Getting the drainage right behind a block retaining wall is arguably more important than the wall itself. Without proper drainage aggregate, water builds up behind the wall, creating hydrostatic pressure that can cause bulging, cracking, or outright collapse — even on a well-built structure. Here's what you need to know to do it correctly.
Why Drainage Aggregate Matters
When rain falls, water naturally migrates through the retained soil and collects behind your wall. If it has nowhere to go, pressure mounts rapidly. A saturated soil mass is also significantly heavier than dry soil, increasing the load on the wall beyond its design capacity. Proper drainage aggregate creates a free-draining zone that channels water safely away before pressure can build.
This is a core requirement under the NZ Building Code, particularly Clause E1 (Surface Water) and the structural requirements that apply to retaining walls. Walls retaining more than 1.5 metres typically require a building consent — always check with your local council before starting, as requirements vary by region.
What Type of Aggregate to Use
Not all gravel is equal. You need a free-draining, clean aggregate — one with minimal fines (clay or silt particles) that would clog the drainage zone over time.
- Recommended: 20 mm to 40 mm clean crushed rock or river gravel (often called drainage metal or AP40 in NZ)
- Avoid: Crusher dust, AP20 with high fines content, or any material that compacts tightly
- Some block manufacturers specify particular aggregate grades — always check the manufacturer's installation guide for your specific block system
Products like drainage chip are widely available at Bunnings and Mitre 10, or in bulk from local landscape suppliers.
How Much Aggregate Do You Need?
As a general guide, the drainage aggregate zone should extend at least 300 mm horizontally behind the wall face for the full height of the wall. For taller walls or sites with high groundwater, a wider zone (up to 600 mm or more) may be specified by an engineer.
| Wall Height | Minimum Aggregate Zone Width | Consent Likely Required? |
|---|---|---|
| Up to 0.5 m | 300 mm | Usually no |
| 0.5 m – 1.0 m | 300 mm | Check with council |
| 1.0 m – 1.5 m | 300 mm–450 mm | Often yes |
| Over 1.5 m | Engineer-specified | Yes |
Always confirm consent requirements with your local council — these thresholds can vary.
Step-by-Step Installation
1. Prepare the Base
Before placing any aggregate, ensure your wall foundation is correctly set on firm, compacted ground. The base course of blocks should sit on a compacted gravel pad or concrete footing as required.
2. Lay the Drainage Pipe
At the base of the wall, install a perforated agricultural drain pipe (commonly 100 mm diameter, also called ag-pipe or slotted PVC) along the full length of the wall. The pipe should:
- Sit on a bed of clean aggregate
- Be wrapped in geotextile filter fabric (sock) to prevent fines from entering
- Fall at a minimum grade of 1:100 toward a legal discharge point (stormwater drain, soakpit, or daylight outlet)
Discharge of stormwater must comply with your local council's rules — never direct it onto a neighbour's property.
3. Place the Geotextile Fabric
Line the back of the excavation (the soil face) with a non-woven geotextile filter fabric before placing aggregate. This separates the retained soil from the drainage aggregate, preventing migration of fines into the gravel zone over time. Overlap joins by at least 300 mm.
4. Place and Compact the Aggregate
Backfill the drainage zone with your clean aggregate in 200 mm layers, lightly compacting each layer. Do not use heavy mechanical compaction directly against the wall — this can damage or displace blocks. Hand tamping or a plate compactor kept well back from the wall face is safer.
5. Fold Over the Geotextile
Once the aggregate zone is at the correct height, fold the top of the geotextile fabric over the top of the aggregate before placing any general backfill soil. This creates a complete envelope that keeps soil out of the drainage layer.
6. Backfill with General Fill
Above and behind the aggregate zone, backfill with suitable compacted fill in layers. Avoid placing heavy clay directly against the drainage zone.
Weep Holes — An Additional Safeguard
Many block retaining walls also incorporate weep holes — gaps left in the mortar joints or purpose-built outlets — at the base of the wall every 1.5 to 2 metres. These provide a secondary escape route for any water that reaches the wall face. Check your block system's specifications for guidance.
When to Call a Professional
Retaining wall drainage sounds straightforward, but errors can be costly and dangerous. You should engage a qualified retaining wall builder or structural engineer if:
- Your wall is over 1.0 m high (and certainly over 1.5 m, where consent is almost always required)
- The site has high groundwater, springs, or poor-draining clay soils
- The wall is near a boundary, building, or slope
- You are unsure about stormwater discharge compliance
For walls requiring consent, your council may require plans prepared or reviewed by a Licensed Building Practitioner (LBP) or engineer. Check the LBP register to find qualified practitioners, and find experienced local retaining wall builders on tradedirectory.co.nz.
For guidance on what work requires consent, MBIE Building Performance is the authoritative source.
Licensed Building Practitioner (LBP)
