Joist ends are vulnerable
Floor joists built into an external wall can absorb moisture from masonry. If the wall is wet and ventilation below the floor is poor, the embedded end may decay while the visible floorboard still looks acceptable.
Leaks create very local decay
A small plumbing leak can keep one joist, sole plate or skirting section wet for months. Localised timber softness often deserves a search for a similarly local moisture source.

Repair includes drying the timber environment
Replacing rotten wood without correcting the leak, masonry moisture or lack of airflow leaves the new timber exposed to the same conditions.
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