Stormwater Pipe Sizing Calculator

Most single-storey homes with roof areas under 200m² use DN100 stormwater pipe. Once the catchment area goes over 200m² or the local rainfall intensity is high (Brisbane, Darwin, Cairns), you'll likely need DN150. Run the numbers through the calculator above using your actual roof area and city rainfall data.

Use the Rational Method formula: Q = (C × I × A) ÷ 3600. C is the runoff coefficient (0.95 for metal roofing), I is the rainfall intensity in mm/hr for your location, and A is the effective roof catchment area in square metres. The result gives you peak flow in litres per second.

AS/NZS 3500.3 doesn't specify a single minimum grade for all situations, but most councils and plumbing authorities require at least 1:100 (1%) for underground stormwater lines. Steeper grades move water faster and reduce silt build-up. Avoid anything flatter than 1:200 for any stormwater pipe.

Divide the total peak stormwater flow (L/s) by the capacity of your downpipe size. A standard DN90 round downpipe handles about 3.5 L/s and a DN100 handles about 5.0 L/s. A 200m² metal roof in Sydney generates roughly 10.2 L/s, so you'd need at least 3 × DN90 downpipes or 2 × DN100.

ARI stands for Average Recurrence Interval. A 20-year ARI rainfall event means a storm intensity that is statistically expected to happen once every 20 years on average. Australian residential stormwater systems are typically designed to handle 20-year ARI events. Overflow systems and flood-prone areas often need 100-year ARI.

The 5-minute duration, 20-year ARI rainfall intensity for Sydney is approximately 194 mm/hr. For a 100-year ARI event, it jumps to around 264 mm/hr. These values come from the Bureau of Meteorology's Intensity-Frequency-Duration (IFD) data.

Stormwater carries clean rainwater from roofs, driveways, and land surfaces to council drains, creeks, or absorption pits. Sewer drainage carries wastewater from toilets, sinks, showers, and laundry to the sewer main or septic system. They are completely separate pipe systems and must never be connected to each other.

No. Connecting stormwater into the sewer is illegal in every Australian state and territory. Doing so overloads the sewage treatment system during heavy rain and causes sewer overflows into streets, waterways, and private property. Water authorities actively investigate and fine properties found with illegal cross-connections.

The runoff coefficient (C) represents the fraction of rainfall that becomes runoff. A metal roof sheds almost all rainwater, so C = 0.95. A tiled roof absorbs slightly more, so C = 0.90. A concrete driveway is about 0.85, and a grass lawn absorbs heavily at C = 0.35.

Many Australian municipal councils (particularly in WA, SA, and parts of NSW and VIC) prohibit direct stormwater discharge into street kerbs without on-site retention or detention (OSD). In sandy soils, soakwells or infiltration trenches recharge groundwater on-site. In clay soils, detention tanks store peak runoff and release it at a controlled council-specified rate (e.g. 5–10 L/s) via orifice plates.

It depends on your city's rainfall intensity. In Melbourne (144 mm/hr, 20yr ARI), a single DN90 downpipe draining a metal roof can handle about 93m² of roof. In Brisbane (242 mm/hr), that same downpipe only covers about 55m². Higher rainfall intensity means each downpipe drains less area.

Yes. Stormwater drainage work must be done by a licensed plumber or drainer in every Australian state and territory. In most states, a compliance certificate must be lodged after the work is complete. Council inspections are often required before backfilling underground stormwater trenches.

Underground stormwater pipes are almost always uPVC (unplasticised polyvinyl chloride). SN4 stiffness class is standard for residential trenches under gardens and lawns. SN8 is required under driveways, parking areas, and any surface subject to vehicle loading. HDPE and corrugated poly are sometimes used for agricultural stormwater.

The two most common causes are flat pipe grade and leaf debris. Pipes laid flatter than 1:100 don't move water fast enough to flush out sediment and organic matter. Over time, silt and leaves build up and form a dam that blocks the pipe. Installing leaf guards on gutters and maintaining a minimum 1:100 grade prevents most blockages.

A 20-year ARI system handles the rainfall intensity expected once every 20 years on average — this is the standard residential design level. A 100-year ARI system handles a much heavier event and is typically required for overflow routes, flood-prone properties, and commercial developments. The 100-year intensity is roughly 35-40% higher than the 20-year value.