Gutters are sized for the roof area draining into them and the rainfall intensity where you work. Placing the outlets well matters as much as the gutter size: a big gutter with one outlet at the far end can overflow where a smaller gutter with two well-placed outlets copes easily.
Common sizes
Australian houses usually have quad (115 or 125 mm) or half-round eaves gutters, with high-front and slotted designs common. Downpipes are typically 90 mm round or 100 × 50 mm rectangular; the NCC Housing Provisions accept 75 and 90 mm round and 100 × 50 or 100 × 75 mm rectangular. Match the downpipe size to the gutter outlet; an undersized pipe throttles the whole system.
Roof area and rainfall
The effective roof area draining to a gutter includes an allowance for pitch, because wind-driven rain hits a steep roof's face. AS/NZS 3500.3 and the NCC Housing Provisions size gutters and downpipes from the roof catchment area and the local design rainfall intensity in mm per hour, and gutter makers publish simple tables based on them. For ordinary houses, the standard sizes above work when outlets are well placed. Large or steep roofs, and valleys that dump water onto one spot, are where a bigger profile or extra outlets pay off.
Outlets
Put an outlet roughly every 12 m (the NCC limit per downpipe) of gutter, and close to valleys, which deliver a lot of water to one spot. A central outlet with the gutter falling both ways doubles the capacity of a run, since each half only carries half the roof. Corners near outlets reduce flow, so keep outlets a little away from angles where you can.
Fall
At least 1:500 (2 mm per metre) for eaves gutters is the usual minimum, and a little more helps on long runs. Set a string line at the right fall before fixing brackets. On long runs, falling both ways to two outlets keeps the gutter from dropping too far below the roof edge.
The full fitting list
- Gutter lengths and joiners (riveted and sealed on steel gutters)
- Brackets at no more than 1.2 m centres
- Stop ends, internal and external corners, and outlets
- Downpipes, bends and clips
- Connectors into the stormwater drain at the bottom
- Leaf guards or outlet cages, if wanted
- Sealant for joints on systems that need it
Measuring
Measure along every eave that needs a gutter, corner to corner. Count separate runs (each with two ends), corners, and the height from gutter to ground at each downpipe. Note where water can discharge: into the stormwater system, a rainwater tank or another legal point of discharge. That decides the bottom fittings.
A worked example
A house with 30 m of eaves in two 15 m runs, 2.6 m to the ground: each run needs two downpipes to stay within 12 m per downpipe, so four in total. That's 6 lengths of 6 m quad gutter, about 27 brackets at 1.2 m, 4 stop ends, 4 outlets, 4 lengths of 3 m downpipe, bends and clips.
Replacing old gutters
Check the fascia before you quote. Rotten fascia boards have to be replaced before new brackets can go on, so price an allowance or exclude it in writing. Look at the drains too. New gutters won't fix water problems caused by blocked or broken underground drains.
The gutter calculator counts every section, bracket, outlet and fitting for your runs.
Common questions
Can I add a downpipe to an existing gutter?
Yes. Cut in a new outlet where the gutter overflows, and fit a new downpipe with somewhere for the water to go.
Why does my gutter overflow at the corner?
Usually a valley is dumping water there, the outlet is too far away, or the gutter is blocked. An extra outlet near the corner, or a deeper profile, normally fixes it.
Materials compared
- Aluminium: light, rust-free, and available in long continuous lengths from specialist installers. Dents more easily.
- PVC / uPVC: cheap, easy to fit, and common on sheds and some homes. It can become brittle with age and expands in heat, so leave expansion gaps at joints.
- Steel: pre-painted (Colorbond) or Zincalume steel is the standard on Australian houses: strong, long-lasting and roll-formed to length. Use the right grade near the coast.
- Copper and cast iron: premium options for period and heritage properties, with very long lives and prices to match.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.