Why roots are at the surface
Roots go where the oxygen and moisture are. In good deep soil that is well below the surface. In compacted suburban soil, on clay, or on a block where the topsoil was scraped off during construction, it is the top few hundred millimetres — so that is where the roots grow.
They then thicken over decades while the soil around them settles, erodes and compacts. What was a root just below turf becomes a root standing proud of it. The tree has not done anything unusual; the soil has.
Some species are far worse than others. Liquidambar, camphor laurel, poplar, willow and the big figs all produce prominent surface roots, and in the older Illawarra suburbs they are the trees people ring about.
What you can safely cut
There is no precise rule, but the general guidance used in arboriculture is conservative for good reason: a root you cut is gone permanently, and the tree does not tell you it is in trouble for another two or three years.
Roots under about 50 millimetres in diameter, at a distance of at least three times the trunk diameter from the trunk, can usually be removed without significant risk. So on a 400-millimetre trunk, that is 1.2 metres out.
Anything thicker than that, anything closer than that, and anything on the side of the tree that faces the prevailing wind should be left alone or assessed first. Structural roots are what stop the tree falling over. Cutting several on one side produces a tree that looks completely normal and is no longer anchored.
If you are dealing with roots against a slab, a footing or a drain rather than a lawn, that is a different and more serious job.
Better options than cutting
Mulch instead of turf. Give up on grass under the tree and make it a mulched bed. Grass never wins that competition anyway. A layer of coarse woodchip covers the roots, stops the mower hitting them, improves the soil and holds moisture. This is the answer for the great majority of surface root complaints.
Ground cover planting. Shade-tolerant ground covers over the root zone achieve the same thing and look better than bare mulch.
Reroute the path. Moving a path around a root zone costs less than repeatedly repairing one that a tree keeps lifting, and the tree stays.
Flexible surfaces. Where a path must cross the root zone, decomposed granite, gravel or a suspended timber walkway tolerates root movement in a way that a concrete slab does not.
Cutting properly if you must
Expose the root by hand or with a garden hose rather than a mattock, so you can see what you are dealing with and how thick it really is. Cut cleanly with a saw or loppers rather than tearing it with a machine. Cut once, at the point furthest from the tree that solves your problem. Do not cut multiple large roots in the same season, and do not cut anything at all in the middle of a hot dry spell when the tree is already stressed.
Water the tree well afterwards and watch it over the following two summers. Crown dieback and small pale leaves are the signs you took too much.
When the tree has to go
Sometimes the roots are a symptom and the real answer is that the tree is in the wrong place. If it is lifting the driveway, entering the stormwater and shading the whole yard, removal and stump grinding is the honest option. Liability where the tree belongs to somebody else is covered in the root damage guide.
Ring (02) 4210 5885 for an assessment before you cut anything substantial.
Frequently asked questions
Why have roots suddenly appeared on the surface of my lawn?
They were probably always near it. Roots grow where oxygen and water are, which in compacted or clay soil is the top few hundred millimetres. Over years the soil around them erodes or compacts down while the roots thicken, and what was just under the surface becomes exposed. It is usually a sign of poor soil conditions rather than an aggressive tree.
Can I cut surface roots to level my lawn?
Some of them, carefully. A useful rule of thumb is that roots smaller than about 50 millimetres, more than three times the trunk diameter away from the trunk, can generally be cut without much risk. Anything thicker, or closer to the trunk, is structural and cutting it risks both the tree's health and its stability. When in doubt, do not.
Will covering roots with soil fix the problem?
Only in small amounts, and it can kill the tree if you overdo it. Roots need oxygen, and burying them under a thick layer of soil suffocates them. Adding more than about 50 to 75 millimetres over the root zone is risky, especially on clay. Mulch is a better answer than soil because it stays loose and lets air through.