Can I Waterproof a Concrete Block Retaining Wall with a Brush-On Membrane?

Yes, a brush-on waterproof membrane is a practical and widely used option for concrete block retaining walls — but success depends heavily on thorough surface preparation, choosing the right product for below-ground or above-ground use, and pairing the membrane with good drainage. Skipping any of these steps is the most common reason waterproofing fails.

Understanding What You're Trying to Achieve

A retaining wall holds back soil, which means one face is constantly in contact with damp earth and hydrostatic pressure. Your goal isn't just to stop water soaking through the blocks — it's to manage hydrostatic pressure (water pushing against the wall from the soil side) and direct water safely away. A membrane alone won't do this; it must work alongside a proper drainage system.

Before you start, check whether your retaining wall required a building consent. In New Zealand, retaining walls over a certain height generally require consent under the Building Act — confirm the current threshold and any conditions with your local council. Always confirm with your local council, as Schedule 1 exemptions have specific conditions. You can find guidance at MBIE Building Performance.

Choosing the Right Brush-On Membrane

Not all brush-on membranes are suitable for below-ground (tanking) applications. Look for products specifically rated for negative-side or positive-side tanking on masonry. Common types include:

• Cementitious waterproofing membranes – powder-based products mixed with water and applied like a slurry. These bond well to concrete block and tolerate damp surfaces. Brands like Ardex and Mapei offer cementitious tanking products widely available in NZ.
• Polymer-modified bitumen coatings – thick, brush- or trowel-applied coatings that are flexible and highly waterproof. Good for below-ground use on the soil-facing side.
• Acrylic or elastomeric membranes – better suited to above-ground or exposed surfaces. Check the manufacturer's datasheet carefully before using these below ground.
• Sika products – Sika offers a range of tanking and waterproofing systems for masonry that are well regarded in the NZ trade.

Always read the technical data sheet for your chosen product. It will specify coverage rates, number of coats, curing times, and whether the product suits your specific application (positive side = dry side, negative side = wet/soil side).

Surface Preparation — The Most Important Step

Poor prep is the number one cause of membrane failure. Follow these steps:

1. Clean the surface thoroughly — remove dirt, efflorescence (white salt deposits), loose mortar, and any existing coatings. A stiff brush, scraper, or pressure washer works well.
2. Repair cracks and voids — fill honeycombing, cracks, and open mortar joints with a suitable repair mortar or hydraulic cement before applying the membrane. Membranes bridge small cracks but won't seal large defects on their own.
3. Round off sharp edges and corners — sharp internal angles can cause membranes to bridge and fail. Apply a cove fillet (a small rounded bead of mortar) at wall-floor junctions.
4. Dampen the surface — most cementitious membranes require a damp (not wet) substrate. Check your product's instructions.
5. Allow new concrete block to cure — if the wall is newly built, allow at least 28 days of curing before applying most membranes.

Application Tips

• Apply a minimum of two coats, working the second coat at right angles to the first.
• Maintain the manufacturer's recommended wet film thickness — too thin and the membrane won't perform.
• Pay extra attention to joints, corners, and penetrations — these are the most vulnerable spots.
• Allow each coat to reach the correct cure stage before applying the next.
• In hot or windy NZ conditions, mist the surface to prevent the membrane drying too fast and cracking.

Don't Forget Drainage

A membrane reduces water ingress but doesn't eliminate hydrostatic pressure. Behind your retaining wall you should have:

BRANZ has useful guidance on retaining wall construction and drainage best practice.

NZ Climate Considerations

New Zealand's high rainfall — particularly in the West Coast, Northland, and Wellington regions — means retaining walls can face significant water loads. In clay-heavy soils, drainage is even more critical. In coastal areas, check that your membrane and any metal fixings are suitable for a salt-laden environment.

NZ Building Code clauses such as E1 (Surface Water) may be relevant here. Your local council can advise on compliance requirements for your specific site.

When to Call a Professional

While applying a brush-on membrane is within the capability of a competent DIYer, there are situations where you should engage a qualified waterproofer or builder:

• The wall is over 1.5 metres high or required a building consent — waterproofing may need to be inspected.
• There is active water ingress or structural cracking — this could indicate a deeper problem needing engineering assessment.
• The wall is part of a habitable space (e.g., a basement) — waterproofing in these situations is more complex and failure is costly.
• You're unsure about drainage design — a poorly drained wall can fail structurally, not just leak.
• The work involves plumbing or drainage connections — these are restricted trades in NZ, requiring a licensed drainlayer registered with the Plumbers Gasfitters & Drainlayers Board.

You can find qualified local waterproofers and builders on tradedirectory.co.nz to get the job done right.

Sources & Further Reading

• MBIE Building Performance – Building Consent information
• BRANZ – Retaining walls and moisture management
• Ardex NZ – Waterproofing and tanking products
• Sika NZ – Waterproofing systems
• Mapei NZ – Cementitious waterproofing
• Standards NZ – NZS 3604 and related standards