Yes, pouring your own concrete deadman anchors is something many capable DIYers can tackle — but whether you're legally allowed to do it without a building consent, and whether your design will actually hold, depends on the height of your retaining wall and your site conditions. Get these details wrong and you could end up with a collapsed wall, a failed council inspection, or worse, a safety hazard.
What Is a Deadman Anchor?
A deadman anchor (sometimes called a deadman tie-back) is a buried concrete block or timber/steel element connected to a retaining wall by a tie rod or strap. It works by using the weight and friction of the surrounding soil to resist the horizontal pressure pushing against the wall. They're commonly used with timber sleeper walls, concrete block walls, and similar gravity-style retaining structures.
The deadman is typically poured in place — you dig a trench perpendicular to the wall, place your formwork or a simple timber box, run your tie rod or strap through, and pour concrete around it. Once cured, the backfill locks it in place.
Do You Need a Building Consent?
This is the most important question to answer before you lift a shovel. In New Zealand, retaining walls are covered by the NZ Building Code and consent requirements depend primarily on wall height and proximity to boundaries or structures.
Under Schedule 1 of the Building Act, retaining walls that retain up to 1.5 metres of ground and are not in a flood-prone area or near a boundary may be exempt from building consent — but this is not a blanket rule. Your local council has the final say, and some councils apply stricter thresholds.
Key rules of thumb (always confirm with your council):
- Walls retaining less than 1.5 m: likely exempt, but check with your council
- Walls retaining 1.5 m or more: almost certainly require a building consent and engineer-designed plans
- Walls near boundaries, slopes, or surcharge loads (driveways, buildings): may require consent regardless of height
If your wall needs consent, your deadman anchor design will need to be specified by a Chartered Professional Engineer (CPEng) or similarly qualified structural engineer, and the work may need to be carried out or supervised by a Licensed Building Practitioner (LBP).
Designing Your Deadman Anchors
If your wall is within the exempt threshold and you're confident in your DIY skills, here's what good deadman design generally involves. Note that these are general principles — always base your actual design on your specific soil type, wall height, and loading conditions.
Sizing and Spacing
- Deadmen are typically spaced 1.8 m to 2.4 m apart along the wall
- The deadman block itself is usually at least 600 mm long and buried at a depth that puts it well behind the wall's failure plane (generally at least 1.5× the wall height back from the face)
- Concrete strength of 17.5 MPa to 20 MPa is commonly used for deadman pours — a standard ready-mix or bagged concrete mix is usually adequate
Tie Rods and Straps
- Galvanised steel tie rods, flat bar, or purpose-made geogrid straps connect the deadman to the wall
- All metal components should be hot-dip galvanised or stainless steel to resist corrosion in NZ's often damp soils
- Connections to the wall must be secure — bolted through sleepers or hooked around blocks, not just surface-fixed
Drainage
Poor drainage is the number-one cause of retaining wall failure in New Zealand. Always install agricultural drain coil (ag pipe) wrapped in geotextile fabric behind the wall, and ensure water has a clear path to exit. BRANZ has useful guidance on retaining wall drainage best practice.
Pouring the Concrete: Step by Step
- Dig your deadman trench perpendicular to the wall at the correct depth and distance
- Set your formwork — a simple timber box works well, or you can pour directly into a neat-sided trench in firm soil
- Thread your tie rod or strap through the form before pouring
- Mix or order your concrete — for small jobs, bagged concrete from Bunnings or Mitre 10 is fine; for larger pours, consider ordering ready-mix
- Pour and consolidate — rod or vibrate the concrete to remove air pockets
- Allow full cure time — at least 7 days before backfilling and loading, ideally 28 days for full strength
- Backfill in layers, compacting as you go to avoid point-loading the wall
When to Call a Professional
You should engage a qualified retaining wall builder or structural engineer if:
- Your wall retains more than 1.5 m of ground
- The wall is near a boundary, building, or driveway
- Your site has poor or unstable soils (clay, fill, or sloping ground)
- You're unsure about your design or the council requires consent
- The wall will support a surcharge load such as a vehicle or structure
A professional can also help you avoid costly mistakes — a failed retaining wall can damage neighbouring properties and create significant legal liability. You can find qualified retaining wall builders in your area on tradedirectory.co.nz.
Safety Note
Excavation work carries real risks. Never work in an unsupported trench deeper than is safe, and follow WorkSafe NZ guidelines on when shoring or battering is required, and be aware of underground services before you dig. Check WorkSafe NZ guidelines on safe excavation practices.
Licensed Building Practitioner (LBP)
