Gutter Slope Calculator Open Calculator
Australia · roof gutter

Roof Gutter Fall Calculator Australia

Fall an Australian roof (eaves) gutter toward its outlets in millimetres per metre and 1:X ratios — and split long runs into evenly draining sections.

Roof gutter fall & outlets
Live

Set the roof gutter length, fall and the most one outlet should serve — the tool splits the run and gives the fall per section.

Live preview — one fall segment to its outlet
level reference drop run HIGH END LOW END →
Gutter (sloped) Water flow Level reference
Set the length, fall and outlet spacing to plan the run.
Copied

This roof gutter fall calculator covers the eaves gutter that runs along the edge of an Australian roof. It works in metric, quotes fall as a 1:X ratio and mm per metre, and includes an outlet planner so long roof gutters can be split into sections that each drain to a downpipe or rainwater head. Roof drainage is governed by AS/NZS 3500.3; eaves gutters are commonly fallen around 1:500 toward outlets, while concealed box and valley gutters use a stricter minimum.

On the roof line

Falling a long roof gutter

Interactive — split a long roof gutter to outlets
total gutter length outlets spaced evenly each section falls to its outlet

For concealed box or valley gutters, use the stricter box gutter fall calculator. For the general eaves case, the Australian gutter fall calculator covers the same ground.

Frequently asked

Questions answered

What fall does a roof gutter need in Australia?
An Australian roof gutter — the eaves gutter along the roof edge — is laid to a gentle continuous fall toward its outlets, commonly around 1:500 (2 mm per metre). Box and valley gutters within the roof use a stricter 1:200 minimum. Roof drainage is set by AS/NZS 3500.3.
How do I calculate roof gutter fall?
Multiply the run in metres by the fall rate. A 9 m roof gutter at 1:500 falls 9 × 2 = 18 mm to the outlet. Enter your run and a ratio above to get the fall in millimetres, plus mm/m, percentage and angle.
How many outlets does a long roof gutter need?
Keep each outlet serving a sensible length and split long runs. The Outlets mode rounds up the number of rainwater heads or downpipes a length needs and gives the fall per segment; on long walls, fall both ways from a high point.
Does roof pitch change the gutter fall?
The roof pitch sets how much water reaches the gutter and where, but the gutter’s own fall is a separate, gentle gradient along its length. Steeper roofs shed water faster, so make sure the gutter and outlets are sized for the flow as well as fallen correctly.
What about overflow on Australian roof gutters?
Roof drainage design includes overflow provisions so a blocked or undersized gutter sheds water away from the building — for eaves gutters this is often a slotted (overflow) gutter or sized gaps. Set the fall here, then confirm sizing and overflow against AS/NZS 3500.3.

Need the full gutter slope toolkit?

Switch between drop, slope check, max run and downspout planning on the main calculator — with the same live diagram.

Open the main calculator