The requirement
Gutters and downpipes carry the rain off the roof and away from the house. Get them too small and the water backs up, spills over, and ends up inside. The rule book — the National Construction Code, or NCC — sets out how to size them so that doesn’t happen (NCC 2025, Housing Provisions Part 7.4).
There are really three things to know: the one simple number you can carry in your head, the method the code uses to size everything, and the one job you hand off to someone else.
The 12 m rule. One downpipe can serve no more than 12 m of eaves gutter length. “Eaves gutter” is the normal gutter that runs along the fascia. This is the easy one — a DTS rule, which just means the tick-box path in the code that’s taken as meeting the rules. If a run of gutter is longer than 12 m to a single downpipe, add another downpipe. Simple.
| Rule of thumb | Limit |
|---|---|
| Gutter length per downpipe | ≤ 12 m |
| Box gutters | design to AS/NZS 3500.3 (or a Performance Solution) |
| Everything else | size by catchment area × local rainfall, off the NCC tables |
Box gutters are not your call. A box gutter is the wide gutter that sits inside the roof — in a valley or behind a parapet — not out on the eaves. It carries a lot of water. The NCC does not size a box gutter for you. It must be designed and installed to AS/NZS 3500.3, or by a Performance Solution (a one-off design an engineer signs off). If the job has a box gutter, hand the sizing to a plumber or engineer. Don’t guess it.
How the NCC sizes a house
For normal eaves gutters and downpipes, the code doesn’t give you one magic number. It works off how much roof water each gutter has to catch. The more roof draining into a gutter, and the harder it rains where you are, the bigger the gutter and downpipe have to be.
Here’s the method the code uses:
- Work out the catchment area. That’s the patch of roof draining into your run of gutter — the gutter length × the distance from the ridge down to the gutter. It gives you an area in square metres.
- Find your local rainfall. The code uses the 5-minute design rainfall for your site — basically, how hard it buckets down in a short, heavy burst. Darwin gets a lot more than Hobart. You read this off the NCC’s rainfall table.
- Read the size off the tables. You take your catchment area and your rainfall into the NCC’s gutter and downpipe tables, and they tell you the size to use. The code even sets an overflow check for the really big storms, so the water has somewhere to go instead of into the house.
That’s the whole idea: bigger roof + harder rain = bigger gutters and more downpipes. Because it’s site-specific, there is no fixed “one downpipe per so many square metres” — the right answer in one town is the wrong answer in the next.
Work it out by hand
You’ve got a run of eaves gutter along one side of a house. It’s 9 m long, and the ridge sits 4 m back from the gutter (measured flat, on the plan). How much roof is that gutter catching, and how many downpipes?
- Catchment area. 9 m × 4 m = 36 m² of roof draining into that gutter. That’s the number you carry forward.
- Size it. You do not read a downpipe size off the 36 m² alone. Take that 36 m² and your local 5-minute rainfall into the NCC gutter and downpipe tables — the right size is whatever those tables give you for your rain. That’s the only place the size comes from.
- Check the 12 m rule. The gutter is 9 m long, under 12 m, so one downpipe could serve it for length. But the tables might still call for more than one downpipe if the catchment and rainfall are high. The 12 m rule is a ceiling on length, not the full answer.
So: the catchment is 36 m², the size comes from the NCC tables for your local rainfall, and no single downpipe covers more than 12 m of gutter. The 36 m² and the 4 m here are just example numbers to show the sum — they’re not code minimums.
Related
- Part of the NCC rules for Australian chippies — the pillar, with the state-by-state adoption tracker.
- Working the roof above these gutters? Minimum roof pitch: what the NCC needs for your profile.
- Setting out the roof structure? NCC stair requirements: riser, going & the 2R+G rule covers the same “read it off the code, don’t eyeball it” habit.
Sources
- NCC 2025, ABCB Housing Provisions Standard, Part 7.4 — Gutters and downpipes. The 12 m-per-downpipe DTS rule, the catchment-area-plus-rainfall sizing method, and the gutter, downpipe, rainfall and overflow tables all live here. Read it at ncc.abcb.gov.au.
- AS/NZS 3500.3 — Plumbing and drainage: Stormwater drainage. Box gutters must be designed and installed to this standard (or by a Performance Solution), and it’s also an alternative route for sizing gutters and downpipes. It’s a paywalled Australian Standard — the sizing figures must be checked there, or with your plumber or engineer. Don’t take a box-gutter size from anywhere else.