How to Remove Tree Roots Before Building a Retaining Wall

Removing tree roots before building a retaining wall is one of the most important preparation steps you can take. Roots left in place can rot and create voids, or continue growing and push against your wall structure — both scenarios can cause costly damage or even wall failure. With the right approach, most homeowners can handle smaller roots themselves, but larger or more complex situations are best left to a qualified professional.

Why Tree Roots Are a Problem for Retaining Walls

Tree roots are opportunistic — they grow toward moisture and nutrients, and a retaining wall provides both a physical barrier and a moisture gradient that roots love to exploit. Over time, roots can:

• Undermine footings by displacing compacted fill or subgrade material
• Create voids as they decompose, causing the wall to settle or lean
• Crack or displace timber, concrete block, or stone wall materials
• Interfere with drainage systems behind the wall, leading to hydrostatic pressure build-up

For these reasons, BRANZ and standard NZ trade practice both recommend clearing the wall zone of significant roots before construction begins.

Check Before You Dig: Consents and Protected Trees

Before you pick up a spade, check two things:

1. Building consent: Retaining walls over 1.5 metres in height generally require a building consent, but lower walls may also require consent depending on ground conditions, surcharge, and proximity to boundaries. Always confirm the specific thresholds with your local council. Even exempt walls must still comply with the code. Always confirm requirements with your local council — Schedule 1 exemptions vary. See MBIE Building Performance for general guidance.
2. Protected or notable trees: Many councils have Notable Tree schedules in their district plans. Removing roots from a protected tree without resource consent can result in significant fines. Check your council's district plan or GIS maps before doing anything.

Tools You'll Need

• Spade and mattock
• Pruning saw or reciprocating saw with a pruning blade
• Loppers and hand pruners for smaller roots
• Stump grinder (available for hire from tool hire companies or equipment hire outlets) for larger stumps
• Safety glasses, gloves, and steel-capped boots
• Spray paint or chalk line to mark your excavation zone

Step-by-Step: Removing Tree Roots

1. Mark Your Excavation Zone

Mark out the full footprint of your retaining wall, including the required setback behind the wall for drainage aggregate and any geotextile fabric. You need to clear roots from this entire zone, not just the wall face.

2. Expose the Roots

Using a spade and mattock, carefully excavate the topsoil within your marked zone. Work methodically to expose roots without damaging them prematurely — cutting a root before you've exposed it fully can make removal much harder.

3. Cut and Remove Roots

• For roots under 50 mm diameter, loppers or a pruning saw will do the job cleanly.
• For roots 50–150 mm diameter, use a reciprocating saw with a pruning blade or a chainsaw (if you're trained and equipped).
• For roots over 150 mm diameter or large stumps, hire a stump grinder or engage a professional arborist or earthworks contractor.
• Always cut roots as far back from the wall zone as practical. Leaving long root stubs that extend under your wall is not ideal — they will still decompose.

4. Remove All Organic Material

Once roots are cut, remove every piece of organic material from the excavation zone. Decomposing wood causes settlement and can harbour pests. Don't be tempted to bury cut roots under fill — this is a common mistake that causes problems years later.

5. Treat Remaining Stumps

If a stump or large root system sits close to (but outside) your wall zone, consider treating it with a stump killer to prevent regrowth. Follow the product label carefully and observe any waterway setback requirements under the Resource Management Act.

6. Compact and Prepare the Subgrade

Once roots are removed, the disturbed soil must be properly compacted before you place any footings or base material. Loose or disturbed ground is a leading cause of retaining wall movement. For walls requiring consent, your engineer or LBP will specify compaction requirements.

Dealing With Live Trees Nearby

If the tree is staying (e.g., it's on a neighbour's property or is a protected specimen), you need to design around it. Options include:

• Root barriers — physical membranes installed vertically to redirect root growth away from the wall
• Adjusting wall alignment to avoid the main root zone (generally within the drip line of the tree)
• Consulting a structural engineer about wall design that can tolerate some root pressure

Always consult a qualified arborist if you're unsure how much root removal a live tree can tolerate without becoming unstable — a falling tree is a serious WorkSafe NZ hazard.

When to Call a Professional

Call a qualified retaining wall builder, arborist, or earthworks contractor if:

• Roots are larger than 150 mm in diameter or deeply embedded
• The tree is large, leaning, or shows signs of instability
• You're working near underground services (always call Before U Dig on 0800 248 344 first)
• The wall requires building consent and engineering sign-off
• You're on a steep or unstable slope
• The tree may be protected under your district plan

You can find experienced local retaining wall builders and arborists on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – Retaining Walls and Consents
• BRANZ – Build Magazine and Technical Resources
• WorkSafe NZ – Safe Tree Work
• Licensed Building Practitioners – Find an LBP
• Bunnings – Tool Hire and Garden Equipment
• Mitre 10 – Tools and Landscaping Supplies