How to Seal a Crack in a Concrete Water Feature Yourself

Sealing a crack in a concrete water feature is a satisfying DIY job that most homeowners can tackle themselves, provided the crack is minor and the structure is sound. With the right preparation, a quality waterproofing product, and a bit of patience, you can restore your water feature to a watertight finish that lasts for years.

Assess the Crack First

Before reaching for the sealant, take a good look at what you're dealing with. Not all cracks are equal:

• Hairline cracks (less than 0.3 mm wide) are common in older concrete and are usually straightforward to seal.
• Structural cracks (wider than 3 mm, or cracks that run through the full depth of the concrete, or are actively moving) may indicate a more serious problem and may need professional assessment.
• Active leaks — if water is visibly seeping through, you'll need a product rated for wet or damp conditions.

If the water feature is large, heavily cracked, or the concrete is crumbling, it's worth getting a waterproofing professional to assess it before you spend money on products. You can find qualified local waterproofers on tradedirectory.co.nz.

Tools and Materials You'll Need

• Wire brush or angle grinder with a grinding disc
• Cold chisel and hammer (for opening up the crack)
• Vacuum or compressed air
• Clean water and a sponge
• Waterproofing crack filler or hydraulic cement
• Waterproofing membrane or slurry coat
• Paintbrush, roller, or trowel (depending on product)
• Safety glasses and gloves

Step-by-Step: How to Seal the Crack

1. Drain and Dry the Water Feature

Drain the water feature completely. For best results, allow the concrete to dry for at least 24–48 hours. Some products can be applied to damp surfaces, but always check the manufacturer's instructions.

2. Open Up the Crack

This step surprises many DIYers, but it's critical. Use a cold chisel and hammer (or an angle grinder) to widen the crack slightly into a V-shape or U-shape — at least 6–10 mm wide and deep. This gives the filler something to grip. Brush out all loose material with a wire brush, then vacuum or blow out the dust.

3. Dampen the Surface

Lightly dampen the crack and surrounding concrete with clean water. This prevents the dry concrete from drawing moisture out of your repair product too quickly, which can cause it to shrink or fail.

4. Apply a Crack Filler or Hydraulic Cement

For cracks with active water ingress, a hydraulic cement (sometimes called plug cement) is ideal — it expands as it sets and can stop active leaks. Mix according to the instructions and press firmly into the crack, holding it in place for a minute or two until it sets.

For dry cracks, a flexible polyurethane or epoxy crack filler works well. Products from Sika and Ardex are widely available in New Zealand and are well-regarded for concrete repair and waterproofing applications. Mapei also offers suitable repair mortars and waterproofing systems. Always read the product datasheet to confirm it's suitable for water features and potable or fish-safe use if relevant.

5. Apply a Waterproofing Membrane or Slurry Coat

Once the crack filler has fully cured (check the product instructions — this can range from a few hours to 24 hours), apply a cementitious waterproofing slurry or liquid membrane over the entire interior surface of the water feature, not just the crack. This creates a continuous waterproof barrier and helps prevent future cracking from causing leaks.

Apply at least two coats, allowing each coat to dry before applying the next. Pay extra attention to corners and joints — these are common weak points.

6. Cure and Test

Allow the waterproofing to cure fully — typically 48–72 hours minimum, but check your product's datasheet. Fill the water feature with water and monitor it over 24–48 hours for any signs of leaking before adding plants, fish, or a pump.

Choosing the Right Product

Always check that any product you use is safe for aquatic life if you plan to keep fish or plants. Product datasheets from Sika, Ardex, or Mapei will specify this. You can source many of these products at Bunnings, Mitre 10, or PlaceMakers.

NZ Climate Considerations

New Zealand's variable climate — particularly in regions with cold winters or high UV exposure — can accelerate concrete cracking. Freeze-thaw cycles in places like the South Island or Central Plateau can cause cracks to expand over winter. If you're in one of these areas, choose a flexible waterproofing product that can accommodate slight movement, rather than a rigid one.

When to Call a Professional

Call a qualified waterproofer if:

• The crack is wider than 3 mm or appears structural
• The concrete is spalling, crumbling, or heavily deteriorated
• You've attempted a repair and it's still leaking
• The water feature is large or complex (e.g., a pond with a liner system or integrated plumbing)
• You're unsure whether the structure is safe

For any plumbing connections to the water feature (such as mains water top-up lines or drainage to the stormwater system), note that this is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board. Similarly, any electrical work for pumps or lighting must be done by a registered electrician — check the Electrical Workers Registration Board for details.

Find a trusted local waterproofer or concreter on tradedirectory.co.nz.

Sources & Further Reading

• Sika NZ — Concrete Repair & Waterproofing Products
• Ardex NZ — Waterproofing Systems
• Mapei NZ — Repair Mortars & Waterproofing
• BRANZ — Building Research Association of NZ
• Bunnings NZ — Concrete Repair Products
• WorkSafe NZ — Safe Use of Tools and Materials