Footings and Slabs Home Maintenance and Repairs

Slab Heave Explainer Video

What is slab heave and how does it affect houses?

Hi. I’m structural engineer Matt Cornell and this is my explainer video on slab heave.

G’day. I’m Matt Cornell from Cornell Engineers.

Today I’m answering a question about slab heave.

Before a house is built on a site, soil moisture conditions are fairly uniform across the site.

That changes when a house is built.

The uniform soil moisture conditions are interrupted by the house.

Slab heave occurs in clay soils when the soil moisture conditions under the house are no longer uniform.

When the soil around the outside of the house becomes wet the clay soils absorb moisture and expand.

The expansive forces are large enough to lift a house.

Under the house stays relatively dry.

There is a transition zone between the wet and dry soils as soil moisture transfers sideways.

When the soil around the outside of the house becomes dry the clay soils release moisture and shrink.

The ground surface moves down and the soil under the house is relatively wetter compared to the dry soils around the outside of the house.

Slab heave is caused by uneven movement of clayey soil. Dry ground causes the ground surface to drop

Again there’s the transition zone between dry soils and wetter soils as moisture traverses from under the house towards the dry zone.

Soil moisture conditions aren’t only affected by changing seasons.

Other sources of soil moisture changes are garden irrigation, broken pipes, poor drainage and tree roots.

This has been Matt Cornell from Cornell Engineers talking about slab heave.

Don’t forget to check out our website for more information and to follow us on Facebook.

Just a reminder:

  • Not everyone who thinks they have slab heave does.
  • Not everyone who thinks they have other problems does.
  • Only use experienced structural engineers and geotechnicians to diagnose and fix house movement.

Contact Cornell Engineers for more information about slab heave and how to fix it.

7 replies on “Slab Heave Explainer Video”

Hi Matt.
Thanks for your great website and the excellent information. We moved to Islington in London UK after 50 years in Sydney. London is mostly clay+silt+sand and our 9M long rear garden backs onto a park with large trees 10M away from the house. Mud for 5 months of the year and dry for the rest. All the 120 year old brick houses built here suffer from heave (and inadequate footings) so thank you for the clear, well presented and invaluable information and links.
John – Retired Architect.

Hi, I have just received my soil test report which indicates class P soil with highly reactive clay. The cost to build has increased by over $6000, which is substantial. Before I put down any confirmation, can I just inquire if this is going to an issue in the long term for a house built on this type of site please? Thank you.

Hi Matilda
The Class P could be due to a either soft soil, uncontrolled fill, current or future trees. it can be managed with good engineering design.
The highly reactive classification means that your builder and you will need to carefully manage the site during construction (builder) and afterwards (you).
Good drainage, careful planning and an appropriate footing system are all needed. Check out this article:
Matt Cornell

Hi Matt,

Your new video on slab heave is well appreciated. You mentioned abt tree roots as one of the contributing factor to slab heave. As a guide, what is the safe distance for any tree to be oplanted near the house? Example, Castle Wallan. Thank you.

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