How to Stop Water Pooling on Your Concrete Patio

Water pooling on a concrete patio is a common frustration for NZ homeowners, and it nearly always comes down to one of three things: insufficient fall (slope), a blocked or missing drain, or surface deterioration that's created low spots. The good news is that many causes can be fixed with a bit of effort — though some situations do call for a qualified concreter.

Why Water Pools on Concrete

Concrete patios should be laid with a minimum fall of 1:100 (1 cm drop for every 1 metre of run) away from the house, as required under the NZ Building Code (particularly Clause E1 – Surface Water). If your patio was laid without enough fall, or if the ground has settled over time, water has nowhere to go.

Common causes include:

• Insufficient or reversed fall — the slab slopes toward the house instead of away
• Low spots or depressions — caused by ground settlement or poor original finishing
• Blocked or absent drainage — no channel drain, or existing drains clogged with debris
• Surface deterioration — spalling or pitting that creates puddle-prone hollows
• Surrounding soil level too high — garden beds or lawn built up against the slab edge, trapping water

Step 1 – Diagnose the Problem

Grab a long spirit level or a straight piece of timber and a tape measure. Lay it across the patio in several directions to find where the low spots are and which way the slab is actually falling. A simple garden hose test — running water across the surface — will quickly show you where it pools and where it drains.

Check your existing drains: are they clear of leaves, soil, and debris? A blocked channel drain is often the simplest fix of all.

Step 2 – Clear and Improve Drainage First

Before doing anything structural, make sure existing drains are clean and free-flowing. If you don't have a channel drain at the low edge of the patio, installing one is often the most cost-effective solution.

Channel (slot) drains are available from Bunnings and Mitre 10 in standard lengths and connect to your stormwater system. Note: connecting a new drain to the stormwater or wastewater system is restricted drainlaying work under the Plumbers, Gasfitters and Drainlayers Act — you'll need a licensed drainlayer for that connection. Check the Plumbers Gasfitters & Drainlayers Board to find a licensed professional.

Also check that the surrounding garden or lawn isn't sitting higher than the patio edge, effectively damming water in. Cutting back soil levels around the perimeter can make a surprising difference.

Step 3 – Fill Low Spots with a Resurfacing Compound

For shallow depressions (generally up to about 10–15 mm deep), a concrete resurfacer or levelling compound can be trowelled or squeegeed into the low areas to restore a flat, drainable surface.

Products like those from Ardex or Sika are designed for this purpose and bond well to existing concrete when the surface is properly prepared. Key preparation steps:

1. Clean thoroughly — remove all moss, algae, oil, and loose material. A pressure washer works well.
2. Etch or grind the surface so the repair compound can bond.
3. Apply a bonding primer as recommended by the product manufacturer.
4. Feather the compound carefully to achieve the correct fall toward the drain or patio edge.

Always follow the manufacturer's instructions for mixing ratios and curing times — NZ's variable climate (especially in cooler southern regions) can affect cure times significantly.

Step 4 – Seal the Surface

Once drainage and level issues are resolved, applying a quality concrete sealer helps reduce water absorption, prevents further surface deterioration, and makes the patio easier to clean. Look for a penetrating sealer or a topical acrylic sealer suited to exterior use. Resene and Dulux both offer concrete and paving sealers suitable for NZ conditions.

Note that some sealers can make surfaces slippery when wet — choose one with a non-slip additive or apply a non-slip aggregate for safety, especially in our wet NZ winters.

Step 5 – Consider a Full Overlay or Re-Pour

If the fall is fundamentally wrong across the whole slab, or if the concrete is badly deteriorated, a bonded overlay (a thin new layer of concrete or topping compound over the existing slab) may be the best option. This allows you to re-establish the correct fall. For severe cases, the slab may need to be broken out and re-poured entirely.

These are jobs best left to a qualified concreter who can ensure the correct fall is achieved and the work meets the NZ Building Code requirements. You can find experienced local concreters on tradedirectory.co.nz.

When to Call a Professional

Call a licensed concreter or drainlayer if:

• The entire slab has the wrong fall and needs an overlay or re-pour
• You need to connect a new drain to the stormwater system (restricted work under NZ law)
• The patio is attached to the house and any work might affect weathertightness or the building structure
• You're unsure whether a building consent is required — always check with your local council, as some concreting work may fall under Schedule 1 exemptions but others may not

For guidance on consent requirements, visit MBIE Building Performance.

Safety Note

Pooling water on a patio creates a slip hazard, particularly on smooth concrete. Until the problem is fixed, consider placing non-slip matting over affected areas. Moss and algae growth on wet concrete can be extremely slippery — treat with a suitable moss killer and keep the surface clean.

Sources & Further Reading

• MBIE Building Performance – NZ Building Code Clause E1 (Surface Water)
• BRANZ – Concrete and paving guidance
• Plumbers Gasfitters & Drainlayers Board – Find a licensed drainlayer
• Ardex – Concrete repair and resurfacing products
• Sika – Concrete repair solutions
• Bunnings – Channel drains and drainage supplies