Concrete, timber or steel? Choosing the right foundations
 

Kiwi foundations have come a long way since the early days of dirt floors with brick or stone piles, untreated logs or even whalebone like Fyffe House near Kaikoura.

These days, your best options are either a concrete slab or treated timber piles supporting a suspended floor. Concrete is easier to install and more durable, but is seen as less sustainable than timber and not so good in certain earthquake-prone areas.

But are these reputations justified? Whether you’re choosing the materials for new foundations, or making repairs to existing homes, there are a few key things to consider.

Seismic risk

After the Christchurch earthquake, houses on slabs in liquefaction zones were badly damaged. It’s best to avoid slab foundations in areas with a known liquefaction risk. However, BRANZ points out that timber pile foundations that are poorly connected to the building superstructure are also vulnerable, having seen buildings shaken off their foundations. Good subfloor bracing and connections to floor framing are essential.

Weather

New Zealand can also experience some extreme weather. Concrete slabs can shift in frozen soil, so care is needed to ensure the supporting concrete beam goes below the frost line. On the other hand, houses on piles need insulated floor joists, which will add costs. Good drainage is also essential to ensure rain and snow aren’t making the subfloor damp.

The benefit of concrete is that it requires less maintenance over the long term. The downside is, if subsidence occurs due to flooding or erosion, it’s usually cheaper to fix a house on timber piles than a concrete slab.

Sustainability

When it comes to which option is more sustainable, timber is the natural choice. However, it does have some drawbacks. The usual treatments are harmful to the environment – while the greener options such as alkaline copper quaternary and copper azole-based (CuAz) treatments are more corrosive, so you’d need to be careful with metal fixings. As for concrete, low-carbon and recycling options are making it a much more sustainable choice than it used to be.

A third challenger, steel footings , are now available in New Zealand, promising a concrete free alternative with fast installation. While steel can’t match timber for sustainability, the piles don’t damage tree roots and there’s no need to excavate soil to pour concrete, reducing waste. While steel footings have some great benefits, they’ve been in the local market for less than a decade, meaning we don’t have extensive knowledge of how they perform in New Zealand conditions long-term.

The vital ingredients for any type of foundation are good installation, and good maintenance. Wellington City Council has a useful guide on the common foundation problems and how they can be addressed.

Every foundation has its pros and cons, depending on location, site and individual requirements. Before you dig, it pays to do some digging.