Subfloor – damp soil

Cause

Surface water flowing underneath the building if the subfloor ground level is lower than the ground or paving outside the building perimeter, or if the ground slopes towards the building instead of away from it

Repair

  • divert surface water away from the house by lowering the ground levels around the building and sloping the ground away from the house (open the drawing at right for details). Install channels or sub-surface drains and discharge into a stormwater drain or soak pit. 

Cause

High water table or natural spring

Repair

  • if necessary, reshape the ground as in the repair above and drain water away

  • 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

  • ensure good subfloor ventilation with a minimum of 10 air changes per hour (the recommendation for a wet site): 
    • 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 should be more than 7.5 m from a ventilation opening.

Cause

Leaking drain or waste pipe in the subfloor space

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 solutions with a licensed or certifying plumber or NZIBS building surveyor.

Cause

Inadequate subfloor ventilation due to blocked or no ventilation openings

Repair

  • ensure good subfloor ventilation with:
    • not less than 3500 mm² of clear open subfloor venting (grilles or gaps between baseboards) for every 1 m² of floor area
    • 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.
  • improve existing subfloor ventilation by:
    • clearing away obstructions 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
  • where necessary, create new ventilation by:
    • replacing or refixing the baseboards with space for air flow 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 (see drawing). Take care that openings are not formed in the top course of the blockwork and that reinforcing is not cut
  • where it is difficult to improve ventilation, lay polythene sheet over the ground as described above.

Cause

Subfloor airflow restricted by rubbish or items stored under the building

Repair

  • clear away items stored under the building, allowing for free movement of air

  • ensure good subfloor ventilation with:

    • not less than 3500 mm² of clear open subfloor venting (grilles or gaps between baseboards) for every 1 m² of floor area

    • 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 it is difficult to improve ventilation, lay polythene sheet over the ground as described above.