What is Slab Heave and How Does it Crack Houses

Slab heave is uneven movement of a house slab and its footings, caused by reactive clay soil beneath it swelling and shrinking as its moisture content changes. Parts of the house rise or fall relative to each other, and the walls, ceilings and floors crack.

Slab heave typically shows up as cracks in internal walls and ceilings, cracked floor tiles, doors that stick or swing open, and floors that are no longer level. Here’s our video explaining it.

In short, slab heave is:

• uneven movement: different parts of the house moving up and down relative to each other;
• caused by uneven changes in ground moisture: the amount of water in the soil varies from one part of the site to another;
• acting on reactive clay: clay soils swell when they get wetter and shrink when they dry out.

Centre Heave vs Edge Heave

• Centre heave (“doming”) is when the middle of the slab ends up higher than the edges.
• Edge heave (“dishing”) is when the edges end up higher than the middle.

The same shapes can also come from the opposite process. If the perimeter dries and shrinks while the centre stays moist, the edges drop and the slab domes. Either way, it is the difference in moisture between parts of the site that does the damage.

What Causes Slab Heave?

Slab heave is caused by reactive clay soils changing volume as they gain or lose moisture. Sources of extra moisture include:

• rainwater and poor surface drainage around the house;
• leaking or broken stormwater, sewer and water pipes;
• garden irrigation and garden beds against the slab;
• groundwater and water moving through the ground from uphill; and
• badly installed ag drains and unsealed joints between paths and the house.

Drying causes uneven movement too. Tree roots, long dry seasons, switching off irrigation that the ground had become used to, and uneven shading can all dry part of the site and make it shrink.

How much a site can move depends on how reactive its clay is. That is set out in the site classification from your soil test.

Why Does Slab Heave Affect Houses?

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

Right up to the minute your slab was poured, ground moisture across your block was pretty consistent, right? All of the ground got wet when it rained and dried out when it was hot and sunny.

Now pour a concrete house slab over part of it. The slab covers some of the ground and stops it getting wetter or drier. Around the edge, the ground still gets wet and dry.

That variation in soil moisture under and around the house is what causes slab heave. If you could somehow keep all the ground equally wet, or equally dry, you would minimise it.

An Experiment: See for Yourself

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

What happens?

• Where the tea towel is uncovered, it dries out.
• Around the edges of the plate, it dries out a little.
• In the middle, under the plate, it stays wet.

Moisture evaporates where the tea towel is uncovered. Some of the moisture under the plate wicks sideways and evaporates too. But the moisture in the very middle stays put.

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

Whether the site starts wet and dries out, or starts dry and gets wetter, the effect is the same. Soil that had consistent moisture now has uneven moisture, and uneven moisture means reactive clay swells and shrinks unevenly.

Moisture under the slab changes slowly, while moisture around the outside changes quickly. Every moment they differ, the soil is swelling or shrinking at different rates, and that is what is moving your walls up and down.

Signs of Slab Heave

• diagonal cracks running from the corners of doors and windows;
• cracks in internal walls and cornices, often worse in the middle of the house (centre heave) or near external walls (edge heave);
• cracked or drummy floor tiles;
• doors that stick, or no longer latch;
• gaps opening under skirting boards or between walls and ceilings; and
• cracks that open and close with the seasons.

Not every crack is slab heave. Concrete shrinkage, settlement of fill and frame movement can look similar. Working out which one you have takes a proper investigation. See our article on cracks and gaps.

Slab Heave vs Subsidence

Heave is upward movement, usually from clay swelling. Subsidence or settlement is downward movement, from clay shrinking, fill consolidating, or loss of support. Houses on reactive clay often experience both in different places at different times of year.

The distinction matters because the fixes are different. Heave is usually managed by fixing the moisture source. Permanent settlement on poor ground sometimes needs underpinning, which is the wrong answer for most heave problems.

Heaving Concrete Driveways, Paths and Patios

External slabs heave for the same reasons, often more dramatically, because they are thin, lightly reinforced and sit right in the zone where the ground wets and dries fastest. Tree roots, leaking pipes and water running under the slab edge are common culprits.

A heaving path or driveway beside the house can also push against the house and direct water to its footings. That is one reason an isolation joint, properly sealed, is worth getting right.

How to Fix Slab Heave

Fixing slab heave is mostly about removing the sources of uneven soil moisture: repairing leaks, improving surface drainage with tools like spoon drains, managing gardens and trees, then giving the ground time to stabilise before repairing the cracks. We cover this in detail in how to fix slab heave.

The type of slab matters too. Waffle slabs are particularly vulnerable to moisture getting underneath. See beware waffle slabs and slab heave: waffle slab vs conventional raft slab.

For background, the QBCC also has homeowner information on subsidence and building movement.

Is Your House Cracking?

Cornell Engineers investigates slab heave and other building movement. We work out what is actually causing the cracking, whether it matters structurally, and what needs to happen next. Where there is a dispute, for example with a builder or insurer, we also prepare independent expert reports. See our forensic structural engineering and drainage advice pages.

Send us the property address, photographs of the cracking and of the ground around the house, a short description of when you noticed it and how it has changed, and any soil reports or drawings you have. We will review the information and let you know whether the matter is a good fit for Cornell Engineers before preparing a fee proposal.

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