What is an OSD tank?

OSD stands for on-site stormwater detention. It's a tank or pit, usually underground or under a driveway, that temporarily holds stormwater from your roof and paving during heavy rain and releases it slowly into the council drain. Councils often require them on new homes and larger developments, so the street's stormwater system isn't overwhelmed in storms. If you've bought a newer home, you may have one without knowing. Here's how it works and how to look after it.

How it works

• Stormwater from the roof and hard surfaces flows into the OSD tank.
• A small outlet, often a plate with a set-size hole, lets water out at a controlled rate.
• In heavy rain the tank fills up and holds the excess, then empties after the storm.
• An overflow takes water away safely if the tank is completely full.

Maintenance

• Check and clean the grates, pits and inlet screens every few months and after big storms.
• Keep the outlet orifice and its screen clear. A blocked outlet means the tank won't empty.
• Have the tank inspected and silt removed periodically.
• Don't remove, alter or cover the system. Changes can breach council requirements.

Common problems

A tank that doesn't empty, water ponding over the grates, or overflow in ordinary rain usually means a blocked outlet screen or a blocked pipe downstream. We clear and inspect OSD systems and the pipes connected to them, with a fixed price before any work starts. See stormwater.

In southern Eastern Suburbs homes

Newer homes and dual occupancies in Pagewood, Eastgardens, Matraville and Maroubra often have OSD tanks under driveways or in the front yard. Sand and leaves fill the screens quickly near the coast and under big trees.

Related help: stormwater, stormwater pits, backyard flooding and stormwater rules guide. Local pages: Pagewood, Eastgardens, Matraville and Maroubra.