OSD Tank Design Sydney — Council-Approved On-Site Detention Plans

OSD (on-site detention) tank design in Sydney requires hydraulic calculations to determine the correct tank volume and outlet size. RI ENG PTY LTD designs OSD systems for residential properties using council-specific flow rate targets. Turnaround: 2–5 business days.

An undersized OSD tank gets rejected. An oversized one wastes thousands of dollars of concrete and excavation. RI ENG PTY LTD sizes on-site detention tanks for Sydney properties using real hydraulic calculations, so you build what the council requires and nothing more.

What Is OSD and Why Sydney Properties Need It

OSD stands for on-site detention. It is a system that temporarily holds stormwater on your property and releases it slowly into the street drainage network.

Sydney’s drainage network cannot absorb the fast peak flows created by large impervious surfaces — roofs, driveways, concrete yards.

Councils manage this by requiring OSD on properties that exceed a certain impervious area threshold. The tank limits the peak flow rate leaving your site.

Without an OSD system, your development approval will not be issued in areas where it is required.

RI ENG PTY LTD calculates the required tank size and outlet restriction for your specific site, council and development type.

How We Size an OSD Tank for Your Sydney Property

OSD sizing is a hydraulic calculation — not a guess. We use the rational method and the council’s specified permissible site discharge (PSD) rate.

The PSD rate varies by council. Some councils use 10 litres per second per hectare for a 1-in-10-year storm. Others use different values.

We know the PSD rates for all Greater Sydney councils. We apply the correct rate to your site area and calculate the required storage volume.

The design specifies the tank dimensions, inlet, outlet orifice size and overflow arrangement.

We also check whether a rainwater tank can double as an OSD tank — in some cases this saves cost and space.

What the OSD Design Package Includes

Our OSD design package covers everything your certifier and council need.

We provide a hydraulic calculation report, OSD tank specification, outlet pit detail and a drainage plan showing the full system.

The package includes pre-development and post-development flow comparisons, which councils require to confirm the OSD achieves the required flow reduction.

We coordinate with your builder or plumber on tank placement and outlet installation details.

Most OSD designs for single dwellings and duplexes are completed within 2–5 business days.

Common Questions About OSD Tank Design in Sydney

Q: Does every Sydney property need an OSD tank?

A: Not every property. OSD is typically required when impervious area exceeds a council threshold, often around 30–40% of the lot. We check your council’s specific rule.

Q: Can a rainwater tank replace an OSD tank?

A: In some councils, yes. A rainwater tank with a restricted outlet can serve as an OSD tank if sized correctly. RI ENG PTY LTD checks this for your site.

Q: What is PSD and how does it affect OSD sizing?

A: PSD is permissible site discharge — the maximum flow rate your property can release to the street drainage. The lower the PSD, the larger the OSD tank needs to be.

Q: Do you provide OSD designs for duplexes?

A: Yes. We design OSD systems for duplexes, townhouses, houses and granny flats across Greater Sydney.

Q: How long does OSD design take?

A: Most OSD designs are ready within 2–5 business days of receiving your site plan.

Ready to Get Your Sydney OSD Tank Designed?

RI ENG PTY LTD designs on-site detention systems for houses, duplexes, townhouses and granny flats across Greater Sydney. We size each tank to your council’s PSD and storage requirements, not to a rule of thumb.

Send us your survey, architectural plans and council name. We will confirm whether OSD applies, what size you are likely looking at, and how quickly we can deliver.

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📞  +61 0451 452 932

📧  riengineering2155@gmail.com