Minimising foundation movement and damage to your house
In both newly built and older homes, it can be common for cracks appear in the walls and floors. This sometimes happens as a result of foundation movement, also known as slab heave. When the footing lifts excessively (heaves) in one section of the house and not another, your walls will usually develop cracks.
Foundation movement is often caused by changing moisture conditions in the soil, but can also be caused by soil settling after a house is constructed, erosion of sandy soils, or tree roots growing under footings.
Understanding soil conditions
If you’re the owner of a newly built home, a soil test will have been carried out before a building permit was issued for the construction of your house. Soil tests classify how reactive the soil is, indicating how much the soil will shrink or swell with a change in moisture. The more reactive the soil, the greater the possibility of excessive movement in the footings of the house.
An engineer will design the footings of a house to suit the classification of the soil and likely movement. You should be able to find a copy of the soil report for your property with your approved building permit documents.
- A – non‐reactive – sand and rock sites – little or no ground movement
- S – slightly reactive clay sites
- M, M–D – moderate reactive clay or silt – movement due to moisture changes
- H1, H1–D – highly reactive clay sites
- H2, H2–D – very highly reactive
- E, E–D – extremely reactive
- P – soft soil, vary depths of fill – reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise.
Movement of footings
Some movement of footings is normal and within design tolerances. In general, cracks in walls less than 1 mm wide are considered part of the normal foundation movement a house may experience.
If your home is showing signs of significant damage, this should be investigated. If your home is less than 10 years old, contact your builder for advice in the first instance.
Cracks in walls between 1 mm and 5 mm wide should be monitored during all weather conditions over a full year.
In moderate or severe cases, the cracking may require substantial repair work. Cracks more than 5 mm wide are considered significant and outside the tolerances for footing movement. Foundation movement may also cause bulging walls, and windows and doors that stick or distort.
The cause of this excessive movement should be further investigated by a registered civil or structural engineer. If defective work is determined and it can be attributed to the actions of your builder, then the builder will be responsible for any necessary remedial works and repairs.
Ways to prevent foundation movement
As a homeowner, there are several things you can do to reduce the risk of damage caused by foundation movement. It is important to ensure that the foundation soil is not subject to significant moisture changes. If any damage to your house is found to be a result of your failure to properly maintain the house, you may be responsible for paying for repairs.
- If you want to grow, keep or remove trees near your house, let your builder know before you sign the building contract. The builder will then advise the engineer, who will take this into account when designing your home's footings.
- If you're planting new trees, plant them away from the house to allow for root growth.
- When carrying out work around your home and garden, make sure you don't change the moisture conditions of the foundation.
- Avoid placing garden beds alongside the house if possible, and don't overwater them.
- Slope soil, paving, paths and garden beds away from the building to prevent water flowing towards your home's foundations.
- Ensure you properly maintain any drainage installed by your builder.
- Promptly repair any damaged or leaking gutters, hot water systems, air conditioners, water tanks, taps or hoses.
- Avoid excavating close to building footings. If you do need to carry out excavations next to your house, make sure you don’t dig deeper than the base of the footing.