Subfloor – timber decay

Cause

Damp subfloor space, lack of ventilation

Repair

  • rotten timber should be replaced. New timber piles must be treated to hazard class H5. H1.2 is the minimum treatment acceptable for subfloor framing, but H3.2 for framing within 300 mm of the ground is recommended where ventilation requirements are met. If there is poor ventilation, using H3.2 for all replacement subfloor framing would be prudent. Replacing a few old rotten wooden piles under a house with new treated timber piles in the same positions does not require building consent, although complete or substantial replacement would.

  • 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 rubbish or 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
  • where necessary, create new ventilation by:

    • replacing or refixing the baseboards with space for air movement between them

    • installing new ventilation grilles in sheet cladding/bracing on sloping sites

    • cutting additional ventilation openings in concrete or concrete masonry foundation walls and installing surface-mounted proprietary grilles. Take care that openings are not formed in the top course of the blockwork and that reinforcing is not cut 

  • 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 75 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

Leaking water or waste pipe; overflowing or leaking from wet area fixtures such as bath, shower, handbasin, dishwasher, clothes washer, hot water cylinder

Repair

  • identify the source and location of the leak. Make a permanent repair, or call a licensed or certifying plumber to make permanent repair. Work on waste pipes and drains must by law be carried out by a licensed or certifying plumber.

  • establish whether the leak is likely to be a one-off problem or stems from old deteriorated pipework/drains or a design or materials fault that may recur. If the latter, discuss potential long-term solutions with a licensed or certifying plumber or NZIBS building surveyor.

Cause

Subfloor framing is too close to or in contact with the ground

Repair

  • where all or part of a house is sitting on the ground, seek expert advice. Lifting the house and setting it on new piles will almost certainly be required. This 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 repiling will require building consent (and lifting the house may require a resource consent).

  • 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

  • rotten timber should be replaced. New timber piles must be treated to hazard class H5. H1.2 is the minimum treatment acceptable for subfloor framing, but H3.2 for framing within 300 mm of the ground is recommended where ventilation requirements are met. If there is poor ventilation, using H3.2 for all replacement subfloor framing would be prudent. Replacing a few old rotten wooden piles under a house with new treated timber piles in the same positions does not require building consent, although complete or substantial replacement would.

  • where there is a subfloor space but it is small, if possible excavate to increase ground clearance to the subfloor framing without undermining the piles. Clearances should be:

    • 150 mm minimum

    • 300 mm below bearers (recommended)

    • 450 mm below joists (to provide a crawl space)

  • where possible, provide subfloor ventilation as described in the first repair above

  • where physically possible, lay polythene sheeting to completely cover the ground under the house, as described in the first repair above.