Your home's foundations

Foundation types

It's good to know the type of foundation(s) your home has and to think about whether you can do anything to improve or maintain their condition and strength.

Different types of foundations perform differently, including in a natural disaster, and can be affected by their geographical location, ground conditions and any land features (for example, sloping).

The most common foundation types in New Zealand are suspended timber floors with a crawl space underneath (called a sub-floor) and concrete slabs cast on the ground. Some properties have a mixture of foundation types.

Suspended floor foundation

Suspended floor foundations usually consist of timber boards or sheets fixed to a series of joists. The joists sit on timber beams called bearers, which are fixed to piles in the ground.

There are two main types of suspended floor foundations, those supported on piles and those with perimeter concrete foundations.

Pile foundation (left) and piles with concrete perimeter foundation (right).

If your home has a suspended floor, you can check the foundations. Alternatively you might ask a professional (for example, an engineer or licensed building practitioner) to do that for you, assessing their overall condition and the way the house is connected to its foundations.

Concrete slab foundation

If your home has a concrete slab foundation there isn’t a lot you can do yourself to check its strength as the slab sits directly on the ground.

If your house sits on or near a slope, look for evidence under the slab of any washouts from heavy rainfall. Washouts can severely compromise the strength of your slab and you may need to get them fixed before they do too much damage.

If you’re concerned or need work done, talk to a licensed building practitioner or engineer. You may need a building consent if work is required.

The Renovate.org website has more information about concrete slab floor construction(external link).

Mixed foundation types 

If your house has a mix of different foundation types (for example, pile foundations with a concrete slab foundation extension) they are likely to perform differently in an earthquake, landslide or other natural disaster.

That's because the two parts of the house might move in different ways and the connection between the two foundations may be weaker than a single foundation. Make sure that the foundations and the connections between them are in good order. Get professional advice from an engineer or licensed building practitioner if you need help.

Simple checks around the house

It’s worth having a look around your home occasionally and creating a list or photographic record of anything that might indicate problems with your foundations. That way it should be easier to identify any changes to the foundations as the house ages or following a natural disaster. Check the cause of any changes as soon as you can, or get professionals to help you.

Inside the house

You can make some basic checks inside your house for signs that might indicate a foundation problem.

Some things to look for include:

• gaps or cracks where the walls and ceilings meet
• doors and windows that stick or don’t quite fit
• sloping or uneven floors
• uneven or cracked ceramic floor tiles
• distressed vinyl floor coverings or carpet (for example, stretched or loose)
• distressed wallpaper in room corners (for example, coming away from the wall)
• signs of borer or rotten timber.

Some of these may be signs of other issues, so check the cause as soon as you can.

Outside the house

You can make some basic checks outside your house for signs that might indicate a foundation problem.

Some things to look for include:

• cracked, crumbling, or exposed piles (sitting under the outer wall)
• cracked, crumbling or weakened foundation walls
• signs of borer or rotting timber
• water damage and ponding water near the perimeter foundation or under any deck
• soil movement or slumping.

Checking under the house

If your house has a suspended floor foundation, you may be able to inspect under your house. Don't go under the house if you think the foundations have weakened or there's a health and safety risk.

If you decide to go under your house, make sure you do this safely.

Different parts and fastenings of a suspended floor foundation.

Are the piles in good condition?

You can make some basic checks under your house for signs that could indicate a foundation problem. Some things to look for include:

• piles on a tilt or not standing upright
• piles unevenly supporting the bearers, possibly with gaps
• timber piles, bearers and joists showing signs of rot or borer
• concrete piles showing signs of cracking or crumbling
• piles exposed by ground excavation or slips
• where piles are missing, or other objects have been used to support joists or bearers.

Piles should be firmly embedded in the soil without gaps around their bases. Piles that are damaged, misaligned or missing need to be repaired or replaced. These aren’t easy tasks, so get advice or help from a licensed building professional.

Is the house properly connected to the piles?

Piles that are well connected to a house will help minimise movement and damage during an earthquake and some other natural disasters.

Check that connections are in good condition (not loose, rusted, broken or missing) between:

• bearers and piles
• bearers and joists.

If the piles are not sufficiently connected to the bearers, special fixings such as Z-nails that are made specifically for this purpose, wire, bolt or bracket bearers (all available from building suppliers) can be fitted to existing concrete or timber piles and bearers.

Be sure to use fittings suitable for your house’s environment. For example, any steel connections within 600mm of the cleared ground level need to be a minimum of type 304 stainless steel.

For detailed information on fixings, see the Foundation and Subfloor section of the NZS 3604 Standard for Timber-framed buildings(external link) and, if in doubt, contact a licensed building professional.

Are the piles supported by bracing?

Piles need support to prevent them tilting in an earthquake. Bracing can provide the piles with support. Seek advice on bracing from a licensed building practitioner or engineer, particularly if your home has either:

• a concrete perimeter foundation wall that is not continuous
• a subfloor more than 600mm above ground (especially one with pole foundations).

If necessary, a chartered professional engineer could design the right bracing solution for your house. Installing bracing is a job for a licensed building practitioner.

If your house has bracing, you can complete a basic bracing review by checking that:

• any timber framing is still adequately connected to the wall and not damaged
• any fasteners are in good condition (not loose, rusty, broken or missing)
• if you have a concrete perimeter foundation wall, there are no signs of cracking or crumbling.

Further guidance and information on foundations affected by the Canterbury earthquake sequence can be found in Part A of this document from MBIE’s building website(external link).

While it has been prepared for Canterbury homeowners, it has relevant information for all New Zealanders on foundations following an earthquake.

Staying safe under the house

Some key steps:

• Find the access point to your foundations.
• Ask someone to watch while you’re underneath and let them know which areas you will be inspecting.
• Check it's safe to work around any power and gas services. If not, turn them off. If you have aluminium foil insulation turn the power off to avoid an electric shock. 
• Wear appropriate safety gear for the task, such as hard hat, mask, overalls, kneepads, boots and protective eyewear.

If you decide to go under your house, make sure you do so safely. You can read more about entering confined spaces(external link) on the WorkSafe website.

If you notice any problems or changes to the foundations, ask a licensed building practitioner or an engineer whether your foundations need work.

If in doubt, get a licensed building practitioner to check the foundations for you - they will know what to look for and how to safely carry out an inspection under the house. They may also be able to suggest some foundation repairs to be carried out, either by you or by a licensed building practitioner.