Roof runoff flows from the roof into a gutter, through a gutter outlet into a downpipe, and then to a ground-level outlet, a subsurface stormwater drain, or a rainwater storage tank. The exact connection depends on the property’s drainage design and the rules adopted by the local authority; there is no single fitting or pipe size that applies everywhere.
How the connection carries water
- The roof sheds rain into the gutter. The gutter collects runoff along the roof edge.
- The gutter outlet feeds the downpipe. An opening at the gutter directs water into the downpipe below. The Australian Building Codes Board (ABCB) defines a downpipe as “a pipe carrying rainwater from a gutter to a sub-surface drainage system or ground level.” ABCB, NCC Navigator
- The downpipe conveys water onward. Depending on the site layout, its lower end may discharge at ground level, connect to a buried stormwater drain, or feed a rainwater storage tank. Victoria’s plumbing practice note describes downpipes as carrying roof water from gutters and roof catchment to drains or storage tanks, including vertical and horizontal systems. Victorian Building Authority, Plumbing RP 04
In the Australian NCC 2019 provision, the acceptable construction practice for gutters and downpipes applies where the roof drainage system connects to a stormwater drainage system complying with the referenced provisions. That describes the relationship between the systems; it does not prescribe one universal connector for every property. ABCB, NCC 2019 Part 3.5.3
What determines the downpipe size and position?
Downpipes and gutters need to be selected as a system. Relevant factors include the roof catchment area, local design rainfall, gutter type, drainage layout, and the rules in force where the building is located. ABCB guidance notes that eaves-gutter sizing depends on building location and rainfall intensity, and recommends confirming installation requirements with the relevant authority. ABCB, Roof Drainage Handbook
Australian Housing Provisions dimensions
The NCC 2022 Australian Housing Provisions list these downpipe forms: 75 mm and 90 mm round, and 100 mm × 50 mm and 100 mm × 75 mm rectangular. They are not interchangeable recommendations: capacity depends on the gutter type and roof catchment. The provisions specifically state that 75 mm round downpipes are not suitable with Type E 150 mm D gutters. ABCB, NCC 2022 Housing Provisions
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Australian location guidance
Under the cited Australian Housing Provisions, one downpipe must serve no more than 12 m of gutter and should be as close as possible to valley gutters. This is Australian code guidance, not a universal rule; confirm the version adopted in your state or territory and any local variations before applying it to a project. ABCB, NCC 2022 Housing Provisions
Why the overflow path matters
A clear downpipe connection does not guarantee that water will always reach a suitable outfall. A drain can become blocked, and rainfall can exceed the system’s intended capacity. Australian Housing Provisions call for overflow measures that direct water away from the building so it does not pond against, enter, or damage it, including when the stormwater system is blocked. The provisions describe eaves-gutter systems as designed for rainfall events with a 5% annual exceedance probability when unblocked, and discuss overflow for events up to 1%; these are code-context design figures, not a performance promise for every installed system. ABCB, NCC 2022 Housing Provisions
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The NCC explanatory material also notes that roof drainage provisions do not remove all water to a suitable outfall during exceptionally heavy rain, particularly in tropical areas. The overflow route should therefore be designed to discharge away from the building rather than allow water to back up or collect beside it. ABCB, NCC 2022 Housing Provisions
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Check local rules before changing a connection
Requirements differ by jurisdiction. Australian NCC provisions reference AS/NZS 3500.3 for gutters and downpipes, and the Victorian Building Authority also points to that standard in its downpipe practice note. In New Zealand, E1/AS2 cites AS/NZS 3500.3:2021; the standard listing describes coverage of materials, design, installation, and testing for roof, surface, and subsoil drainage to a point of connection, and says it is to be read with the New Zealand Building Code.
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- Victorian Building Authority: Plumbing RP 04
- New Zealand Building Performance: E1/AS2
- Standards New Zealand: AS/NZS 3500.3:2021
Before altering a buried or building-integrated connection, check the locally adopted code, the plumbing or building authority’s requirements, and the property’s drainage design. The sources cited here do not establish one underground pipe diameter, fitting, fall, permit rule, or installation method for all properties.
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