Piled foundations – rusted fixings

Cause

High subfloor moisture levels; exposure to air-borne salt, dust or dirt; trapped moisture and dirt

Repair

  • for high subfloor moisture levels, try to identify the reason behind the moisture and deal with that first

  • identify and fix leaky pipes. You can fix a broken downpipe that is discharging rainwater under the house yourself, but if the leak is in a waste pipe or drain, under law that must be fixed by a licensed or certifying plumber

  • if surface water is getting under the house, lower the ground levels around the building and slope the ground away from the house. Install channels or sub-surface drains and discharge into a stormwater drain or soak pit

  • if constructing a soak pit, follow the guidance in New Zealand Building Code Verification Method E1/VM1. Be aware that construction of a soak pit may require a building consent

  • remove anything that is stored under the house and blocking free movement of air through the subfloor

  • remove anything outside the house perimeter such as raised garden beds or dense planting that is blocking ventilation

  • ensure good subfloor ventilation by providing 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 should be 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

  • remove and replace rusted fixings. In NZS 3604:2011 Timber-framed buildings, open subfloors are regarded as ‘sheltered’ environments. Steel fixings should at the least be hot-dip galvanised. All connections to treated timber piles within 600 mm of the ground, and/or in coastal areas, must be 304 stainless steel.

  • 50 x 50 x 3 mm washers must be used with the bolts

Cause

Copper-based timber treatments reacting with the metal of fixings

Repair

  • remove and replace rusted fixings with 304 stainless steel fixings

Cause

Inadequate corrosion protection for the environment/location 

Repair

  • remove and replace rusted fixings with 304 stainless steel fixings

Cause

Damp subfloor space, lack of ventilation

Repair

  • follow the guidance given in High subfloor moisture levels above

  • ensure good subfloor ventilation by providing 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 should be 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.

  • remove and replace rusted fixings as described in the first repair above.

Cause

Rusted wire ties, nails, nail plates, joist hangers, bolts

Repair

  • for high subfloor moisture levels, try to identify the reason behind the moisture and deal with that first
  • 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
  • remove and replace rusted fixings. In NZS 3604:2011 Timber-framed buildings, open subfloors are regarded as ‘sheltered’ environments. Steel fixings should at the least be hot-dip galvanised. All connections to treated timber piles within 600 mm of the ground, and/or in coastal areas, must be 304 stainless steel.
  • 50 x 50 x 3 mm washers must be used with the bolts.