Retaining walls: what you can build without an engineer

A retaining wall holds back ground that’s higher on one side than the other. As a builder the first thing you need to sort is whether you can build it off the shelf or whether it needs an engineer — NZS 3604 effectively caps prescriptive design at 1.5m retained height with no surcharge, and anything taller (or with a driveway or building above) needs an engineer’s specific design (PS1).

What you can build without engineering

You can build off prescriptive rules when all of these hold true:

• Retained ground is ≤1.5m, measured from finished ground on the low side to finished ground on the high side.
• No surcharge — no building, driveway, retaining wall or vehicle within the zone of influence above.
• Stable soil — not on filled or unstable ground.
• Drainage installed — backfill with free-draining metal plus an ag drain to an outlet.

Schedule 1 item 20 exempts a wall like this from building consent — but it still has to comply with the Building Code.

A typical timber retaining wall (≤1.5m, no surcharge)

A standard timber build off these rules looks like this:

• Posts: 125×125mm or 150×150mm H5 treated, at roughly 600–900mm centres.
• Post depth: the rule of thumb is ⅔ above ground, ⅓ in ground — so for a 1.5m wall, the posts sit 1.0m below ground.
• Footings: 600mm diameter × 600–900mm deep concrete (17.5 MPa+) around each post.
• Rails: 200×50 H4 or H3.2 horizontal between the posts.
• Drainage: 100mm Novacoil ag drain at the base, daylighted to the surface or stormwater.
• Backfill: 300mm scoria or 20mm metal aggregate behind, then filter cloth, then native soil.
• Waterproofing: a waterproof membrane on the back face of the timber (or use a butynol blanket).

When you need an engineer (PS1)

Bring in an engineer for a specific design when any of these apply:

• Wall over 1.5m retained height.
• Surcharge above — driveway, building, vehicle or another retaining wall within the influence zone.
• Slope above the retaining wall steeper than 1:5.
• Poor soil — TC2/TC3, fill or peat.
• Adjacent to a public area where failure could harm people.
• Stepped or terraced walls where a lower wall gets surcharge from the upper one.

Don’t underestimate water

The leading cause of retaining-wall failure is water pressure behind the wall (hydrostatic) — not the weight of the soil. Even a small wall fails fast without drainage. So on every job:

• Ag drain.
• Free-draining backfill.
• Waterproof membrane on the back of the timber.

BRANZ Bulletin BU562 (Low Retaining Walls) is the bible on this.

Common materials

Timber isn’t the only option — here’s the range you’ll come across:

• H5 timber — the most common, 30–40yr life if drained. CCA / Tanalith E treated.
• Concrete crib walls — modular concrete units stacked; 1–3m heights typical, run as a modular system from suppliers.
• Concrete block (NZS 4229) — reinforced concrete masonry on a foundation strip footing.
• Allan Block / Anchor walls — interlocking concrete blocks, dry-stacked with geogrid reinforcement.
• Gabion baskets — wire mesh filled with rock; rustic look and free-draining.
• Boulder walls — large stones placed by excavator, for a rural or landscape look.
• Concrete poured-in-place — engineered cantilever or counterfort; premium urban.