A deadman anchor is a horizontal timber or other structural member buried into the retained soil behind a retaining wall and connected to the wall face, preventing the wall from rotating or sliding forward under soil pressure. They are one of the most common and effective stabilising methods for timber sleeper retaining walls, and installing them correctly is essential for a safe, long-lasting structure.
Why Retaining Walls Need Deadman Anchors
Soil exerts significant lateral (sideways) pressure against any retaining wall — a force that increases with wall height, soil type, and moisture content. Without adequate anchorage, even a well-built wall can slowly lean forward and eventually fail. Deadman anchors work by transferring that lateral load back into the stable soil mass behind the wall, effectively using the weight of the earth itself as a counterbalance.
For timber sleeper walls taller than about 600–800 mm, deadman anchors are generally considered best practice. For walls over 1.5 m in height, they are typically essential — and building consent is almost certainly required (more on that below).
Do You Need a Building Consent?
In New Zealand, retaining walls are regulated under the NZ Building Code. As a general guide:
- Retaining walls up to 1.5 m in height may be exempt from consent under Schedule 1 of the Building Act, provided they meet certain conditions (no surcharge loading, not near boundaries or structures, stable ground).
- Walls over 1.5 m, or any wall with additional loads (driveways, buildings, slopes above), will almost certainly require a building consent.
Always confirm with your local council before starting work — exemptions have conditions that vary by site. Check MBIE Building Performance for guidance on Schedule 1 exemptions.
What You'll Need
- H5-treated timber for deadman members and wall sleepers (ground contact, high-decay-risk environment)
- Galvanised or stainless steel coach screws or bolts (minimum 12 mm diameter)
- Geotextile fabric (optional but recommended to prevent soil migration)
- Compactable backfill (gravel or AP20 aggregate)
- Spade, level, tape measure, drill, and appropriate bits
- Safety glasses and gloves
Safety note: Retaining wall construction involves heavy materials and excavation. Be aware of underground services before digging — call Dial Before You Dig (0800 248 344) first. If working near slopes or unstable ground, assess the risk of collapse carefully.
How a Deadman Anchor Works
A typical deadman assembly consists of:
- A tie-back member — a horizontal timber running at 90° back into the hillside from the wall face.
- A deadman log or plate — a shorter timber fixed perpendicular to the tie-back at its buried end, forming a T or L shape that grips the soil.
- A connection to the wall face — usually bolted through the face sleeper.
| Component | Typical Size (H5 Timber) | Spacing |
|---|---|---|
| Tie-back member | 200×75 mm or 200×50 mm | Every 1.2–1.8 m along wall |
| Deadman plate | 600–900 mm long, same section | One per tie-back |
| Coach bolts | 12–16 mm galvanised | 2 per connection point |
Sizes and spacing should be confirmed by a structural engineer for walls over 1.5 m or in challenging ground conditions.
Step-by-Step Installation
1. Plan Your Layout
Mark deadman positions along the wall every 1.2–1.8 m. They are typically installed at one-third to one-half the wall height up from the base — so for a 1.2 m wall, roughly 400–600 mm up from the footing course.
2. Excavate the Tie-Back Trench
Dig a trench running horizontally back into the slope at each deadman location. The trench should extend at least 1.5× the wall height back into the retained soil — for a 1.2 m wall, that's at least 1.8 m back. The trench depth should match the timber height plus a little clearance.
3. Assemble the Deadman
Cut your tie-back timber to length. Fix the deadman plate perpendicular across the far end using galvanised coach screws, forming a T-shape. Pre-drill to avoid splitting.
4. Connect to the Wall Face
Before placing the course of sleepers that will sit at deadman height, position the inner end of the tie-back so it overlaps the back of the wall sleeper by at least 300 mm. Bolt through with two galvanised coach bolts.
5. Backfill and Compact
Lay geotextile fabric over the deadman assembly if desired, then backfill the trench with compactable gravel in 150 mm layers, compacting each layer firmly. Avoid using expansive clay soils as backfill — they retain water and increase pressure on the wall.
6. Continue Building the Wall
Continue laying sleeper courses above the deadman level, staggering vertical joints for strength. Ensure adequate drainage (agricultural drain pipe or gravel drainage layer) behind the wall to prevent hydrostatic pressure build-up.
When to Call a Professional
While a straightforward single-tier sleeper wall with deadman anchors can be a rewarding DIY project, there are situations where you should engage a qualified retaining wall builder or structural engineer:
- The wall exceeds 1.5 m in height
- There are structures, driveways, or slopes above the wall
- The ground is soft, wet, or unstable
- You need a building consent (an LBP may need to sign off restricted building work — check Licensed Building Practitioners)
- The wall is near a property boundary
You can find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
