Why have cracks suddenly appeared in my house?
Cracking in wall plaster, cornices, ceiling plaster and brickwork are among the most common forms of damage to occur within houses. But why does this damage occur in the first place? In 99% of cases, the answer is differential movement.
Differential movement
The term differential movement is used to describe the scenario where two or more points in a structure aren’t moving in the same direction at the same rate. An example of this would be one area of a house sinking, while other areas rise, or remain at the same location. A visual depiction of this scenario is shown below.
Figure 1. Typical examples of cracking within houses.
What causes differential movement?
Houses are typically supported by either a concrete slab or a series of stumps, which are in turn supported by the soil below it. This soil is commonly referred to as the foundation or founding soil. When the water content in this soil changes, the soil may swell (increase in volume) or shrink (decrease in volume). These changes in volume cause the stumps or slab to lift or fall, but only in specific areas. Adjacent areas may be unaffected; therefore, the house has undergone differential movement.
Figure 2. An example of differential movement within a dwelling built on a slab. The edge of the slab on the left-hand side is dropping, while the adjacent section of the slab is lifting, and the remaining areas remain the same. The differences in vertical movement are causing cracks to form in the external brickwork.
Figure 3. When a body of water rests on the surface of the soil, it can ingress to the soil, causing the soil to increase in volume.
There are other, more complex actions that can cause differential movement, but if the cracking appeared recently and suddenly, swelling or shrinking of the founding soil are the most likely causes.
How do you prevent differential movement (and therefore cracks) from occurring?
The most common causes of differential movement include plumbing leaks, ineffective surface drainage and trees that are placed too close to the dwelling. If you have recently noticed movement-related damage you can:
ensure at least one meter of non-permeable paving exists around the perimeter of the house and falls away from walls (see Figure 4)
remove any trees may be affecting the house, or install root barriers (see Figure 5)
engage a plumber to determine if there are any leaks in the property
engage a forensic engineer to assess the property and provide you with personalised recommendations
Figure 4. An example of effective surface drainage.
Figure 5. The image shows an example of a tree causing damage to a house because it was planted too closely.
What we can do to help
Unfortunately, most of the structural damage we see during our forensic site investigations could have been avoided by taking simple preventative measures when the damage was first noticed. At Arbiter Group, our engineers specialise in assessing structural damage and provide detailed short term and long term plans for damage prevention.
Arbiter Group have extensive experience and a proven track record in finding the causes of damage within Australian houses. If you have any questions relating to the information presented on this page get in touch.
Further reading for crack prevention
The CSIRO’s document ‘Foundation maintenance and footing performance’ is an excellent guide to preventing differential movement.
The VBA’s document ‘Minimising foundation movement and damage in your house’ also contains much of the same information as the CSIRO document.
You can also check out our other posts for simplified, engineering-related information.
