As the land continues to dry out due to the drought, houses are slowly sinking, and as they sink they crack, writes Steve Chandler.
Buildings aren’t flexible like rubber bands, they are designed to be robust and sturdy but with an ability to withstand minor movement. When movement exceeds a property’s ability to accommodate this movement, cracks form.
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The problem people are starting to experience across the eastern states is that the ground they are built on is moving substantially. Clay soils are highly reactive to the moisture content within them and as this moisture content varies, so too does the quantity of clay. They clay swells as it absorbs water and shrinks as that water dries out.
If a house was built when the moisture content was high, the house is now likely to be sinking and cracking given the drought has reduced the moisture content of the clay.
Many modern houses are built on what is called a raft slab. This is a concrete slab that is constructed directly on top of the ground across the entire footprint of the house. A raft slab is one way that engineers design to minimise movement of the house if moisture content in the clay the house is built on changes.
Many older homes are not built on raft slabs; they are built on strip footings. Strip footings are concrete footings built under the walls of a house. Strip footings only exist where walls exist. Houses with strip footings are generally built with an under floor cavity and will have either timber strip flooring or sheet flooring and carpet finish within the home.
Homes with strip footings are more likely to crack due to changes in moisture content as each strip footing can move independently from the others as it is not connected by a raft slab, which helps to control cracking.
Regardless of the manner of construction of a house, cracking can, and will, occur if there is enough change in moisture content of the ground below. Raft slabs often have cracks appear in the slab itself and then cracks can appear in walls, skirtings where they abut the floor, and cornices where they cover the intersection of the wall and ceiling. Houses with strip footings can have similar cracks.
When you see cracks appear in your house there usually isn’t too much to worry about. Many cracks open and close by themselves over time as movement occurs however, when you can fit the thickness of a credit card into a crack then it could be time to call an expert to review the crack and determine whether it is structural or cosmetic.
Repairs to consider to drought houses sinking
When considering repairs to cracks caused by shrinkage of the clay beneath your house, you must be careful not to fill the cracks with solid material such as sand and cement mixture, as when the drought eventually breaks, the house will rise as the clay absorbs water and expands causing the cracks to close. If you have used a solid repair material then the repairs may cause further damage as the expanding clay under your house will force it to move in other locations.
A number of years ago, during a drought, my wife and I purchased an older style home that had been built in the 1960’s on strip footings. There was one major crack in a masonry wall on one of the corners inside the house. I’d say it was about 8mm wide. I chose to fill this crack with a flexible, paintable, sealant.
I recall some time later during a period of about a week of rain, we heard a massive crash sound in the house. It sounded like a car had run into the house. When we looked through the house my wife and I found that the crack had closed completely and the flexible sealant in the crack had been crushed and squeezed out of the crack.
The clay that our house was built on had absorbed water over the week of rain and had expanded eventually causing the corner of the house to “pop” back up to its original position, closing the crack.
If I had used a solid material to fill the crack there could have been some serious other damage caused when the house returned back to its original position.
And if you’re reading this article thinking to yourself that you are safe because your house has a raft slab and is built on sand, you too are subject to potential cracking as the height of the water table below your house will affect its stability. Think of sand on the beach, when it is wet it is firm but when it dries out it becomes soft, and when it is soft your feet sink into it when you walk. The same can be said about your house.