Mortaring concrete blocks for a retaining wall is a satisfying DIY project when done correctly — but it requires careful preparation, the right mortar mix, and an understanding of NZ building regulations. Done properly, a mortared block retaining wall can last decades; done poorly, it can fail and cause serious property damage or injury.
Check If You Need a Building Consent First
Before you mix a single batch of mortar, check your consent obligations. In New Zealand, retaining walls are subject to the NZ Building Code and may require a building consent depending on their height and the surcharge they carry.
As a general guide under Schedule 1 of the Building Act, retaining walls up to 1.5 metres in height (confirm the measurement methodology with your local council, as this can vary) that do not support a surcharge (such as a driveway, building, or slope) may be exempt from consent — but this is not guaranteed in every situation. Always confirm with your local council before starting, as ground conditions, proximity to boundaries, and other factors can affect requirements. You can find more information at MBIE Building Performance.
Gather Your Materials and Tools
For a mortared concrete block retaining wall you will need:
- Concrete blocks (standard 390 × 190 × 190 mm hollow or solid blocks are common in NZ)
- Mortar mix — typically a 1:4 or 1:5 ratio of Portland cement to washed sand (by volume), or a bagged pre-mixed mortar product
- Plasticiser (optional, improves workability)
- Trowels, spirit level, string line, rubber mallet, jointing tool
- Reinforcing rod (rebar) and grout if required by design
- Drainage aggregate (20 mm chip) and agricultural drain pipe for the back of the wall
You can source blocks, cement, sand, and tools from Bunnings, Mitre 10, or PlaceMakers.
Prepare a Solid Foundation
A mortared retaining wall is only as good as its footing. Excavate down to firm, undisturbed ground — typically at least 200–300 mm below finished ground level on the low side. Pour a concrete footing (at minimum 300 mm wide and 150 mm deep for a modest wall) and allow it to cure fully — at least 24–48 hours — before laying blocks. A level footing is critical; use a spirit level and string line to confirm.
Mix Your Mortar Correctly
A standard mortar mix for concrete block retaining walls is 1 part Portland cement to 4–5 parts washed sand (by volume), with enough clean water to achieve a firm but workable consistency — it should hold its shape when squeezed but not be crumbly or sloppy. Adding a small amount of plasticiser (available at hardware stores) improves workability without weakening the mix.
Do not use too much water — a wet mix is easier to spread but significantly reduces the final strength of the mortar joint.
Mix only what you can use within about 30–45 minutes, especially on warm or windy days.
Laying the Blocks: Step-by-Step
1. Lay the First Course
Spread a 10–15 mm bed of mortar on the footing using a trowel. Set the first block firmly into the mortar, checking it is level in both directions. Continue along the footing, buttering the vertical (perpend) joints on the end of each block before placing it.
2. Maintain Consistent Joint Thickness
Aim for 10 mm mortar joints throughout — both horizontal (bed joints) and vertical (perpend joints). Use a string line stretched between corner profiles to keep each course straight and level.
3. Stagger the Joints (Running Bond)
Offset each course by half a block length — this is called a running bond and is essential for structural strength. Never allow vertical joints to line up between courses.
4. Strike the Joints
Once the mortar is firm enough to hold a thumbprint (usually 30–60 minutes after laying), use a jointing tool or the edge of your trowel to compress and shape the joints. This improves weather resistance and appearance.
5. Keep Blocks Clean
Wipe mortar smears off block faces with a damp cloth before they harden. Dried mortar is much harder to remove.
Install Drainage — This Is Critical
A mortared retaining wall must have adequate drainage behind it. Without it, water pressure (hydrostatic pressure) will build up and can push the wall over. Install:
- A layer of free-draining aggregate (20 mm chip) at least 300 mm wide behind the wall
- A perforated agricultural drain pipe at the base of the wall, sloped to daylight or a stormwater outlet
- Weep holes through the wall at regular intervals (every 1–2 metres) at the base course, formed by leaving perpend joints open or inserting short sections of pipe
For guidance on drainage and moisture management, refer to BRANZ.
Curing and Finishing
Keep the finished wall damp for at least 3–5 days after laying to allow the mortar to cure properly — cover with hessian or plastic if conditions are hot, windy, or frosty. Avoid loading the wall with backfill for at least 7 days, and ideally longer.
When to Call a Professional
For walls over 1.0 metre in height, walls retaining a slope or surcharge, or any wall requiring a building consent, it is strongly recommended — and may be legally required — to engage a qualified retaining wall builder or structural engineer. Retaining wall failures can be dangerous and costly. A Licensed Building Practitioner (LBP) can carry out or supervise restricted building work where required — find one at Licensed Building Practitioners. You can also find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
