Crumbling concrete piles

Cause

Poor quality concrete

Repair

  • all damaged piles should be replaced. Install new H5-treated timber piles or precast concrete piles. Follow NZS 3604:2011 Timber-framed buildings, Section 6

  • getting the fixings right is important:

    • concrete piles are fixed to bearers with 4 mm wire threaded through a hole in the top of the pile and then stapled to the bearer with 4 staples at each end of the wire. The wire is hooked to form a U around the end staple to allow the wire to be pulled tight before the remaining staples are driven across both wires of the U

    • with 125 mm square H5‐treated timber piles, use 2/4.9 mm wire dogs plus 2/100 x 3.75 mm nails skew fixed into the top of the pile or 4/100 x 3.75 mm nails skew fixed into the top of the pile.  

    • fixings should be hot-dip galvanised, or stainless steel in exposure zone D, or in zones B and C where the top of the timber pile is less than 600 mm from the ground or the floor has more than 7000 mm2 of clear vent area per m2 of floor

  • insert DPC (damp-proof course) between the pile and the bearer

  • repiling falls into the category of ‘restricted building work’. When done by a contractor, it must be carried out or supervised by a licensed building practitioner with a Foundations licence
    • if only one or two concrete piles are crumbling, replacing these with new piles in the same location will not require a building consent (but still needs to comply with the Building Act). Repiling all of a house or a substantial area will require building consent.

Cause

The reinforcing is rusting

Repair

  • replace the damaged piles as described in the first repair above

  • the rusting may be the result of a damp subfloor space. Once the damaged piles are replaced, remove anything that limits subfloor ventilation:

    • clear away obstructions around the house perimeter such as planting or soil build-up outside the vents

    • for painted vents or grilles, remove paint build-up that may reduce the air path space

    • clear 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.

Cause

Damp subfloor space, lack of ventilation

Repair

  • replace the damaged piles as described in the first repair above

  • once the damaged piles are replaced, remove anything that limits subfloor ventilation:

    • clear away obstructions around the house perimeter such as planting or soil build-up outside the vents

    • for painted vents or grilles, remove paint build-up that may reduce the air path space

    • clear 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.