Poor drainage isn’t just a nuisance; it’s one of the most common and costly causes of damage to Australian homes. From soggy lawns and flooded patios to cracked foundations and rising damp, water that isn’t properly managed can undermine your home’s structure, ruin landscaping and lead to expensive repairs down the line.
The good news? With the right design and maintenance, you can keep stormwater moving where it should; safely away from your house.
Understanding where the water goes
Every site has a natural fall that determines where water will flow. The goal of a good drainage system is to work with this fall, not against it. When you build or renovate, it’s important that your site’s levels are set correctly from the start so water drains toward the street or a designated stormwater point, not back toward the home.
If you’ve noticed puddles forming near your walls, a damp garage or water flowing across pathways, those are early warning signs that your site’s grading isn’t doing its job. Before landscaping or pouring new concrete, get a licensed plumber or drainage specialist to assess the slope and adjust levels if necessary.
Even a small change — as little as 1:100 (1cm of fall per 1m of length) — can make a big difference in how water moves across your property.
The importance of surface drainage
Surface water is the most visible kind of drainage problem. Heavy rain that pools in low spots or runs off hard surfaces like driveways and patios can quickly overwhelm your property if not properly managed.
To prevent flooding, surface water needs to be intercepted and redirected. Common solutions include:
- Grated trench drains along driveways, paths and patios
- Yard drains or pits installed in low-lying garden areas
- Swales (shallow, grassed channels) to guide runoff toward the street or stormwater connection
These systems are designed to capture water before it reaches your home’s foundations. Just remember that anything connected to the stormwater system must comply with AS/NZS 3500.3, which covers stormwater drainage design and installation.
A good rule of thumb is to inspect surface drains at least twice a year. Clear away leaves, mulch and soil that can block grates, because even the best-designed system won’t work if it’s clogged.
Managing subsoil drainage
Surface drains deal with runoff, but what about the water you can’t see? In many parts of Australia, especially on clay or reactive soils, water can accumulate below ground and cause swelling or movement in the soil. Over time, this can lead to cracking in walls, uneven floors, and foundation problems.
That’s where subsoil drainage comes in. Commonly known as ag pipes (short for agricultural pipes), these perforated pipes are buried in trenches around footings or behind retaining walls. They collect groundwater and channel it safely away from the structure.
If you’re building on reactive soil, your engineer will likely specify a subsoil drainage system in your foundation design. It’s important these are installed correctly — pipes should be laid with a consistent fall, wrapped in geotextile fabric to prevent silt blockages, and connected to the stormwater system rather than just left to drain “somewhere downhill.”
More details on the correct installation can be found in AS/NZS 3500.3 and AS 2870 (Residential slabs and footings).
Roof and gutter drainage
Roof water management is another critical piece of the puzzle. Gutters and downpipes have to handle huge volumes of water during heavy rain, especially in tropical or coastal regions. If your gutters overflow or your downpipes can’t keep up, water can spill into the eaves or pool around the slab edge, leading to long-term structural damage.
Regular maintenance is key. Check gutters for blockages every few months and ensure downpipes discharge into a lawful point of discharge — typically a street kerb, stormwater pit or absorption trench approved by your local council.
If you’re upgrading your roof, consider installing larger-capacity gutters or additional downpipes. In many newer homes, roof catchment areas are bigger than the systems designed to drain them, so increasing pipe size or adding extra outlets can make a huge difference in storm performance.
Retaining walls and garden beds
Retaining walls are another common weak point when it comes to drainage. When soil behind a wall becomes saturated, the hydrostatic pressure increases dramatically — and without proper drainage, that pressure can cause the wall to bulge, crack or collapse.
Every retaining wall higher than 600mm should include:
- A perforated drain (such as an ag pipe) behind the wall
- A layer of free-draining gravel
- A geotextile fabric between the gravel and soil to prevent clogging
Garden beds can also create problems if built too close to the house. Raised beds that trap water against exterior walls can lead to rising damp or termite risk. Always leave a small gap — around 50–100mm — between your garden edging and walls to allow airflow and drainage.
Common DIY drainage mistakes
It’s tempting to try fixing drainage issues yourself, but many quick fixes end up making things worse. Common mistakes include:
- Connecting stormwater to sewer pipes (which is illegal and can cause blockages)
- Installing ag pipes without a proper fall
- Using solid PVC pipes instead of perforated ones for subsoil drainage
- Covering inspection openings or stormwater pits with paving or turf
If you’re unsure, get a licensed plumber or drainage contractor to inspect your system. They can use CCTV cameras to locate blockages or collapsed pipes without having to dig up your whole yard.
Keeping your home dry and damage-free
A well-planned drainage system works quietly in the background — you’ll hardly notice it when it’s doing its job. But when things go wrong, the impact can be major.
Before your next renovation or landscaping project, take the time to understand how water moves across your site and make sure every element, from roof gutters to garden drains, has a clear path to discharge.
Good drainage isn’t just about preventing puddles; it’s about protecting your home for the long haul.
