The best gravel to use as backfill behind a retaining wall is a free-draining aggregate — typically a clean, crushed angular gravel in the 20–40 mm range, sometimes called drainage metal or AP40 in New Zealand. Getting this right is one of the most important factors in how long your retaining wall lasts, because poor drainage is the leading cause of retaining wall failure.
Why Backfill Material Matters So Much
Water is the enemy of retaining walls. When water saturates the soil behind a wall, it dramatically increases the hydrostatic pressure pushing against the structure. Over time, this can cause walls to lean, crack, or collapse entirely. Free-draining gravel allows water to move quickly down through the backfill and out through weep holes or a drainage pipe at the base, keeping pressure manageable.
The BRANZ guidelines and the NZ Building Code (particularly Clause E1 – Surface Water) both emphasise the importance of managing water around structures. If your retaining wall is over 1.5 metres high, it will almost certainly require a building consent — and the drainage design will be part of what council assesses.
The Best Gravel Types for Retaining Wall Backfill
Clean Crushed Aggregate (AP40 or AP20)
AP40 (all-passing 40 mm crushed rock) is the most commonly recommended backfill material for retaining walls in New Zealand. It's angular, which means the pieces lock together well, and it drains freely because it has very little fine material (clay or silt) mixed in.
- AP40 – ideal for the main drainage zone directly behind the wall
- AP20 – a slightly finer crushed aggregate, also suitable and sometimes easier to compact in tighter spaces
- Pea gravel (10–14 mm) – round and smooth, drains well, but less stable under load; sometimes used around drainage pipes
Avoid using AP65 (larger roading aggregate) directly against the wall as it can be difficult to compact and may not sit well against geotextile fabric.
What to Avoid
Never use the following as primary backfill behind a retaining wall:
- Clay or silty soil – retains water and swells, massively increasing pressure
- Topsoil or garden mix – breaks down, compacts unevenly, and holds moisture
- Recycled concrete rubble – variable quality and drainage characteristics
- Beach sand – can liquefy under saturation and provides poor structural support
How to Install Gravel Backfill Correctly
Getting the gravel in place properly is just as important as choosing the right material.
| Step | What to Do |
|---|---|
| 1. Drainage pipe | Lay a 100 mm perforated ag-pipe (agricultural drain) at the base of the wall, wrapped in geotextile sock, sloping to an outlet |
| 2. Geotextile fabric | Line the excavation face with non-woven geotextile fabric to prevent native soil migrating into the gravel over time |
| 3. Gravel placement | Backfill with AP40 in 200–300 mm layers, compacting each layer lightly with a plate compactor or hand tamper |
| 4. Transition zone | If you need to return to topsoil or garden soil above, taper the gravel zone and use the geotextile to separate materials |
| 5. Weep holes | Ensure weep holes through the wall (if block or concrete) are clear and unobstructed |
The width of the drainage gravel zone behind the wall should generally be a minimum of 300 mm, though your engineer or council may specify more depending on wall height and soil conditions.
Geotextile Fabric — Don't Skip It
Geotextile separation fabric is a non-negotiable part of a well-built retaining wall drainage system. It acts as a filter, letting water pass through while stopping fine soil particles from slowly washing into and clogging your gravel over the years. Look for a non-woven geotextile rated for drainage applications — these are widely available at Bunnings and Mitre 10, or through civil suppliers.
NZ Climate Considerations
New Zealand's variable climate — particularly in high-rainfall regions like the West Coast, Northland, and parts of Wellington — means drainage behind retaining walls must be designed to handle significant water volumes. In expansive clay soil areas (common in Auckland and parts of Hawke's Bay), the contrast between wet and dry seasons puts extra stress on walls, making free-draining backfill even more critical.
If you're in an earthquake-prone area (much of NZ falls into this category), your retaining wall design may need to meet additional requirements under NZS 3604 or require a specific engineering design. Check with your local council.
Building Consent — Do You Need One?
In New Zealand, retaining walls over 1.5 metres in height generally require a building consent. Some walls under 1.5 m may be exempt under Schedule 1 of the Building Act, but this depends on factors like proximity to boundaries, other structures, and surcharge loads. Always confirm with your local council before starting work — don't assume your wall is exempt. You can find guidance at MBIE Building Performance.
When to Call a Professional
Retaining wall construction — especially walls over 1 metre high — involves real structural risk. If a wall fails, it can damage property and seriously injure people. You should engage a qualified retaining wall builder or structural engineer if:
- Your wall is or will be over 1 metre high
- The wall is near a boundary, driveway, or structure
- You're on a steep or unstable slope
- You're in a high-rainfall or earthquake-prone zone
- A building consent is required
A licensed builder or engineer will specify the correct backfill, drainage design, and compaction requirements for your specific site. You can find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
