Gutters, downpipes and roof plumbing: what the code requires, and why

Gutters and downpipes are the part of a house most people only notice when they fail. On a dry day they look like a simple metal channel and a pipe. In a summer storm in Sydney they are carrying a large volume of water off the roof in a few minutes, and if that water can't get away fast enough, it has to go somewhere. Too often, that somewhere is back over the fascia, into the eaves and down inside the wall.

This article covers what the code requires for roof drainage on houses, why those requirements are set the way they are, where roof plumbing commonly goes wrong, and how we build it.

What the code requires

For houses, the performance requirement sits in the National Construction Code (NCC) 2022, Volume Two, Part H2 Damp and weatherproofing. Requirement H2P1 sets two storm levels:

• water from a storm with a 5% annual exceedance probability (roughly a one-in-20-year storm) must be disposed of without damage or nuisance to any other property; and
• water from a storm with a 1% annual exceedance probability (roughly a one-in-100-year storm) must not enter the building.

There are two Deemed-to-Satisfy ways to meet that for gutters and downpipes. The first is to design and build them to AS/NZS 3500.3 Plumbing and drainage, Part 3: Stormwater drainage. The 2021 edition is the one referenced in NCC 2022. The second, for houses within its limits, is to follow the ABCB Housing Provisions, Part 7.4 Gutters and downpipes. Part 7.4 sets out, among other things:

• Design rainfall intensities by locality. Eaves gutters are sized for the 5% storm, and the overflow measures for the 1% storm, both based on five-minute rainfall intensities.
• Gutter sizing from tables, based on the roof catchment area draining to each downpipe and the local rainfall intensity.
• Fall and support. Eaves gutters must have a fall of not less than 1:500, with brackets at stop ends, corners and no more than 1.2 m apart.
• Overflow measures. Eaves gutters need either a continuous overflow (such as a front-face slotted gutter, a controlled gap behind the gutter, or a front bead set below the top of the fascia) or dedicated overflows (such as end-stop weirs, front-face weirs or rainheads) sized to the roof.
• Downpipes. A downpipe must not serve more than 12 m of gutter, and must be placed as close as possible to valley gutters.
• Valley gutters, sized by rainfall intensity with a minimum freeboard.
• Box gutters are sized, graded and given overflows using AS/NZS 3500.3.
• Materials. Gutters, downpipes and flashings must meet the relevant product standards and be compatible with the roofing upstream of them.

In New South Wales, roof plumbing is licensed trade work. It covers fixing, installing, altering and repairing gutters, downpipes, roof flashings and roof coverings, and a contractor licence is needed for residential roof plumbing valued over $5,000.

Why the code is this specific

It is sized to the storm, not the average day. A gutter that copes with steady rain can be overwhelmed in minutes by an intense storm. That is why the sizing is based on short, five-minute rainfall intensities for each locality rather than annual rainfall. The short, heavy burst is what overfills a gutter.

It plans for the gutter being overwhelmed anyway. Even a correctly sized gutter will eventually see a storm bigger than its design, or a downpipe blocked by leaves. The code's answer is the overflow measure: a deliberate path that sends excess water outward, away from the building, instead of letting it rise over the back of the gutter and into the eaves or wall. This is the single most important idea in Part 7.4. A gutter fixed tight to the fascia with its back higher than its front has nowhere safe to overflow, so the water finds the unsafe path.

It recognises that metals react with each other. The Housing Provisions require metals in a roof system, including flashings, fasteners, gutters and downpipes, to be compatible. Water running off one metal onto another can cause corrosion. For example, the provisions do not allow copper upstream of galvanised materials, or lead upstream of zinc or aluminium-coated steel. A gutter can rust through years early because of what is upstream of it, not because of the gutter itself. See Part 7.2 of the Housing Provisions.

Box gutters fail inside the building. An eaves gutter that overflows does so outside. A box gutter sits within the roof, often above a ceiling, so when it overflows or leaks, the water goes straight into the building. That is why box gutters must be designed to AS/NZS 3500.3, with proper sizing, falls and overflows, rather than worked out on site.

Where roof plumbing commonly goes wrong

• No overflow path. Gutters installed hard against the fascia, with the back higher than the front, so excess water goes into the eaves and wall cavity.
• Too few or undersized downpipes. Long runs of gutter feeding one downpipe, often to save a pipe on an elevation. When the downpipe is overloaded, the gutter overflows.
• Flat or back-falling gutters. Gutters without enough fall hold water, collect debris and corrode faster.
• Dissimilar metals. Copper, lead or the wrong fasteners in contact with or upstream of coated steel, causing early corrosion.
• Valleys and box gutters worked out on the day. Valley gutters without enough capacity or freeboard, and box gutters without designed overflows.
• Leaves and debris. Even a correctly designed system fails if gutters, rainheads and downpipes are never cleaned. This is a maintenance item, and our post on maintenance schedules covers how often to check.
• Downpipes that go nowhere. Downpipes discharging onto the ground beside the house rather than into a stormwater system, so the roof water ends up against the footings. Our post on site stormwater drainage explains where that water should go.

Many of these show up first as what looks like a wall or ceiling leak. Our post on where buildings leak covers how to trace a stain back to its source.

Questions worth asking your builder

1. Is the roof drainage designed to AS/NZS 3500.3 or to Part 7.4 of the Housing Provisions, and what design rainfall intensity was used for this site?
2. What overflow measure is used on the eaves gutters, and where does the overflow water go?
3. How many downpipes are there, and how much gutter does each one serve?
4. Are there box gutters or internal gutters, and who designed their size, falls and overflows?
5. Are all the metals in the roof, including flashings and fasteners, compatible with each other?
6. Who is doing the roof plumbing, and do they hold the right licence?
7. Where do the downpipes discharge, and is that connected to the stormwater system?

How we build it

• We design roof drainage before the roof goes on. Rainfall intensity for the site, catchment areas, downpipe numbers and positions, and overflow measures are worked out at design stage, not decided on the day by whoever installs the gutter.
• Every eaves gutter has a planned overflow, using one of the measures the code accepts, so excess water goes outward and away from the building.
• Box gutters are designed to AS/NZS 3500.3, with sizing, falls, sumps and overflows documented.
• We check material compatibility across roofing, gutters, flashings and fasteners, including what is upstream of what.
• Roof plumbing is done by licensed roof plumbers, and connections to the stormwater system are made by licensed trades.
• Roof drainage is part of our hold points. Gutter falls, overflows, downpipe connections and box gutter details are checked and photographed before they are closed in. Our post on hold points and the handover record explains how that works.
• We hand over maintenance notes, so you know which gutters, box gutters and rainheads need regular cleaning.

If you would like to talk through a new build, a re-roof or recurring overflow problems on an existing house, get in touch.

Sources

• NCC 2022, Volume Two, Part H2 Damp and weatherproofing, Australian Building Codes Board
• NCC 2022 Housing Provisions, Part 7.4 Gutters and downpipes, Australian Building Codes Board
• NCC 2022 Housing Provisions, Part 7.2 Sheet roofing, Australian Building Codes Board
• Roof plumbing work, NSW Government

This article is general information, not advice for your project. Roof drainage design depends on the site's rainfall data, the roof form and the products used, so confirm requirements with your designer, roof plumber and certifier. Full website disclaimer