What standard governs stormwater drainage for commercial buildings in Australia?

AS/NZS 3500.3 (Plumbing and drainage - Stormwater drainage) is the primary standard. The NCC 2025 calls it up in clause F1D3 as a Deemed-to-Satisfy solution for all Class 2-9 buildings.

What rainfall intensity are box gutters designed to in Australia?

Internal and box gutters must be sized to the 100-year ARI (1% AEP) rainfall intensity per AS/NZS 3500.3 Table 3.1. External eaves gutters use the less stringent 20-year ARI (5% AEP).

How many downpipes does a 1,000 m2 commercial roof need?

Using AS/NZS 3500.3 Figure 3.5(A), a 90 mm round downpipe serves a maximum of 47 m2 of catchment. For a 1,000 m2 roof, that means at least 22 downpipes at 90 mm, or fewer if larger pipe sizes are used.

Is on-site stormwater detention required for commercial buildings?

Yes. Most Australian councils require on-site detention (OSD) for all commercial, industrial and special-use buildings. The OSD must restrict discharge to the council's permissible site discharge (PSD), typically 20 to 80 L/s per hectare, for storms up to the 100-year ARI.

Do box gutters need overflow devices under AS/NZS 3500.3?

Yes. Every roof drainage system must include an overflow device for storms beyond the design event. For box gutters, the overflow must activate before the gutter overtops, and it must be sized to match or exceed the downpipe design flow.

Commercial Stormwater Drainage: AS/NZS 3500.3 Guide

Section 1

What You Need to Know

Rain hits your roof and needs somewhere to go. AS/NZS 3500.3 (the Australian Standard for stormwater drainage) sets the rules for gutters, downpipes, and overflows. The NCC requires that stormwater from a 1-in-100-year storm must not enter the building. Get the drainage wrong, and you risk water damage, failed inspections, and council hold-ups.

Section 2

The Rules

Stormwater drainage must be designed to AS/NZS 3500.3 (NCC 2025, F1D3)
The drainage system must handle a 5% AEP storm (1-in-20-year event) without damaging neighbouring properties (NCC 2025, F1P1)
Surface water from a 1% AEP storm (1-in-100-year event) must not enter the building (NCC 2025, F1P2)
External eaves gutters are sized to a 20-year ARI rainfall intensity; internal box gutters are sized to a 100-year ARI (AS/NZS 3500.3, Table 3.1)
Box gutters need a minimum fall of 1:200 to outlets and 30 mm of freeboard above the design water level (AS/NZS 3500.3)
Every roof drainage system must include an overflow device for storms beyond the design event (AS/NZS 3500.3)
Most councils require on-site detention (OSD) for all commercial buildings, sized up to the 100-year ARI storm

Section 3

What This Means in Practice

Take a 1,000 m² flat commercial roof in Sydney, where the 5-minute rainfall intensity is around 180 mm/h at the 100-year ARI. The box gutters and downpipes must handle this full intensity. Using the standard's sizing tables, a 90 mm round downpipe serves roughly 47 m² of catchment (AS/NZS 3500.3, Figure 3.5(A)). That means you need at least 22 downpipes for 1,000 m², or fewer if you use larger pipe sizes. For sloped roofs, the effective catchment area increases with roof pitch, so a 22-degree roof gets a 1.2 multiplier applied to the plan area.

Box gutters on commercial buildings are the highest-risk element. They sit inside the roof envelope, so a failure sends water straight into the building. That is why the standard requires 100-year ARI sizing (not 20-year like eaves gutters) plus a dedicated overflow device. The overflow must activate before the gutter overtops. Size the overflow to match or exceed the downpipe design flow.

Council will also require an OSD system. This is a tank or pit that stores runoff and releases it slowly through an orifice plate. Each council sets a permissible site discharge (PSD), typically 20 to 80 L/s per hectare. The OSD must drain within 24 hours. Budget for this early because the tank takes up space, usually below the ground floor slab or in a basement.

Section 4

Key Design Decisions

1

Eaves Gutters vs. Box Gutters

Use eaves gutters where the roof form allows. They overflow safely to the outside of the building. Box gutters are common on commercial buildings with parapet walls or multi-level roofs, but they carry more risk because failure means internal water damage.

Trade-off: Box gutters give architects clean roof lines and allow hidden drainage, but they demand 100-year ARI sizing, overflow devices, and ongoing maintenance access.

2

Conventional vs. Siphonic Drainage

Siphonic systems use fewer, smaller pipes and do not need gravity fall in horizontal runs. They suit large commercial roofs over 150 m² where traditional downpipe counts get impractical. A specialist engineer must design them (AS/NZS 3500.3, Section 10).

Trade-off: Siphonic systems cost more to design and install, but they can significantly reduce the number of downpipes needed, free up ceiling void space, and simplify coordination with structure.

3

Overflow Path: Planned or Unplanned?

Design a clear overflow path for every box gutter and internal drain. Use overflow spitters, scuppers, or secondary drainage. If you do not plan the path, water will find its own way, usually through the ceiling.

Trade-off: Adding overflow devices and secondary drainage costs $500-2,000 per gutter run, but prevents catastrophic water damage in a major storm.

4

On-Site Detention: Above or Below Ground?

Below-ground OSD tanks are the most common for commercial buildings. Above-ground tanks or detention roofs are alternatives where excavation is difficult. Size depends on your council's PSD and the site's impervious area.

Trade-off: Below-ground tanks are hidden but add excavation cost and waterproofing. Above-ground or roof-based detention saves excavation but uses lettable space or adds structural load.

Section 5

Who Needs to Know What

Section 6

References

AS/NZS 3500.3:2021, Plumbing and drainage — Part 3: Stormwater drainage
AS/NZS 3500.3:2025, Plumbing and drainage — Part 3: Stormwater drainage (revised edition, adopted in VIC from October 2025)
National Construction Code 2022, Volume One, Part F1 — Surface water management, rising damp and external waterproofing
National Construction Code 2022, Volume Three, Part E3 — Stormwater roof drainage systems
Sydney Water, On-Site Stormwater Detention Guidelines
Bureau of Meteorology, Rainfall Intensity-Frequency-Duration Data (IFD)

Stormwater Drainage for Commercial Buildings