Backfilling behind a retaining wall is one of the most important — and most commonly mishandled — steps in the whole construction process. Done correctly, it protects the wall from hydrostatic pressure, prevents movement, and ensures the structure performs safely for decades. Done poorly, it can cause the wall to lean, crack, or fail entirely.
Why Backfill Matters So Much
A retaining wall holds back soil, but it's the pressure of water-saturated earth that causes most failures. When water can't escape freely from behind the wall, it builds up hydrostatic pressure that can push even a well-built wall out of position. The backfill material and drainage system you install work together to prevent this.
For walls over 1.5 metres in height, a building consent is generally required under the NZ Building Code. The exact measurement reference point (e.g. from the bottom of the footing or from the retained ground level) can vary — check with your local council to confirm how height is measured for consent purposes in your situation. Even for shorter walls, consent may be needed depending on your site conditions, proximity to boundaries, or surcharge loads. Always check with your local council before starting — Schedule 1 exemptions have specific conditions and your council is the right authority to confirm what applies to your situation. You can find guidance at MBIE Building Performance.
Step 1 — Install Drainage Before Any Backfill
Drainage is non-negotiable. Before a single shovel of fill goes in, you need a proper drainage system at the base of the wall:
- Drainage aggregate (AP20 or similar clean crushed rock): Place a layer at least 300 mm wide directly behind the wall.
- Perforated drainage pipe (ag pipe): Lay a 100 mm perforated pipe at the base of the wall, wrapped in geotextile filter fabric to prevent silt blocking it. The pipe should drain to a legal outlet — a stormwater system, soakage area, or daylight at a lower point on the site.
- Geotextile fabric: Line the drainage zone with geotextile to separate the aggregate from the retained soil. This stops fine particles migrating into the drainage layer over time.
- Weepholes: If your wall is concrete block or timber, ensure weepholes are included at regular intervals (typically every 1–2 metres) at the base to allow any water that does build up to escape.
Step 2 — Choose the Right Backfill Material
Not all fill is equal. The ideal backfill material is:
- Free-draining granular material such as AP20 crushed rock, clean gravel, or coarse sand
- Free of organic matter, clay, or topsoil — these retain water and can swell
- Free of large boulders or debris that could create voids or uneven pressure
Avoid using the excavated clay or silty soil you dug out — it holds water, expands when wet, and dramatically increases pressure on the wall. If you're unsure about your fill material, your retaining wall builder or a geotechnical engineer can advise. BRANZ also publishes guidance on earthworks and retaining structures.
Step 3 — Backfill in Compacted Layers
This is where many DIYers go wrong — dumping all the fill in at once and compacting it in one go. The correct method is:
- Place fill in layers (lifts) of no more than 150–200 mm at a time.
- Compact each layer using a plate compactor or hand tamper before adding the next.
- Work from the drainage zone outward, keeping the granular drainage layer intact against the wall face.
- Avoid heavy machinery close to the wall — the vibration and weight can damage or displace the structure, especially while the concrete or mortar is still curing.
- Continue until you reach the finished ground level.
Compaction Tips
| Zone | Recommended Equipment | Notes |
|---|---|---|
| Within 500 mm of wall | Hand tamper or jumping jack | Avoid plate compactor near wall face |
| General fill area | Plate compactor | Work in 150–200 mm lifts |
| Near services or pipes | Hand tamper only | Prevent pipe damage |
Step 4 — Finish the Surface Correctly
Once backfill reaches the desired level:
- Grade the surface away from the wall to direct surface water away, not toward it.
- Allow for settlement — granular fill will settle slightly over time, so you may need to top up after the first rain season.
- If you're laying lawn or garden over the fill, add a layer of topsoil last — not mixed into the structural fill.
Common Mistakes to Avoid
- Using clay or organic soil as backfill
- Skipping the drainage pipe or aggregate layer
- Compacting in one thick lift rather than layers
- Backfilling before concrete or mortar has fully cured
- Directing downpipes or surface water toward the wall
- Ignoring consent requirements for taller or complex walls
When to Call a Professional
Retaining walls that are over 1.5 m high, on steep or unstable ground, near boundaries, or supporting structures like driveways, buildings, or pools should always be designed and built by a qualified professional. A Licensed Building Practitioner (LBP) or structural engineer may be required depending on the complexity. You can verify LBP credentials at Licensed Building Practitioners.
For larger or more complex sites, a geotechnical engineer should assess the soil conditions and specify the correct backfill and drainage design. Getting this wrong is expensive — a failed retaining wall can cost many times more to repair than it would have to build correctly in the first place.
If you need a qualified retaining wall builder in your area, you can search for local professionals on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
