Waterproofing a retaining wall from the inside (the dry, visible face) is sometimes called negative-side waterproofing. While it's not the ideal solution — waterproofing the soil-side (positive side) is always preferred — it can be a practical option when excavation isn't feasible. Done correctly with the right products and preparation, it can significantly reduce water ingress and protect your wall.
Understanding Positive vs Negative-Side Waterproofing
Before you start, it's worth understanding why this matters. Positive-side waterproofing is applied to the exterior (soil-facing) surface of the wall, stopping water before it enters the structure. Negative-side waterproofing is applied to the interior face, where water pressure is already pushing through. Negative-side products must be able to withstand hydrostatic pressure from behind — not all waterproofing membranes can do this, so product selection is critical.
For retaining walls in New Zealand, moisture management is especially important given our variable climate — from wet West Coast conditions to freeze-thaw cycles in the South Island high country. Poor waterproofing can lead to efflorescence, structural cracking, and undermining of the wall over time.
What Products Work for Negative-Side Waterproofing?
Not every waterproofing product is suitable for negative-side application. Look for these types:
- Crystalline waterproofing compounds – These penetrate concrete and react with moisture to form crystals that block water pathways. They're one of the most effective negative-side options. Brands like Sika offer crystalline-based products suitable for this application.
- Cementitious waterproofing coatings – Two-part polymer-modified coatings that bond well to masonry and concrete. Ardex and Mapei both supply cementitious waterproofing systems available in New Zealand.
- Hydraulic cement – A fast-setting product used to plug active leaks before applying a full waterproofing system. Useful as a first step if water is actively seeping through.
Avoid standard bitumen-based membranes or flexible sheet membranes on the negative side — they are not designed to resist hydrostatic pressure pushing from behind and will likely fail.
Step-by-Step: Applying Negative-Side Waterproofing
1. Assess the Wall and Fix Active Leaks
Start by identifying any cracks, joints, or holes where water is actively entering. Use a hydraulic cement or fast-setting plug compound to stop active water flow before applying your main waterproofing system. Allow this to cure fully per the manufacturer's instructions.
2. Prepare the Surface Thoroughly
This is the most important step. The waterproofing coating must bond directly to sound, clean masonry or concrete:
- Remove all loose material, paint, efflorescence, and contamination
- Use a wire brush, angle grinder, or pressure washer to open up the surface
- Fill any voids, honeycombing, or large cracks with a suitable repair mortar
- The surface should be damp but not wet before applying most cementitious coatings — check your product's data sheet
3. Apply the Waterproofing System
Follow the manufacturer's instructions precisely — mix ratios and application thickness matter. General guidance:
- Apply the first coat by brush, working the product firmly into the surface
- Allow to reach the correct cure stage (check data sheet — often a few hours)
- Apply a second coat in the opposite direction to the first for even coverage
- Typical dry film thickness for cementitious coatings is 1–2 mm over two coats, but always confirm with the product data sheet
- Pay extra attention to wall-floor junctions, corners, and any penetrations — these are common failure points
4. Allow Proper Curing
Keep the coating damp during curing (mist with water) if using a cementitious product — this helps it reach full strength. Avoid foot traffic or loading until fully cured.
5. Consider Drainage
Waterproofing alone won't solve the problem if hydrostatic pressure is extreme. Make sure your retaining wall has adequate weep holes or a drainage layer on the soil side. If water is building up significantly behind the wall, address the drainage issue as well — otherwise even good waterproofing will be under constant stress. BRANZ has useful guidance on moisture management in structures.
NZ Building Code Considerations
Retaining walls over 1.5 metres high (or in certain ground conditions) generally require a building consent in New Zealand. If your wall was consented, any significant remediation work may need to comply with the NZ Building Code, particularly Clause E2 (External Moisture). Always check with your local council before starting work, especially if the wall is structural. Visit MBIE Building Performance for guidance on consent requirements.
When to Call a Professional
Negative-side waterproofing can be a DIY project for a capable homeowner on a smaller, non-structural wall — but there are situations where you should bring in a qualified waterproofer:
- The wall is actively leaking heavily or shows signs of structural movement or cracking
- The retaining wall is over 1.5 m high or is a consented structure
- You're unsure whether the wall is concrete block, poured concrete, or timber — each needs a different approach
- Previous waterproofing attempts have failed
- The wall is part of a basement or habitable space where moisture damage could be significant
A licensed waterproofer will assess the root cause, recommend the right system, and apply it correctly. You can find qualified local waterproofing tradespeople on tradedirectory.co.nz.
Safety Note
When grinding or preparing surfaces, wear appropriate PPE — safety glasses, a dust mask (P2 rated minimum), and gloves. If the wall is old and you suspect the paint contains lead, treat it as hazardous material. Check WorkSafe NZ for guidance on working safely with hazardous substances.
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