STORMWATER DRAINAGE

On-site stormwater detention (OSD) for plumbers

On-site stormwater detention, or OSD, is a tank or system that holds back stormwater during heavy rain and lets it out slowly, so the council network is not overloaded. As the plumber you install it to the engineering design and connect it to the legal point of discharge.

What OSD actually does

When you cover a site with roof, paving and concrete, rain that used to soak in now runs straight off, faster and in greater volume. On-site stormwater detention holds that surge back in a tank or storage system and releases it slowly through a small controlled outlet, so the flow leaving the property stays at or below the pre-development peak. The council network downstream is sized for the old flow, and OSD is what keeps the new development from overwhelming it.

When councils require it

Detention is typically a condition of approval where a development adds impervious area, a new dwelling, a unit development, a large extension or extra hardstand. The council or the engineer sets whether it is needed and to what standard. The detention storage volume and the permitted release rate are design figures, worked out by the hydraulic or stormwater engineer and approved by council. They are not yours to assume, so always work from the stamped drawings.

The plumber's role

• Install the detention tank or storage system in the right location, at the right level, to the engineering design.
• Fit the outlet control, the orifice plate or restrictor, exactly as specified. This is the part that limits the release rate, so substituting a different size defeats the whole system.
• Provide the high-level overflow path so that in a storm beyond the design event, water spills safely rather than backing up into the building.
• Connect the controlled outlet to the legal point of discharge nominated by council.
• Keep the inlet, outlet and overflow accessible for cleaning, because a blocked orifice stops the system working.

Common failures and what to document

• Wrong orifice or outlet size, so the release rate does not match the design. Photograph the orifice plate or restrictor in place and its marked size.
• No overflow path, or an overflow that discharges somewhere unsafe.
• The controlled outlet teed straight into the connection without the restrictor, which turns the detention tank into nothing more than a pit.

Photograph the tank, the inlet, the outlet control with its size, the overflow, and the connection to the legal point of discharge, against the approved drawing. That set is what proves the system was built to the design. Elemetric keeps that with the job.

Common questions

Who sets the OSD tank size and outlet?

The hydraulic or stormwater engineer does, and the council approves it. The required storage volume, the release rate, and the orifice or outlet size all come from that design. Your job is to install it exactly as drawn, not to pick the figures yourself.

When does a council require OSD?

Usually when a development increases the hard surfaces on a site, like a new build or a large extension, so more water runs off faster than before. The council asks for detention to keep the peak flow leaving the site at or below what it was. The planning or building approval will state it.

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.