Yes, you can spot many of the early warning signs of poor drainage around your home's foundation yourself — no special tools required. A systematic walk-around after heavy rain is one of the most useful things a homeowner can do to protect their property. Catching drainage problems early can save you thousands in repairs and help keep your home compliant with the NZ Building Code.
Why Foundation Drainage Matters
New Zealand's varied climate — from the wet West Coast to Auckland's subtropical downpours and Wellington's wind-driven rain — means foundations are regularly tested by water. The Building Code clause E1 (Surface Water) requires that surface water and subsoil water be managed so it doesn't damage the building or neighbouring properties. When drainage fails, water can undermine foundations, cause subsidence, drive moisture into subfloor spaces, and lead to mould — all of which affect both structural integrity and the health of occupants.
BRANZ research consistently identifies moisture ingress as one of the leading causes of building defects in NZ homes, making drainage a priority for any homeowner.
Do a Walk-Around After Rain
The best time to inspect is during or shortly after a significant rainfall event. Walk the full perimeter of your home and look for the following:
1. Pooling or Ponding Water
Standing water near the foundation after rain is the clearest red flag. Water should drain away from the house within a few hours. If puddles persist close to the foundation, the ground may be graded incorrectly or the soil may be compacted and unable to absorb water.
2. Soil Grading Sloping Towards the House
The ground around your foundation should slope away from the house — a general rule of thumb is at least 50 mm drop over the first metre. If the soil, lawn, or garden beds slope inward, water is being directed straight at your foundation.
3. Overflowing or Blocked Gutters and Downpipes
Check that gutters are clear and downpipes discharge water well away from the foundation — ideally into a stormwater drain or at least 1.5–2 metres from the building. Downpipes that dump water right at the base of the wall are a very common cause of localised foundation saturation.
4. Erosion Channels or Bare Soil
Visible erosion channels, ruts, or patches of bare, washed-out soil near the foundation indicate that water is flowing with enough force to move soil — and likely carrying it away from where it's needed to support your foundation.
5. Damp or Wet Subfloor Space
If your home has a suspended timber floor, check the subfloor space (if safely accessible). Signs of concern include:
- Standing water or mud on the ground
- Efflorescence (white salt deposits) on concrete piles or block foundations
- A persistently musty smell coming from floor vents
- Condensation or mould on floor joists or piles
Poor subfloor ventilation combined with drainage issues is a known contributor to unhealthy homes. The Healthy Homes Standards require rental properties to have adequate subfloor moisture barriers, but owner-occupiers should take this equally seriously.
6. Cracks in the Foundation or Exterior Walls
While not all cracks are caused by drainage, stair-step cracks in brick or block, or diagonal cracks running from window corners, can indicate differential settlement caused by soil movement — often linked to water saturation or erosion. Horizontal cracks in concrete or block foundation walls can indicate lateral soil pressure, which is more serious.
7. Rust Stains or Efflorescence on Concrete
Rust-coloured streaks on concrete suggest water is penetrating and corroding reinforcing steel inside. White powdery deposits (efflorescence) mean water is moving through the concrete and depositing minerals on the surface — a sign of ongoing moisture movement.
8. Retaining Walls Leaning or Bulging
If you have retaining walls near the house, check for leaning, bulging, or cracking. These can indicate waterlogged soil building up pressure behind the wall. Note that retaining walls over a certain height typically require a building consent — check with your local council for the current threshold — check with your local council.
Simple Checks You Can Do
| What to Check | What to Look For | When to Check |
|---|---|---|
| Ground slope | Slopes away from house | Any time |
| Gutters & downpipes | Clear, discharging away from foundation | After rain |
| Subfloor space | Damp, mould, standing water | After prolonged rain |
| Foundation walls | Cracks, efflorescence, rust stains | Any time |
| Retaining walls | Leaning, bulging, cracking | After heavy rain |
| Lawn/garden beds | Pooling, erosion, soggy patches | During/after rain |
Quick DIY Improvements
Some minor drainage improvements are within a confident homeowner's reach:
- Re-grading garden beds away from the house using topsoil
- Cleaning gutters and downpipes and extending downpipe outlets further from the foundation
- Adding a splash block under a downpipe outlet
- Installing a subfloor vapour barrier (polythene sheeting on the ground) if the subfloor is accessible and dry enough to work in safely
Anything involving connecting to the stormwater or drainage system is restricted work under the Plumbers, Gasfitters and Drainlayers Board and must be carried out by a licensed drainlayer.
When to Call a Professional
Call a licensed building inspector or building surveyor if you notice:
- Structural cracks in the foundation or walls
- Persistent subfloor dampness despite basic improvements
- Significant soil movement or settlement
- Retaining walls showing signs of failure
A qualified building inspector can assess the full picture and recommend whether you need a drainlayer, geotechnical engineer, or other specialist. For drainage work connecting to the stormwater network, you'll need a licensed drainlayer. If you're unsure whether any work requires a building consent, always check with your local council — MBIE Building Performance has useful guidance on Schedule 1 exemptions.
You can find qualified local building inspectors and drainage professionals on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
BCITO Qualified
