"That tree's going to damage my foundations." Sometimes true, often not. Here's how to tell.
How tree roots actually cause building damage
Two mechanisms:
- Direct physical damage: roots growing under shallow foundations, paths, drains or slabs and lifting them. Most common in NZ with shallow-rooted species near unrestrained slabs and concrete paths.
- Soil-moisture damage: tree roots drawing water out of clay soils, causing the soil to shrink. As soil shrinks, foundations sink unevenly, causing cracking. This is mostly a problem in clay-soil regions (parts of Auckland, Waikato, eastern South Island) and mostly with heavy water users like willows, poplars and large eucalypts.
Species that genuinely cause problems
- Willows (Salix species). The number one culprit. Massive water demand, vigorous roots, attracted to drains and soakage lines like a magnet. Never plant a willow within 30m of any underground service.
- Poplars (Populus species). Similar to willows. Avoid near buildings.
- Eucalyptus / gums. Large mature gums in clay soils can dry out the soil under nearby buildings. Less aggressive than willows but the trees are bigger.
- Liquidambars. Surface-rooting species. Lift concrete and paths.
- Plane trees. Surface roots, especially in shallow-soil street plantings.
- Norway maple, sycamore. Surface-rooting and aggressive in clay.
Species that rarely cause problems
- Most NZ natives: pohutukawa, totara, kauri, kowhai, lemonwood, griselinia. These have less aggressive root systems than the introduced problem species.
- Fruit trees at typical residential sizes.
- Magnolia, cherry, dogwood, smaller maples. Generally well-behaved.
- Most oaks: large root systems but deep, not aggressive.
Signs roots are actually causing damage
- Cracks in walls that open and close seasonally (suggesting clay-soil shrinkage).
- Doors and windows that bind seasonally.
- Floor slabs lifting along clear lines pointing at a tree.
- Sewer or stormwater drain failures, blocked or crushed pipes containing tree roots.
- Lifting concrete paths, driveway slabs or retaining walls.
Cracks that don't open and close seasonally probably aren't soil-moisture related. Slow general subsidence is more likely a foundation or geotechnical issue than a tree.
What to do about it
Before removing the tree:
- Get an arborist's report identifying the species and assessing root behaviour.
- Get a builder or engineer's report identifying the actual cause of the building damage. Sometimes trees get blamed for foundations that were never adequate.
- Consider root barriers, vertical HDPE barriers installed between tree and structure can divert roots without killing the tree.
- Consider crown reduction, reducing canopy reduces water demand, which can help on clay soils.
Sometimes removal is the right answer. Often, mitigation is. Don't take down a 50-year-old tree if a $4,000 root barrier solves the problem.
If you're planting new
The most useful preventive measure is choosing the right species for the position. Don't plant a willow near a soakage line. Don't plant a liquidambar against a paved area. Don't plant a gum within fall distance of the house.
Worried about a specific tree? Send us photos through the quote form, including any building damage. We can come and have a look on site and tell you what we think.