Expansive Clay: Why Some NZ Soils Crack Floors and Walls
Cracks appearing in a concrete floor or along an interior wall can be unsettling, especially if they seem to open up after rain and close again when it dries out. In parts of New Zealand, the cause isn't poor workmanship — it's the soil itself. Some clay soils swell when wet and shrink when dry, and that movement can be enough to crack floors, jam doors, and strain walls.
What Makes a Soil "Expansive"?
Under the Building Code's B1/AS1 Acceptable Solution, a soil is classed as expansive when it has a liquid limit of more than 50% and a linear shrinkage of more than 15%, tested in accordance with NZS 4402. Soils that meet this definition are excluded from the "good ground" category that most standard house foundations rely on under NZS 3604, so a different foundation design approach is needed.
How Much Can It Move?
BRANZ's Level guidance notes that reactive clay can swell and shrink enough to produce surface movement of up to 50 mm between wet and dry conditions. Building a house also changes how moisture moves through the ground beneath it compared with the open site it used to be, which is part of why the movement can continue, or even change pattern, after construction.
What Makes It Worse
- Trees close to the house draw moisture out of the soil unevenly.
- Poor drainage or leaking pipes keep one part of the ground wetter than the rest.
- Paving and other impermeable surfaces change how rain reaches the soil near foundations.
Official guidance recommends keeping trees back from foundations by roughly one and a half times their mature height, for this reason.
How Foundations Are Designed For It
Where site testing confirms expansive soil, B1/AS1 sets out severity classes — from Slightly to Extremely expansive — based on the amount of ground movement expected. Each class carries its own foundation requirements, such as thicker reinforced concrete slabs. The Acceptable Solution also limits what can be built this way: generally single-storey homes with simple rectangular, L, T or boomerang-shaped plans, floor dimensions up to 24 m, and modest roof heights and slopes. Anything larger or more complex needs specific engineering design rather than the standard acceptable solution.
What To Do If You See Cracking
- Note when the cracks appear or widen — after rain, during a dry spell, or steadily over time.
- Check whether doors or windows have started sticking.
- Get a geotechnical or structural engineer to test the soil and assess the foundation rather than assuming it's just normal settling.
If you're planning a build on a site with clay soil, or you've noticed cracking in an existing building and aren't sure what's causing it, get in touch with Fil Engineers — we're happy to help you work out what's going on.
This article is general information, not a substitute for site-specific engineering advice. Every site is different, so talk to a suitably qualified engineer before making building decisions.
