Waterproofing a concrete drinking water tank is absolutely achievable as a DIY project, but it must be done correctly — the wrong product or poor preparation can contaminate your drinking water or cause the lining to fail within months. The key is using a potable-water-approved (food-safe) waterproofing system and following the manufacturer's instructions to the letter.
Why Concrete Tanks Need Waterproofing
Concrete is naturally porous. Over time, untreated concrete can leach lime and minerals into stored water, alter the pH, and allow algae or bacteria to take hold in micro-cracks. Older tanks may also have hairline cracks that allow seepage in or out. A proper internal waterproofing membrane seals the concrete, protects water quality, and extends the life of the tank significantly.
In New Zealand, drinking water quality is governed by the Health (Drinking Water) Amendment Act and associated standards. If your tank supplies a household, you have a responsibility to ensure stored water is not contaminated by the tank itself. Always check with your local council if your tank is part of a consented water supply system.
Choosing the Right Waterproofing Product
This is the most critical decision. You must use a product that is certified safe for contact with potable (drinking) water. Look for products that comply with NSF/ANSI Standard 61 or carry equivalent potable-water approval documentation from the manufacturer.
Commonly used product types include:
- Cementitious waterproofing coatings — cement-based slurries or renders that bond directly to concrete. Brands such as Ardex and Mapei offer cementitious systems; always confirm the specific product is approved for potable water contact before purchasing.
- Epoxy coatings — two-part epoxy systems designed for water tanks. These form a hard, smooth, non-porous film. Again, confirm potable-water certification with the supplier.
- Crystalline waterproofing — products that penetrate concrete and form crystals within the pores, self-sealing over time. Some crystalline systems carry potable-water approval.
Important: Never use a standard paint, bitumen coating, or general-purpose waterproofing membrane inside a drinking water tank. Always read the product's technical data sheet and safety data sheet, and confirm potable-water suitability with the manufacturer or supplier before purchase.
Tools and Materials You'll Need
- Angle grinder or wire brush (for surface preparation)
- Pressure washer
- Crack filler or hydraulic cement (potable-water compatible)
- Mixing paddle and drill
- Stiff-bristle brush or masonry brush
- Appropriate PPE: gloves, safety glasses, dust mask or respirator
- The chosen waterproofing system (primer + membrane/coating)
Step-by-Step: How to Waterproof the Tank
1. Empty and Clean the Tank Thoroughly
Drain the tank completely. Remove all sediment, algae, and debris. Pressure wash the interior walls and floor. Allow to dry, or follow the manufacturer's guidance — some cementitious products require a damp (not wet) substrate.
2. Inspect and Repair Cracks
Check all surfaces carefully for cracks, honeycombing, or spalling. Use a potable-water-compatible crack filler or hydraulic cement to repair any defects. Pay particular attention to wall-floor joints and any pipe penetrations. Allow repairs to cure fully before proceeding.
3. Prepare the Surface
Grind or wire-brush any loose material, efflorescence (white salt deposits), or contamination. The surface must be structurally sound, clean, and free of oils or curing compounds. Good surface preparation is the single biggest factor in coating longevity.
4. Apply the Waterproofing System
Follow the manufacturer's instructions exactly — mixing ratios, number of coats, and drying times are critical. Most systems require:
| Step | Typical Requirement |
|---|---|
| Primer coat | Brush-applied, allow to tack off |
| First membrane coat | Apply evenly, work into corners |
| Second membrane coat | Apply at right angles to first coat |
| Curing time before filling | Typically 7–28 days (check datasheet) |
Pay extra attention to corners, joints, and pipe penetrations — these are the most common failure points. Some systems include a reinforcing fabric tape for these areas.
5. Cure and Test Before Use
Allow the full curing period specified by the manufacturer before refilling the tank. Once filled, it is good practice to flush and discard the first fill of water, then test water quality before using it for drinking. BRANZ provides useful guidance on water storage best practice.
Safety Considerations
Working inside a confined concrete tank carries real risks. Ensure adequate ventilation at all times — some coatings release fumes during application and curing. Never work alone inside a tank; have someone outside who can assist in an emergency. Refer to WorkSafe NZ guidelines on confined space entry before starting work.
Rainwater Collection and Healthy Homes
If your tank collects roof rainwater, ensure your collection system (gutters, first-flush diverters, filters) is also in good order. For rental properties, Tenancy Services Healthy Homes standards may apply to water supply requirements.
When to Call a Professional
Consider engaging a qualified waterproofing tradesperson if:
- The tank is large, deep, or difficult to access safely
- There is significant structural cracking or concrete deterioration
- You are unsure which product system is appropriate
- The tank is part of a consented or community water supply
- Any plumbing connections need to be modified — plumbing work on water supply systems is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed tradesperson registered with the PGDB
A qualified waterproofer will have experience with potable-water systems and can provide a warranty on their work. You can find experienced local waterproofing professionals on tradedirectory.co.nz.
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