Gutter Fall Calculator NZ

The Gutter Fall Calculator NZ exists for New Zealand builders and plumbers who work with spouting under NZBC clause E1 and AS/NZS 3500.3. In New Zealand the eaves gutter is commonly called spouting, and fall is quoted in the same metric terms as Australia — 1:X ratios and millimetres per metre — making a purpose-built NZ page more useful than a generic imperial tool. This calculator finds the fall in millimetres for any spouting run, checks an installed fall against recommended practice, and plans outlet positions so long continuous spouting runs drain correctly to their downpipes.

NZ spouting fall uses the same ratio-based formula as Australian roof drainage:

Fall (mm) = Run (m) × Rate (mm/m)

Where Rate (mm/m) = 1000 ÷ X for a 1:X ratio.

For example, 1:500 gives a rate of 2 mm/m. A 9 m spouting run at 1:500 needs 9 × 2 = 18 mm of fall. To check an existing spouting fall: measure the run in metres and the drop in millimetres, then calculate Rate = Drop ÷ Run and Ratio = 1000 ÷ Rate. A 12 mm drop over 7 m is 12 ÷ 7 ≈ 1.71 mm/m, or approximately 1:584 — within the acceptable eaves spouting range.

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.

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.