Two very different complaints get described with the same sentence: “I’ve got a drainage problem.” One is water arriving somewhere it shouldn’t. The other is water that has arrived normally and has nowhere to go. The fixes are not interchangeable, and installing the wrong one is the most common reason we get called to a yard that has already had drainage work done.
The short version
- Stormwater pipe
- Sealed. Keeps water out of the ground. Carries roof and surface water from downpipes and grates to a legal point of discharge. If water is visibly running or pooling somewhere it shouldn’t, this is your problem.
- Ag drain
- Slotted. Lets water out of the ground. Perforated pipe in an aggregate trench wrapped in geotextile, laid below the surface. If the lawn is spongy for days with no visible source, this is your problem.
A stormwater pipe is a sealed tube whose entire job is to not leak. An ag drain is deliberately full of holes. Putting a sealed pipe in the ground to fix a waterlogged lawn achieves nothing at all, which is a discovery people usually make about a month and one heavy rain after paying for it.
Why Melbourne’s north waterlogs
Heavy clay subsoil. Most of the northern and western suburbs sit on it — Meadow Heights, Roxburgh Park, Craigieburn, Broadmeadows and out towards Sunbury are all clay country.
The mechanism is simple. Rain soaks through 200 or 300 mm of topsoil, hits clay it cannot pass through, and stops. It cannot go down, so it sits in the root zone and moves sideways at the speed of treacle. The surface looks dry. Step on it and the whole lawn moves.
What follows is predictable: grass thins out because roots are drowning, moss takes over the shaded parts, the lawn stays unusable for days after rain, and anything planted in the low corner dies twice. Nothing is broken. There is simply no exit, and an ag line is an exit.
How to tell which one you have
Go and look during the next decent rain. It is the single most useful diagnostic there is.
- Water visibly running or gushing from a downpipe, a blocked grate, or over a paved surface → stormwater.
- A puddle that appears fast and drains within hours → surface water, and often a levelling or falls problem rather than a drainage one.
- No visible water, but the ground is saturated and stays that way for days → groundwater on clay. Ag drain.
- A wet strip along a fence line or the low side of the block → groundwater moving sideways, frequently from a neighbour’s property. Ag drain, run to intercept it.
- Water against the house or under the slab → get it looked at properly and soon. That is not a lawn problem.
What a properly built ag line looks like
An ag drain is not a difficult thing to build and it is a very easy thing to build badly. Four details separate a line that works for decades from one that fails in three years.
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Consistent fall
The trench has to run downhill the whole way to its outlet. Fall matters far more than depth, and a deep trench with a low spot in the middle is just a buried puddle. This is the single most common defect.
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Clean angular aggregate
Scoria or a clean crushed rock. Angular, so the pieces lock together and leave open voids for water to travel through. Not sand, and emphatically not crusher dust, which packs down into something close to concrete.
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Geotextile wrap
The fabric that lines the trench and separates aggregate from soil. Skipping it is the classic false economy: fine clay particles wash into the aggregate, fill the voids, and the line silts up and quietly stops draining. You will not notice for a couple of years, and then you are digging it up.
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An outlet that exists
Sort this out before the trench is dug, not after. The line has to discharge somewhere legal — a stormwater connection made by a licensed plumber, a soakwell or rubble pit, or a surface point that does not push water onto a neighbour.
Where the licence line sits
A subsoil drain that stays inside your property is landscaping and earthworks. The moment it connects into below-ground stormwater, it becomes licensed drainage work — in Victoria that work, from the above-ground pipes through to the point of discharge, sits under Part 4 of the Plumbing Regulations 2018 and the Victorian Building Authority requires a licensed plumber to do it.
A new discharge point also needs a legal point of discharge application to your council. Both are worth sorting before anyone starts digging, not after.
Things that are not a drainage problem
Worth ruling out before spending money on a trench:
- Compaction. A lawn that has had vehicles, a building crew or years of foot traffic across it can shed water simply because there is no pore space left. Core aeration is a great deal cheaper than an ag line.
- Grading. If the ground falls towards the house or into a corner, water goes there because you told it to. Sometimes the fix is a bobcat and an afternoon, not a pipe.
- A blocked existing line. Plenty of yards already have drainage that has silted up or been crushed. Finding and clearing it beats installing a second system alongside it.
- One bad week. After a genuinely extreme rain event, everything holds water for a while. Judge a yard on how it behaves in an ordinary wet month.
Getting it looked at
Photos taken during rain are worth more than anything measured afterwards, so if the yard is wet right now, take them now. What we do on the excavation side — trenching, bedding, backfill and compaction, and working alongside your plumber where a licence is required — is set out on the drainage and stormwater page.