Charged Downpipe Systems to Rainwater Tanks: Rules That Pass

Why Charged Systems Are Everywhere Now

BASIX rainwater tank commitments mean most new homes in NSW connect a large share of their roof to a tank — but the tank is rarely next to every downpipe. The solution is a charged downpipe system (also called a wet or pressure system): pipework drops from the gutter, runs underground below the tank inlet level, and rises back up into the tank. Between storms the buried section stays full of water — hence “charged”. Done properly, these systems are reliable and certifiable. Done casually, they leak, breed mosquitoes and overflow at the gutter. Here are the design rules that get them through certification.

A note on terminology: builders often call these “siphonic” systems. True engineered siphonic roof drainage is a specialised full-bore design used on large commercial roofs; the residential tank-feed system is more accurately a charged gravity system driven by head difference. Certifiers care about the design logic either way.

Rule 1: The Hydraulics Must Actually Work

A charged system is driven by the height difference between the gutter outlet and the tank inlet. The non-negotiables:

• The tank inlet must be lower than the lowest connected gutter outlet — with enough margin to overcome friction in the pipe run at the design flow. Long runs, small pipes and multiple bends eat head quickly.
• Size the pipe for the connected catchment using the design rainfall intensities in AS/NZS 3500.3:2021, just as you would a conventional downpipe and drain. Undersized charged lines back water up the downpipes and out of the gutters in heavy rain.
• Every connected downpipe shares one water level. In a multi-downpipe charged network, water will rise in all legs to the same level — so all connected gutter outlets need to sit above the tank inlet, not just the nearest one.

Rule 2: Pipework Built for Standing Water

The buried section holds water permanently, which changes the material requirements:

• Pressure-capable, solvent-welded joints below the charged water line. Rubber-ring stormwater fittings that are fine in a dry gravity drain will weep under standing head — slowly saturating the ground beside footings.
• Continuous, robust pipe runs with as few buried joints as practical, and adequate cover where pipes pass under paving or driveways.
• Watch the footings. Charged lines often run close to slab edges; a leaking pressurised joint next to a footing in reactive clay is a structural problem, not just a plumbing one. Keep clearances and document the route.

Rule 3: Overflow — Plan for the Blocked Day

Certification lives and dies on the failure case. The system must shed water safely when the charged line blocks or the tank is full:

• Gutter-level overflow. Each connected gutter needs overflow provision so a blocked charged line pushes water over the gutter front, outside the building — never into the eaves.
• Tank overflow at full bore. The tank overflow must match the inlet capacity and discharge to the lawful stormwater system, not onto the ground around the tank base.
• Mark the system on the plans — the drainage drawings should show the charged sections, levels, pipe class and overflow paths so the certifier can verify the design rather than guess at it.

Rule 4: Maintenance and Health Details

• Flush/drain point at the low point of the charged line, so the stagnant water can be released and the line cleared.
• Mosquito-proofing — screened tank inlets and overflows, consistent with health requirements for rainwater tanks.
• Leaf management — gutter guards or leaf diverters upstream keep the charged line from silting; first-flush devices help water quality where the tank supplies internal reuse.

What Certifiers Commonly Reject

• Charged lines drawn with no levels — no demonstration the gutter outlet sits above the tank inlet.
• Standard stormwater fittings specified below the water line.
• No overflow provision at gutters or an undersized tank overflow.
• No drain-down or inspection point on the buried run.
• Tank overflow discharging to ground instead of the lawful discharge point.

Design It Once, Certify It Once

Contrive Consultants designs charged downpipe and rainwater tank systems as part of council-ready stormwater and roof drainage plans across Sydney and NSW — levels, sizing, overflow and documentation certifiers can sign off without questions. See our services or contact us on +61 497 848 111 or info@contriveconsultants.com.au.