What is pile rot?
Pile rot is the decay of timber foundation piles, causing them to lose their bearing capacity. The main cause is the pile head drying out: as soon as oxygen reaches the timber, wood-rotting fungi take hold.
Timber piles under old buildings last for centuries — but only while they stand permanently below the groundwater table. Under water there is too little oxygen for fungi. When the water table falls, through drought, drainage, a leaking sewer, a construction dewatering scheme or a change in water-level management, decay begins.
Two forms of decay
- Fungal attack (drying out). Fast and destructive: once heads are exposed, a pile can lose most of its capacity in a matter of years to decades. Pine and spruce are especially vulnerable.
- Bacterial degradation. This also occurs under water, but far more slowly — over decades to more than a century. The wood becomes soft and spongy while the shape still looks intact.
Signs in the building
No single sign is proof, but this combination is suspicious:
- Diagonal cracks in the masonry, often above or beside window openings
- Doors and windows that stick, or a noticeably sloping floor
- Different settlement between the main house and a rear extension
- A facade leaning out of line with the neighbours
Diagnosis and repair
Only a foundation survey gives certainty: an inspection pit beside the foundation, with the pile heads exposed, the groundwater level measured and timber samples assessed. This is usually combined with a cone penetration test and a levelling survey of the bed joints along the facade.
If the foundation really is failing, foundation repair follows — normally new steel tube piles or concrete piles under a new ground beam. This is major, expensive work: costs typically run from the mid five figures to well over a hundred thousand euros or pounds per building, depending on width, method and access. Damage from pile rot is generally not covered by buildings insurance.
Related terms
- Pile foundation
- Timber pile
- Groundwater level
- Foundation repair
- Foundation survey
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