When it is present
Established dry rot requires decisive work. Infected woodwork should be removed completely and destroyed, while adjoining masonry or other affected materials need treatment rather than being left as a source of reinfection. The repair is not complete until the dampness and lack of ventilation that allowed the decay have also been addressed.
Do not trust the name
The name can mislead: the decay is associated with moisture even though the timber may later appear dry. Check the building's ventilation and water paths, then replace compromised wood with properly dried material. Where the extent is uncertain, obtain a specialist assessment before closing the work up.