How trees affect your home's foundations
A guide for homeowners and builders. Why trees move slabs on reactive clay, and what to keep in mind when you plant, remove or build near trees.
Trees near a house can crack walls, jam doors and move slabs, but only on certain soils. Across much of Melbourne and regional Victoria, homes are built on reactive clay, and that is where trees matter most when designing a slab.
How trees affect foundations
Trees affect foundations by drying out the soil underneath them. Reactive clay swells when it gets wet and shrinks when it dries, and a large tree can draw hundreds of litres of water out of the ground on a hot day.
A house slab is designed for the normal seasonal wetting and drying of the soil around it. A tree adds a much stronger, deeper drying effect on one side of the house. The ground under that part of the slab shrinks and drops, while the rest of the house stays put.
This uneven movement is what causes damage. One corner or edge of the house settles more than the rest, and the structure above has to bend to follow it.
The effect reaches further than most people expect. Roots spread well beyond the canopy, and the drying can extend a long way from the trunk, especially during long dry spells and droughts. A tree that caused no trouble for years can start causing problems after a dry summer.
Why your soil type matters
The more reactive your soil, the more a tree can move your slab. Before a house is designed, a geotechnical engineer tests the site and gives it a classification under AS 2870, the Australian Standard for residential slabs and footings.
| Site class | Soil | Ground movement |
|---|---|---|
| A | Sand and rock | Little or none |
| S | Slightly reactive clay | Slight |
| M | Moderately reactive clay | Moderate |
| H1 | Highly reactive clay | High |
| H2 | Very highly reactive clay | Very high |
On sand and rock (Class A), trees have little effect on the slab. On reactive clay, from Class M upwards, trees become a real consideration, and the effect grows with the class. Much of Melbourne’s growth areas sit on M, H1 and H2 sites.
You’ll find your site class in the soil report for your property. If you’re building, your builder or engineer will have a copy.
Removing a tree can cause problems too
Removing a tree can affect the ground as much as keeping one. Once the tree stops drawing water, the dried-out clay slowly takes up moisture again and swells. This is called heave, and it can push part of the slab upwards over several years.
The same applies on a new building site. If trees were removed shortly before construction, the soil underneath may still be drier than normal and will swell back over time. If trees have been or will be removed from your building site, tell your engineer before the slab is designed.
Trees you don't own still count
Roots don’t stop at the fence line. A large tree in your neighbour’s yard or on the nature strip can dry the soil under your house just as much as one in your own garden.
Groups and rows of trees have a bigger effect than a single tree. Several trees together draw more water over a wider area, so their influence reaches further from the house.
When your house is designed, your engineer should be told about every significant tree nearby, including those on neighbouring properties and council land, and any you plan to plant later.
Planting near a new home
Most house slabs are not designed for trees planted after the house is built. If you’re landscaping a new home on a reactive site, where you plant matters as much as what you plant.
- Think about mature height, not current size. A young gum tree sapling can grow well over 20 m tall. What matters is how big it will be when fully grown.
- Keep trees well away from the house. On reactive clay, keeping trees at least their mature height away from the slab reduces the risk of damage, though it doesn’t eliminate it.
- Plant groups and rows even further away. Hedges and clusters of trees draw more water than a single tree.
- Keep the ground near the house evenly moist. Paths, lawn and garden beds watered consistently help avoid one side of the house drying out more than the rest. Avoid leaking taps and pipes that wet one spot.
If you want a large tree close to the house, talk to your engineer before you plant.
Planning to plant a tree?
Use our free tree planting distance check before you plant. Enter your site class, the tree’s mature height and how far from the house you plan to plant it, and it will show whether that spot meets the recommended planting distance.
The check is for new planting only. It is not an assessment of existing trees. If there are already trees near your building site, or trees have been or will be removed, send us your project and an engineer will take them into account in your slab design.
Planning a new home or extension?
ENGPRO has designed slabs and footings for more than 5,000 projects across Victoria and New South Wales, many on reactive clay sites with trees nearby. Send us your drawings or project brief and one of our engineers will respond with a free quote.