This is the serious end of root problems
Surface roots in a lawn are an inconvenience. Roots in a sewer line, under a slab or hard against a strip footing are a building problem with a tree attached, and the tree is often not the main thing that needs fixing.
Roots in pipes
Roots do not break into sound pipework. They find a defect — a cracked earthenware line, a perished joint seal, a poorly bedded connection — and then proliferate inside because a sewer or a leaking stormwater line is the best growing environment available.
So the sequence matters. Clearing roots mechanically or chemically restores flow, but if the defect is not repaired the roots return, usually within a couple of years. A CCTV inspection to find the defect, then patch repair or relining, is what actually fixes it. Removing the tree helps and is sometimes justified, but a defective pipe will eventually find another root.
Where the tree is worth keeping and the pipe can be relined, that is generally the better outcome than losing a mature tree.
Roots against slabs and footings
The mechanism here is usually not roots physically pushing concrete. It is moisture.
On the reactive clay soils west of the lake — through Dapto, Horsley and the Albion Park flats — a large tree draws substantial water out of the soil in a dry spell. The clay shrinks. A footing sitting on it settles. When the rain returns the clay swells and lifts. Do that over enough cycles and you get cracking in brickwork and slabs.
Roots do also lift and displace lighter structures directly: paths, driveways, paving, retaining walls and fence footings all go that way.
The distinction matters because the remedies differ. Moisture-driven movement is addressed by root barriers, by managing soil moisture, sometimes by underpinning, and occasionally by removing the tree. Direct displacement is addressed by root pruning, barriers, or rebuilding the surface in something that tolerates movement.
Root barriers
A barrier is a rigid sheet set vertically in a trench between the tree and the structure, typically 600 to 1000 millimetres deep depending on soil and species, run continuously along the line being protected.
Done properly it redirects roots down and away. Done badly — too shallow, in disconnected sections, or with the trench cut carelessly through major structural roots — it either fails or destabilises the tree. Trenching near a mature tree is itself a root-pruning operation and should be planned rather than improvised.
Root pruning near structures
Where roots have to be cut, the same principles apply as anywhere: clean cuts, not tearing; as far from the trunk as the problem allows; nothing structural without an arborist's assessment; and not several major roots on the same side.
Get the assessment in writing. If the tree later fails, the question of whether the root cutting caused it will be asked, and "the builder said it would be fine" is not an answer. Liability is covered in the root damage guide.
When removal is the answer
A tree planted a metre from a slab in 1975 is in the wrong place and will stay in the wrong place. If the structure is worth more than the tree and the conflict is fundamental, removal followed by stump grinding is the resolution.
Do it with advice on a large tree near a house on clay, because the soil rebound after removal is a real effect. And check the approval position first — damage to a structure is a recognised ground, but it needs to be documented.
Ring (02) 4210 5885 for an assessment.
Frequently asked questions
How do roots get into a sewer or stormwater pipe?
Through a defect. Roots do not drill through sound PVC. They find an existing crack, a failed rubber ring, a badly bedded joint or an old earthenware pipe that has already shifted, then exploit the moisture and nutrients inside. That is why clearing the roots without repairing or relining the pipe simply resets the clock — the defect is still there and the tree is still nearby.
Do root barriers actually work?
Installed properly, yes. A root barrier is a rigid physical sheet set vertically in a trench between the tree and the thing being protected, deep enough that roots cannot simply go under it, usually 600 to 1000 millimetres. Installed shallow or in short disconnected lengths it does very little. It is a trenching job, and digging the trench itself cuts roots, so it is best done with advice.
Will removing the tree stop the damage?
It stops it getting worse. Existing roots die and decay in place over several years, which does not reverse a lifted slab and can occasionally cause minor settlement as the root mass breaks down. On heavy clay there is a second effect: removing a large tree stops it drawing water out of the soil, the clay rehydrates and swells, and footings can move again. It is worth engineering advice on a big tree near a house.