STORMWATER DRAINAGE

Subsoil and ag-drain rules under AS/NZS 3500.3

A subsoil or ag drain takes groundwater away from footings, retaining walls, and paving before it can sit and cause damage. It is a stormwater job, not a sewer job, and the rules keep those two worlds apart.

What a subsoil drain is for

A subsoil drain, often called an ag drain, sits below ground and collects the water that builds up in the soil around footings, behind retaining walls, and under paving. Left there, that water softens the ground, pushes on walls, and works its way into the building. The ag line gives it a path out before it can do that.

Keep it out of the sanitary system

Where it may and may not discharge

The collected water has to go somewhere sensible: an approved stormwater connection or a legal point of discharge. What it must not do is discharge back against the structure it was meant to protect, onto a neighbouring property, or across paving and paths where it becomes a hazard. A drain that just relocates the problem is a common pull-up.

Inspection and flushing, and what to photograph

An ag line silts up over time, so it needs points where it can be inspected and flushed clear rather than being sealed away with no access. Photograph the drain run and its surround before backfill, the inspection or flushing points, and where it discharges, so it is clear the line is separate from the sanitary system and goes to an approved point. Elemetric keeps that with the job.

Common questions

Can a subsoil drain connect into the sewer?

No. A subsoil or ag drain carries groundwater and must stay part of the stormwater system. Connecting it to the sanitary drainage is not allowed, because it floods the sewer with ground and rain water it was never meant to take.

Where can a subsoil drain discharge to?

To an approved stormwater point such as the property's stormwater drainage or a legal point of discharge. It must not run to the sanitary system, and it must not just dump where it will undermine a footing, a neighbour, or a path.

General information for licensed tradespeople, not legal or regulatory advice. The licensed plumber remains solely responsible for compliance. Refer to the current AS/NZS 3500 standards and the Building and Plumbing Commission (formerly the VBA) for authoritative requirements.