Rotten timber piles

Cause

Untreated piles in older houses (such as totara or puriri)

Repair

  • you can check for rot in timber piles or other timber with a screwdriver. Try to push it into the timber – with timber piles, do this just below the ground surface – and if the screwdriver easily enters the timber, then the timber is rotten
  • all rotten piles should be replaced. Use H5 treated timber piles (or precast concrete piles). Replacing just a few old rotten wooden piles under a house with new treated timber piles in the same positions does not require building consent. More extensive repiling falls into the category of ‘restricted building work’ and must be carried out or supervised by a licensed building practitioner with a Foundations licence. Total or extensive repiling will require building consent
  • the Building Act states that after the repairs/renovations are finished, the building must comply with the Building Code to at least the same level as it did before the work started
  • earthquake bracing should be added to the piles: find details on this page.

Cause

Damp subfloor space, lack of ventilation

Repair

  • test for rotten piles and then replace as necessary, as detailed in the repair above
  • once the rotten piles are replaced, remove anything that limits ventilation by:
    • clearing away obstructions around the house perimeter such as planting or soil build-up outside the vents
    • for painted vents or grilles, removing paint build-up that may reduce the air path space
    • clearing items stored under the house to improve air flow
  • ensure good subfloor ventilation:
    • provide not less than 3500 mm² of clear open subfloor venting (grilles or gaps between baseboards) for every 1 m² of floor area

    • make ventilation openings high in the subfloor space, near to the underside of bearers

    • spread vents evenly around the building perimeter to allow cross-ventilation

    • no part of the subfloor more than 7.5 m from a ventilation opening

  • lay polythene sheeting at least 0.25 mm thick to completely cover the ground under the house. Ensure that the ground is shaped so no water accumulates on top of the polythene. Polythene sheets should be lapped a minimum of 150 mm (preferably taped) and tightly butted up to foundation walls and piles. Weigh down the sheets with bricks or concrete to avoid them being displaced by air movement from subfloor ventilation.