Getting the base course of a concrete block retaining wall perfectly level is the single most important step in the whole build. If the first course is off, every block above it will compound the error — and you could end up with a wall that leans, cracks, or fails. The good news is that with the right preparation and a few basic tools, most homeowners can tackle this themselves for smaller walls.
Know the Rules Before You Dig
Before you lift a spade, check whether your retaining wall needs a building consent. In New Zealand, retaining walls over 1.5 metres in height (measured from the bottom of the footing to the top of the wall) generally require a consent, as do walls of any height that support a surcharge such as a driveway or building. Schedule 1 of the Building Act lists some exemptions, but the rules can vary — always confirm with your local council before starting. For walls over 1.5 m, you'll likely need engineering input as well. See MBIE Building Performance for guidance on consent requirements.
Tools and Materials You'll Need
- String line and line level (or a laser level)
- Builder's square
- Tape measure
- Rubber mallet
- Spade and hand tamper or plate compactor
- Crushed aggregate (AP20 or similar compactable fill)
- Concrete blocks (standard 390 × 190 × 190 mm hollow blocks are common in NZ)
- Spirit level (at least 1.2 m long)
- Mortar or dry-stack adhesive depending on your block system
Step 1 — Excavate and Prepare the Trench
Dig your trench to the required depth. As a general guide, bury the first course of blocks at least one-tenth of the total wall height below finished ground level — so for a 1 m wall, bury at least 100 mm. For taller or engineered walls, your engineer will specify the footing depth.
The trench bottom must be firm and undisturbed. Remove any loose soil, roots, or organic material. If the ground is soft or clay-heavy (common in many parts of NZ), you may need to dig deeper and bring in compacted aggregate.
Step 2 — Lay a Compacted Aggregate Base
Spread a 100–150 mm layer of compactable aggregate (AP20 crushed rock is widely available from landscape suppliers and Bunnings or Mitre 10) across the trench floor. Compact it thoroughly with a hand tamper or plate compactor. This layer provides drainage and a stable, level platform for your blocks.
Step 3 — Set Up Your String Line
This is where accuracy really counts. Drive stakes into the ground at each end of the wall and run a taut string line at the height of the top of your first course of blocks. Use a line level or laser level to make the string perfectly level. Check it from multiple points — string can sag in the middle on longer runs, so keep spans short or use intermediate stakes.
Step 4 — Place and Level the First Block
Start at the lowest point of your trench (if the ground slopes, you'll need to step your footing). Place your first block on the compacted aggregate and check it for level in two directions — along the length of the wall and front-to-back. Use a long spirit level for accuracy.
Adjust by:
- Adding aggregate underneath if the block is too low
- Tapping down gently with a rubber mallet if it's slightly high
- Scraping away aggregate if it's too high
Never use soil to pack under blocks — it will compress and settle over time.
Step 5 — Work Along the Course
Once your first block is set, use it as your reference. Place each subsequent block tight against the previous one, checking constantly against your string line and with your spirit level. The top of every block should just kiss the string line — not push it up or sit below it.
| Check | Tool | Acceptable Tolerance |
|---|---|---|
| Level along wall | Spirit level / string line | ±3 mm over 3 m |
| Level front to back | Spirit level | ±2 mm per block |
| Alignment (straight) | String line | ±5 mm over full length |
For dry-stack block systems (such as segmental retaining wall blocks), no mortar is used in the base course — the weight and interlocking design hold everything together. For mortared block walls, bed each block in a consistent mortar mix and check level before the mortar sets.
Step 6 — Check the Whole Course Before Moving On
Once all base course blocks are placed, step back and check the entire course with your string line and spirit level. Fix any issues now — it's much easier than trying to correct problems once you've built up several courses.
Drainage Is Critical
NZ's Building Code (specifically Clause E1 — Surface Water) requires that retaining walls manage water effectively. Install agricultural drain pipe (ag pipe) behind the base of the wall, wrapped in geotextile fabric, and ensure it outlets to a suitable discharge point. Backfill behind the wall with free-draining gravel, not clay. Poor drainage is the number one cause of retaining wall failure in New Zealand. BRANZ has useful guidance on drainage behind retaining structures.
When to Call a Professional
For walls over 1 m in height, walls retaining a slope or surcharge, or any wall on unstable or sloping ground, it's strongly recommended — and often legally required — to engage a qualified retaining wall builder or structural engineer. Retaining wall failures can be dangerous and costly. Licensed Building Practitioners (LBPs) can carry out and sign off restricted building work where required — find one at LBP. You can also find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
