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.

Edge heave experiment
Plate on wet teatowel experiment simulates moisture changes under house slab

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.

14 replies on “What is Slab Heave and How Does it Crack Houses”

Matt, I live in NW Florida. We bought the house 8 years ago and it has always had a hump at the front door and the living room slopes to the exterior wall at the rear of the house. It is a monolithic slab. The house does not have any cracks anywhere and all of the windows and doors work properly. The only thing I have noticed, recently, is the front porch slab is separating more from the house. There is a gutter downspout in front of the porch but it has always been there. The other downspouts all drain into a drainage system out to the ditch in front yard. Here’s where I need help. We just finished an addition 15’x30′ on the side of the house. Poured the foundation 6/23 and started framing about a week later and it is finished now 9/23. I did not notice any heaving after the pour but when I started laying the floor I really noticed what looks like a heave. Along the 30′ exterior wall it rises in the middle of the span and slopes down to the corners and slopes down across the 15′ width. We did add a clay and sand mixture to the base but no gravel. There was a driveway there prior to this addition. There was a vapor barrier put down across the flat surface but not in the footer area. Question: would an exterior monolithic slab heave in just a month? I need to mention that the condensate drain for my ac unit was dumping right about the center of that 30′ span. I have since routed it to the front of the house. It was only draining there for about a month. There is no plumbing in the slab. Along with this being monolithic, on the 30′ side of the addition we poured a step down slab that is a driveway. It was all done in one pour. The condensate was actually dumping onto that step down slab and draining away from the house. There is no vapor barrier under the driveway slab. The addition has lap siding and when I look down the 30′ side it seems like there is a rise in the middle on some of the rows but not on others. Any help would be greatly appreciated.

Hi Scott
Timeframe for slab heave is a funny thing – but let’s first rule out ‘heave’ which is lifting. Is there potential another part of the slab has dropped relative to the high area rather than the high area going up? Could some of the ground have dried out after you poured the slab?
Unfortunately there isn’t much you can do if the slab is no longer level. You copuld maybe try a self-levelling compound over the top of the slab but if the movement is ongoing this will be just wasted when the slab moves again.
Fix all and any drainage issues and possible variations in moisture. Build a flexible superstructure (walls). Monitor. Hope….
Not much help I know but I hope it points you in the right direction.
Matt Cornell

We got a heaps slave but we cannot find out the exist cause of it! We inspect our proper more than two times even they cannot find out the reason! We got big zig zag crakes inside the house as well! Is any other reason of it!

High Matt it’s Pamela Crane again I forgot to tell you we have a Waffle slab and it was built in 2008 .I know we will be untouched with you if the insurance company won’t help us thank you .

High Matt my name is Pamela Crane we resently found cracked on the out side the house don’t get started the inside is bad we contacted the insurance company they sent out engineering company he told we have a fall of 30ml in the middle of house and 35ml on the far side we think the insurance company won’t help us we are pensioners and we no this is going to cost what I want to know what will the cost be please .

Can a root barrier dug to the depth to reach the natural soil be as effective as a cut off trench?
On one side of my home the neighbours are slightly higher and although I will be paving the concrete up to the fence line I want to stop any water from next door just in case.
Another engineer suggested a cut off drain also which I read on your site is not ideal.

Hi Danny
A cut off trench works fine so long as it can be positioned away from your dwelling and it can be laid in a trench with a slope so that it is free-draining. Root barriers aren’t necessarily designed to remove water from the ground. You’re in the right track by investigating your options and using an experienced, local engineer who has knowledge of your site is definitely a good ideas.
Matt Cornell

Thanks Matt, I have consulted a few engineers all with different opinions which leaves me confused actually. The site cut slopes back and its about 900mm of fill at the front and 500mm of fill at the rear before it reaches the natural soil. The neighbours have fenced right up to the boundary so there isn’t really alot of water but as they are a little higher I just wanted to eliminate ANY chance of moisture coming in horizontally to the expansive fill soil so I was thinking of using the root barrier film like a vapour barrier between the two properties trenched to the natural soil level.

Hi Danny
I cut off drain will work well in that situation as long as it is dug down into the natural ground.You can slope the plastic
liner from the house down into the house side trench wall .
depending on the width of your trench it can be tricky getting the plastic liner positioned and sitting all the way to the bottom of the trench.

Hi, we had a water pipe burst under our driveway and didn’t know due to it being enclosed on three sides, the only indication of water pipe being broken was from water coming up through the slab of driveway in areas of cracking and where the cracking is the driveway has a heave. The driveway has been dowled to front of shed slab and no sign of heaving at that point. Would the expanding clay soil have caused the heave as we guess the water may have been leaking from the pipe for up to one year as our water bill was high during that parried. Thanks Aaron

Hi Peter
Slab heave is caused by clays swelling when there is more soil moisture available. The swelling causes concrete footing and slabs to lift unevenly. Yes. Walkways can be affected just as easily as house footings. The difference is that the resultant damage isn’t as bad, but this depends on what the walkway slab is near and whether it is connected to any other structures.
Matt Cornell

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