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New Zealand · metric

Gutter Fall Calculator NZ

Fall New Zealand spouting toward its outlets in millimetres per metre and 1:X ratios — find the fall, check one you have, or plan outlets on a long run.

NZ spouting fall finder
Live

Enter the gutter run and a fall rate — the calculator returns the total fall to build in.

Live preview — the gutter tilts to your fall
level reference drop run HIGH END LOW END →
Gutter (sloped) Water flow Level reference
Enter a run and a fall rate — the fall appears here.
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In New Zealand the eaves gutter is usually called spouting, and this gutter fall calculator is set up for it. It works in metric, quotes fall as a 1:X ratio and mm per metre, and includes an outlet planner for long runs. Spouting is fallen toward its outlets at a gentle gradient — commonly around 1:500 — under NZBC clause E1 and AS/NZS 3500.3. Concealed internal and box gutters use a stricter minimum and need overflow design.

NZ reference

Spouting falls

Ratiomm per mFall over 9 mUse
1:6001.7 mm/m15 mmGentle / short
1:5002.0 mm/m18 mmCommon spouting fall
1:3003.3 mm/m30 mmLong / heavy rain
1:2504.0 mm/m36 mmFast drainage
1:2005.0 mm/m45 mmInternal gutter min
Interactive — pick a spouting fall, drag the run
level reference fall HIGH
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Falls shown are common practice for guidance. Confirm roof drainage design — grades, spouting size, outlets and overflow — against NZBC E1/AS1, AS/NZS 3500.3 and the manufacturer's details for your product.
Frequently asked

Questions answered

What fall should spouting have in New Zealand?
New Zealand spouting (eaves gutter) is laid to a gentle continuous fall toward the outlet, commonly around 1:500 — about 2 mm per metre — with a range of roughly 1:600 to 1:300 depending on run length and rainfall. Roof drainage follows NZBC clause E1 and AS/NZS 3500.3.
How do I calculate spouting fall in mm?
Multiply the run in metres by the fall rate. A 9 m spout at 1:500 (2 mm/m) falls 18 mm to the outlet. Enter the run and a ratio above to get the fall in millimetres, with mm/m, percentage and degrees alongside.
Can spouting be installed level in NZ?
Some short runs of spouting are fitted near level relying on channel depth, but a small continuous fall toward the outlet drains better and reduces standing water and overflow. A fall is recommended, especially on longer runs and in high-rainfall regions.
What is the minimum fall for an internal gutter in NZ?
Internal and box gutters carry concentrated flow and use a stricter minimum — typically 1:200 — with overflow provisions, under AS/NZS 3500.3. The gentle ~1:500 figure is for external eaves spouting, not concealed internal gutters.
How many outlets does NZ spouting need?
Keep each outlet serving a sensible length and split long runs, falling both ways from a high point on long walls. The Outlets mode rounds up the number of outlets and gives the fall per segment.

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