Waterproofing the back (retained soil) face of a concrete block retaining wall is one of the most important steps you can take to extend its life and prevent costly structural damage. Without proper waterproofing and drainage, hydrostatic pressure builds up behind the wall, leading to cracking, leaning, and eventual failure. The good news is that much of this work is DIY-friendly — provided the wall itself was built correctly and doesn't require a building consent.
Does Your Wall Need a Building Consent?
Before you start, it's worth confirming whether your retaining wall required — or requires — a building consent. Under the NZ Building Code and Schedule 1 exemptions, retaining walls up to 1.5 metres in height (measured from the bottom of the footing) that do not support a surcharge (such as a driveway, building, or slope) may be exempt from consent. However, rules vary by council and site conditions, so always check with your local council before proceeding. You can find general guidance at MBIE Building Performance.
Why Waterproofing Matters
Concrete blocks are porous. Moisture from the retained soil passes through the blocks over time, causing:
- Efflorescence (white salt deposits on the face of the wall)
- Freeze-thaw cracking (less common in NZ but relevant in colder regions like Otago and Southland)
- Corrosion of any steel reinforcing inside the wall
- Hydrostatic pressure build-up that can push the wall over
A good waterproofing system works alongside proper drainage — one without the other is rarely effective.
Step 1: Prepare the Surface
Proper surface preparation is critical for any waterproofing product to bond correctly.
- Clean the block face thoroughly. Remove loose mortar, dirt, efflorescence, and any organic growth using a stiff brush or pressure washer.
- Allow the surface to dry to the level specified by your chosen product (some products are designed for damp surfaces — check the datasheet).
- Fill any cracks or voids with a suitable cementitious repair mortar before applying a membrane. Products from Sika or Ardex are widely available in NZ and well-suited to this kind of repair work.
Step 2: Apply a Waterproofing Membrane
There are several types of waterproofing membranes suitable for below-ground retaining wall applications in NZ:
| Type | How It's Applied | Best For |
|---|---|---|
| Bituminous/Asphalt coating | Brush or roller | Budget-friendly, widely available |
| Cementitious membrane | Brush or trowel | Damp surfaces, good bond to concrete |
| Liquid-applied polymer membrane | Brush, roller, or spray | Flexible, good for irregular surfaces |
| Sheet membrane (dimple mat) | Mechanically fixed | Often used as a drainage layer too |
For most DIY applications, a two-coat bituminous waterproofing compound or a cementitious slurry membrane is the most practical choice. Apply strictly according to the manufacturer's instructions — coverage rates and drying times matter.
A dimple drainage mat (sometimes called a drainage cell or egg-crate membrane) is often applied over the waterproofing layer. It creates an air gap that channels water downward toward the drainage system rather than letting it press directly against the wall.
Step 3: Install Drainage Behind the Wall
Waterproofing alone is not enough. You must manage the water that collects behind the wall. The NZ Building Code (in particular Clause E1 – Surface Water) requires that surface and subsoil water is managed so it doesn't cause damage.
- Place a layer of free-draining aggregate (typically 20 mm clean chip) directly behind the wall, at least 300 mm wide.
- Install a perforated agricultural drain pipe (ag pipe) at the base of the wall, wrapped in geotextile filter fabric to prevent silt ingress.
- Ensure the ag pipe drains to a suitable outlet — a soakpit, stormwater drain, or daylight at the base of a slope. Check with your council about stormwater connection requirements.
- Weep holes through the base of the wall (typically 75–100 mm diameter, spaced every 1.5–2 m) provide a secondary drainage path and are often required by council.
Step 4: Backfill Carefully
Once the membrane has cured fully (check the product datasheet — this can be 24–72 hours), backfill in compacted layers of no more than 200 mm using suitable fill material. Avoid using heavy machinery directly against the wall, as impact can damage the fresh membrane.
Place geotextile fabric between the drainage aggregate and the general fill to prevent fine soil particles from migrating into and blocking the drainage layer.
Choosing the Right Products in NZ
You'll find a good range of waterproofing membranes, ag pipe, drainage aggregate, and geotextile fabric at Bunnings, Mitre 10, and PlaceMakers. For more technical or commercial-grade systems, specialist suppliers stocking Sika, Ardex, or Mapei products are worth considering. Always read the product technical data sheet before purchasing.
When to Call a Professional
While waterproofing itself is generally not restricted work, there are situations where you should engage a qualified retaining wall builder or geotechnical engineer:
- The wall is over 1.5 m high, or supports a surcharge (driveway, building, or slope above)
- The wall is showing signs of movement, cracking, or leaning
- You're unsure about drainage design or soil conditions
- The site has a high water table or is in a flood-prone area
- A building consent was required and the work needs to be inspected
For walls that involve plumbing connections to stormwater systems, a licensed drainlayer may be required — check with the Plumbers Gasfitters & Drainlayers Board. You can find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
