Foundation Options: RibRaft, Rafts and Piles — How We Choose the Right One
Every house needs a foundation, but not every house gets the same one. The ground under your section, the slope, and whether the land is prone to liquefaction all change what a builder or engineer will specify. Here's a plain-language look at the two foundation types you'll hear about most on a typical build: engineered raft slabs (often sold under brand names like RibRaft) and piles.
The starting point: NZS 3604 and "good ground"
Most timber-framed NZ homes are built using NZS 3604:2011, the standard that sets out generic foundation and framing details for sites on what it defines as "good ground." If your site doesn't meet that definition — because of liquefaction risk, soft or variable soil, or a steep slope — the standard details no longer apply, and a specific engineering design is needed instead.
Raft slabs (RibRaft-type systems)
A raft foundation is a reinforced concrete slab that spreads the building's load over the whole floor area rather than at isolated points. Rib-and-pod raft systems such as RibRaft are one of the most widely used residential floor systems in New Zealand, and manufacturers also produce versions specifically engineered for TC2 and TC3 land — areas identified as having minor to moderate potential for liquefaction-related ground damage.
- Spreads load evenly, which helps where some differential ground movement is expected
- Reinforced with ductile, seismic-grade steel mesh, a requirement that followed lessons from the Canterbury earthquakes
- Commonly used on flat sites with reasonable, if not perfect, ground conditions
Importantly, Ministry of Business, Innovation and Employment (MBIE) guidance is clear that enhanced raft slabs without deep piles are only appropriate where liquefaction-related lateral ground movement is minor (broadly, up to about 50mm). Where the potential settlement or lateral spread is greater than that, a raft alone isn't considered appropriate, and the site needs a geotechnical investigation plus a specific engineering design — which often means piles.
Pile foundations
Piles are posts — timber, concrete or steel — that carry the building's loads down to the ground at individual points, lifting the floor clear of it. Under NZS 3604, timber piles must be a minimum size and treated to hazard class H5 for in-ground durability, and different pile types (ordinary, driven, braced or anchor piles) are used depending on whether they only need to carry vertical weight or also need to resist wind and earthquake loads sideways.
- Suit sloping sites, where a slab would mean significant cut and fill
- Raise the floor for subfloor clearance, drainage and ventilation
- Can be designed to resist lateral loads where ground movement is expected to be more significant
So what decides which one you get?
In practice, the choice comes down to a few questions an engineer will work through with you:
- What does your geotechnical information say about the ground — is it "good ground" under NZS 3604, or does it need specific design?
- What's your site's technical category (TC1, TC2 or TC3), and what liquefaction potential does that imply?
- Is the site flat or sloping, and how far is it to stable ground?
- Do you need extra floor clearance for drainage, ventilation or access?
None of these questions have a one-size-fits-all answer, which is exactly why foundation design on anything other than a straightforward, flat, good-ground site should go through a chartered professional engineer rather than being assumed from a neighbour's house or a generic plan.
Thinking about foundations for your next build or renovation? Get in touch with Fil Engineers — we're happy to talk through what your site actually needs.
This article is general information only and is not a substitute for site-specific engineering advice. Always consult a chartered professional engineer for your particular site and project.
