Remedies: foundations and subfloors

Most foundations will be in good condition, though some may lack adequate earthquake bracing. Check for unevenness, moisture and other signs of damage or deterioration.

Original reinforced concrete foundation walls and precast concrete or timber piles are likely to be in good condition. There will be a higher risk of potential damage in coastal areas because of the higher levels of atmospheric salts, particularly if the original concrete was of poor quality.

There may also be problems such as inadequate framing or bracing, poor ground bearing, or problems with moisture.

Check for unevenness in floors, cracking in floors or cladding, and evidence of moisture or dampness. If there is evidence of dampness, check:

  • foundation walls for evidence of spalling of concrete and/or corrosion of the reinforcing steel
  • wire ties, bolts or proprietary connectors connecting piles to bearers for corrosion and where this has occurred replace them
  • timber bearers, joists and jack-framing for deterioration - any rotted timber will need to be replaced with new material.

Perimeter baseboards or sheets of fibre-cement or asbestos-cement may be cracked, broken, allow insufficient subfloor ventilation, or may simply look unsightly. Replacing with timber baseboards is usually not difficult. Take great care working with materials you suspect may contain asbestos. This can be dangerous and some types of work with asbestos is restricted to licensed specialists. Find more details on the asbestos page.

Inadequate ground clearance

Reinforced concrete floor slabs were sometimes constructed with insufficient clearance above ground (Figure 1). Also, if foundation walls were low, subfloor framing may have inadequate ground clearance. Subsequent landscaping (paths and driveways), gardening or site work may have reduced the ground clearance even further. Take great care working with materials you suspect may contain asbestos. This can be dangerous and some types of work with asbestos is restricted to licensed specialists. Find more details on the asbestos page.

If this is the case, soil should be removed from around the foundations or floor slab to meet minimum ground clearance requirements. Constructing exterior paving that falls away from the house to drain water may also be required.

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Poor ground bearing

Foundation subsidence, which can be indicated by cracking in concrete floor slabs or veneer cladding, uneven floors and doors that swing open or closed, may mean that the house was built on unsuitable ground such as:

  • deep organic topsoil
  • peat 
  • soft or expansive clay (expansive clay may also cause cracks as a result of dry clay swelling when it absorbs excessive moisture or as a result of the clay drying out and shrinking)
  • poorly compacted fill.

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

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Subfloor moisture and ventilation

Insufficient or restricted subfloor ventilation or very wet soils may result in deterioration of the timber framing, the underside of the particleboard flooring or cupping of floorboards, higher moisture levels in the wall framing and possibly within the in the house.

It may also result in the corrosion or tarnishing of foil insulation and corrosion of metal fixings. Foil insulation should be replaced with a bulk insulation product designed for underfloor use. The practice of repairing or retrofitting foil insulation has been banned.

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. Any leaking pipes or wastes should be repaired.

Where the cause of wet soil is due to ground water, this should be remedied by adding drainage or diverting ground water away from the house. Once the soil has dried out the bearing should improve, but in some cases it may be necessary to install additional piles and bearers.

If the subfloor moisture cannot be minimised through ventilation improvements, cover the ground under the building with polythene to prevent the moisture being emitted from the soil and thus reduce the subfloor moisture levels.

While sufficient ventilation grilles may have been installed, they can become blocked over time, often because of a soil build-up or DIY renovations. Ventilation grilles should therefore be checked and any obstructions should be cleared.

Current subfloor ventilation requirements are for a minimum of five changes of air per hour for dry sites - this figure should be increased to 10 for wet sites. A clear opening area of 3500 mm2 should be provided for each square metre of floor area which is what the design should aim to achieve with a renovation project. A ventilation grille must be located within 750 mm of each corner and grilles should be evenly spaced at 1.8 m centres maximum around the building.

No part of the subfloor should be further than 7.5 m from a ventilation grille.

There may be a small number of houses built during the 1970s that have brick veneer cladding that is open to a subfloor space where damp air from the subfloor may rise up through the veneer cavity and condense within the roof space - where this occurs:

  • cover the ground with polythene
  • block off the cavity from the subfloor if possible
  • form ventilation and drainage slots by raking out the mortar form every third vertical joint in the bottom course of bricks.


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Inadequate foundation bracing

Houses built early in the 1970s may not have lateral subfloor bracing that meets current requirements.

Where modifications are proposed, particularly work that requires removing loadbearing walls or adding an extra floor, the existing piles and their connections to the bearers must be checked by an engineer for their ability to provide the required support and bracing (Figure 2).

Even if renovations are not planned, adding foundation bracing may be a wise investment – the cost is low compared to the likely repair costs and disruption if a house is knocked off its foundations in an earthquake. Timber braces at a slope of not more than 45° can be fixed between piles and bearers/joists or concrete corner walls. Each brace is connected to the underfloor framing that runs parallel to it, whether that is a bearer or joist. The fixing of the bracing to the pile should be 200–300 mm above ground level.