Remedies: foundations and subfloors

Most foundations will be in good condition aside from lacking earthquake bracing. Other common problems include unevenness and corrosion of reinforcing.

Precast concrete piles and reinforced concrete foundation walls are likely to be in good condition. Damage may be more likely where the building is close to the sea or in a geothermal area where reinforcing may have corroded as a result of the salts in the air or the original concrete was poor quality.

Check wire ties connecting piles to bearers for corrosion and where this has occurred, the ties must be replaced.

Specific issues that may occur with foundations include: deterioration of subfloor framing; settlement due to poor ground giving uneven floors; inadequate foundation lateral support; and subfloor moisture and insufficient ventilation.

Failure due to deterioration

Generous ground clearances and generally good subfloor ventilation of houses with suspended timber floors mean that the underfloor is likely to be dry. Although deterioration of subfloor framing is unlikely to be a major problem as dry timber is less susceptible to fungal or insect attack (see Borer, rot and mould), the subfloor framing should still be checked.

Foundation walls should be checked for evidence of corrosion of the reinforcing steel.

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Settlement due to poor ground

Settlement of foundations or internal evidence of settlement may indicate that the house was built on unsuitable ground such as:

  • deep organic topsoil
  • peat 
  • soft or expansive clay 
  • uncompacted backfill.

Poor ground bearing or soft ground may be the result of wet soil which should be addressed if possible (see subfloor moisture and ventilation). Once the soil has dried, the bearing should improve, but it may be necessary in some cases to install additional piles and bearers.

If settlement has occurred, the advice of a structural engineer should be obtained.

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Inadequate foundation bracing

Existing piled foundations (without any in-situ concrete foundation walls) are not likely to have the level of lateral support required by the current version of NZS 3604. Those with foundation walls are likely to have resistance to lateral loads, but the condition of the fixings of the floor structure to the foundation should be checked to ensure that they are not corroded.

Where renovation or alteration work is proposed, particularly work that requires removing loadbearing walls or adding an extra floor, the existing foundations must be checked by an engineer for their ability to provide the required support and bracing.

Additional bracing (which may be required for houses with a piled system only) can be provided by a combination of:

  • timber bracing between piles
  • anchor piles
  • cantilevered piles
  • perimeter walls.

With suspended floor construction, diagonal bracing can be installed between adjacent piles, or between piles and bearers/joists using 12 kN pile fixings or M12 hot-dip galvanised bolts (stainless steel in areas close to the coast or for fixings within 300 mm of the ground) (Figure 1).

Where sufficient ventilation grilles may have been installed, a common problem is that they become blocked over time for a variety of reasons such as soil or paint build-up. Vents should therefore be checked and if there is anything blocking them, it should be cleared.

Current ventilation requirements are for a minimum of five changes of air per hour – this figure should be doubled for wet sites. A clear opening area of 3500 mm2 should be provided for each square metre of floor area. Vents should be located within 750 mm of corners and then evenly spaced around the building at 1.8 m centres maximum. No part of the subfloor should be further than 7.5 m from a ventilation grille.

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Uneven floors

If there has been a failure of the foundations, it will be evident in the unevenness and movement (springiness) of the floor.

A floor that is not level may be due to:

  • soft ground that has resulted in settlement
  • piles or bearers that have been removed during a previous alteration 
  • the floor not being levelled when the building was repiled 
  • the floor joist span being too great for the size of the joists
  • damaged floor joists.

Alternatively, installing anchor piles  embedded more deeply into the ground will provide additional lateral support. The number of piles required depends on the building’s size and location and earthquake and wind zones for the region.

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Subfloor moisture and ventilation

Insufficient subfloor ventilation or very wet soils may result in damp to the underside of flooring and higher moisture levels in the house, and may cause specific problems such as ‘cupping’ of the top surface of floorboards due to a lower moisture content on the upper surface than the underside of the boards.

This problem will be accentuated if the floorboards are covered with linoleum or have an applied finish, since excess moisture is unable to evaporate through the upper surface.

If there is evidence of moisture, check the subfloor space to ensure that the source of the water is not from leaking pipes or drains or from surface water running under the house.

If the cause of subfloor moisture cannot be remedied, options to address the problem include to:

  • cover the ground with polythene sheeting at least 0.25 mm thick (Figure 2). Ensure the ground is shaped so no water accumulates on top of the polythene. Lap sheets a minimum of 150 mm to cover the whole ground and weigh them down with bricks or concrete blocks.
  • install additional ventilation grilles in the foundation walls.