What is Slab Heave and How Does it Crack Houses

Slab heave is the uneven movement of a house footing and slab. Here’s our video on slab heave for an explanation.

Slab heave causes damage to internal walls and ceilings. It can cause cracks in the floor tiles.

“Doming slab heave” is when the slab is higher in the middle than around the edge.

“Dishing slab heave” is when the edges are higher than the middle of the slab.

Slab Heave Is:

• Uneven movement: Different parts of the house moving up and down; caused by
• Uneven changes in-ground moisture: The amount of water in the soil; resulting in
• Swelling of Reactive clays: Clay soils swell (or increase in volume) when they become wet (or absorb moisture) and shrink when they dry out.

What Causes Slab Heave?

Slab heave is caused by clay soils expanding when they absorb moisture. The source of moisture can be rainwater, broken sewer pipes, groundwater, poor surface drainage and garden irrigation.

The amount of water in the ground is often uneven and so the movement in the house is uneven.

Slab heave can also be caused by the ground drying out. Causes of the ground drying out include tree roots, long dry seasons, terminating irrigation and uneven shading of the ground.

Here’s QBCC’s take on the issue: https://www.qbcc.qld.gov.au/home-maintenance/dealing-subsidence

Why Does Slab Heave Affect Houses?

Think about the day or week or month that your house was built.

Right up to the minute your concrete slab was poured, the ground moisture conditions in your allotment were pretty much consistent, right? All of the ground got wet when it rained and dried out when it was sunny and hot.

Now consider what happens when a concrete house slab is poured on your allotment. The slab covers some of the ground and stops it from getting wetter or drier. Around the edge of the slab, the ground still gets wet and dry.

This variation in soil moisture under a house is what causes slab heave. If you could somehow keep all the ground wet or all the ground dry you could minimise the effects of slab heave.

An Experiment – See for Yourself

Try this experiment: Wet a tea towel evenly and lay it flat on a kitchen table. Cover the middle of the tea towel with an upside-down dinner plate. Now, wait.

What happens?

• Where the tea towel is not covered it dries out.
• Around the edges of the dinner plate, the tea towel dries out a little bit.
• In the middle under the plate, the tea towel stays wet.

Moisture evaporates out of the tea towel where it is uncovered. Some of the moisture under the upturned plate wicks sideways and evaporates too. But the moisture in the very middle of the plate remains.

The very same thing happens when you build a house. Uniform moisture conditions are interrupted when you build a house.

Whether the site starts wet and becomes dry or starts dry and becomes wet the effect is the same. Where there were consistent moisture levels in the soil under the house has now become uneven soil moisture levels.

Different levels of moisture in the soil result in uneven swelling and shrinking of reactive clays. Uneven swelling and shrinking of clays are causing your house to crack.

So it doesn’t matter whether your soil was wet or dry on the day you poured your slab, the soil moisture conditions under the slab change slowly and the soil moisture conditions around the outside change quickly.

Every second they are different, the soils are swelling or contracting at a different rate – and that is what is making your walls move up and down.

How to Solve Slab Heave

Check out our page on solving slab heave. It’s all about removing sources of uneven soil moisture.

Cornell Engineers Designs Raft Footings

We structurally engineer house footings to comply with the Australian standard for footings and slabs. We prefer raft slabs over waffle slabs. Want to know why? Click here.

Get a Quote or call us today. Cornell Engineers service all of Queensland and parts of New South Wales.