Installing a French drain to redirect surface water from foundations
When summer storms sweep in off the Coral Sea or the Bass Strait, the water that sheets across a Brisbane block or a narrow Melbourne terrace has to go somewhere. Far too often it ends up soaking the soil directly against the slab, where reactive clay swells and shrinks with each wet-dry cycle. Over years, that pressure opens hairline cracks, lifts footings, and turns a tidy living room into a musty storeroom.
A French drain is a simple answer. It is a gravel-filled trench holding a perforated pipe that intercepts runoff before it reaches the wall, then carries it sideways to a soakwell, kerb connection, or rain garden. The build is within reach of a careful owner-builder, though Australian conditions add a few wrinkles worth knowing before the first shovel goes in.
This guide covers reading the site, choosing materials that suit local soils, digging to the right fall, and finishing the line so it keeps flowing for years. Pairing exterior drainage with a mudroom with built-in lockers keeps the rest of the house drier through the worst of a southerly buster, and the same care applies inside as out.
Reading the block before you dig
Spend a full day watching what the rain does. Walk the yard after a storm and note where puddles sit, where water streaks across paving, and where the soil stays boggy for hours. In a Perth sandy lot, water disappears fast but reappears at the foot of any retaining wall. In a Sydney suburb with reactive clay, the ground stays soft for days and slowly creeps toward the slab.
Lodge a free Dial Before You Dig enquiry on 1100 and wait for the plans. Gas, water, NBN and stormwater assets criss-cross most suburban blocks, and a hit on any of them turns a weekend into an insurance claim. Some Brisbane and Melbourne councils require a permit for new stormwater connections, so check before scheduling the work. If the drain will run past a detached garage that doubles as a workshop, planning the route now helps the next project, and the article on renovating a garage covers how drier footings support a slab-mounted compressor and longer paint cures.
Marking the line and excavating
The trench should fall at roughly 1 in 100 (10 mm per running metre), though 1 in 50 works on short runs where ground is flat. Use a string line and a line level, or hire a small rotary laser, because a flat or back-sloping drain simply moves the problem two metres further down the slope. Width sits between 200 mm and 300 mm for a 100 mm pipe; going wider wastes gravel without improving flow.
Depth depends on what is being protected. For a slab-on-ground home in Adelaide or Geelong, keep the pipe at least 100 mm below the interior floor and well clear of any footing beam. In Hobart's rocky soils, expect to break out more with a mattock or hire a small rock saw. Shore the sides if digging below 1.2 m, and keep the spoil on a tarp so the lawn does not turn to mud.
Pipe, aggregate and fabric choices
| Drain type | Best suited to | Typical cost per metre (AUD) | Service life |
|---|---|---|---|
| 100 mm slotted PVC | Heavy clay or deep runs | $12–$18 | 50+ years |
| 100 mm corrugated ag pipe | Sandy Perth and coastal lots | $5–$9 | 25–40 years |
| Flexi coil with sock | Curved paths around gardens | $7–$11 | 15–25 years |
| Strip or channel drain | Paved driveways and patios | $40–$80 (grate included) | 30+ years |
For most Australian suburban jobs, either slotted PVC or corrugated ag pipe will do the job, and the choice usually comes down to soil type and budget. Ag pipe suits sandy Perth lots where roots push through anything softer; PVC is the better answer in reactive clay because it stays rigid if a section is backfilled too tightly.
Wrap the pipe in a needle-punched geotextile rather than woven weedmat, which clogs with fine silt within a season. Backfill with 20 mm scoria or blue metal, not pea gravel, because open-graded aggregate lets water in freely and resists compaction. River pebble and decorative stone look tidier at the surface but slow the inflow, so keep them to the top 50 mm.
What to gather before the dig:
- Shovel, trenching spade and mattock
- String line, line level or rotary laser
- Wheelbarrow and builder's plastic for spoil
- Gloves, eye protection and sturdy boots
What to order from the supplier:
- 100 mm slotted pipe or ag pipe (allow 10% extra)
- Geotextile filter fabric (twice the trench length)
- 1.5–2.0 m³ of 20 mm drainage aggregate
- Inspection tee at each change of direction
Backfilling and surface finishing
Lay the fabric in the trench with overlap on the banks, drop in 50 mm of gravel, bed the pipe with holes pointing down, then cover with another 150 mm of aggregate before folding the fabric across the top. This envelope keeps silt out of the pipe and turns the trench into a continuous sieve. Top up the remaining depth with the original soil, lightly compacted, and finish with turf or paving to match the surrounding yard.
Where the outlet daylights into a play area used by children, finish the last metre with a safe and durable playground surface so kids are not slipping on wet grass. Run the discharge through a pop-up emitter onto a grassed strip, into a kerb adaptor where the council allows, or into a rainwater tank charged through a downpipe diverter.
Ongoing care and common pitfalls
Flush the line once a year with a garden hose fed into an inspection pit. Slow flow signals silt, root intrusion, or a crushed section under a driveway crossing. Avoid planting willow, fig or bamboo above the run, as their roots find perforated pipe within a season. Keep mulch off the gravel zone too, since fine particles migrate down and clog the fabric.
In Darwin and Cairns, schedule a check before the wet season so a blockage does not arrive mid-downpour. In the Blue Mountains and Adelaide Hills, clear leaf litter from the outlet grate so embers cannot ignite a blocked line of dry material. Watch any nearby retaining walls, because a working French drain adds load to the wall's own ag drain, which may need upgrading at the same time.
The finished drain sits hidden under turf or mulch, and the yard returns to a usable surface within hours instead of days. The footing stays drier through each storm, the slab stops sweating after rain, and the most expensive repair of all — a foundation that has shifted because the ground around it kept moving — quietly never happens.
