Roof Gutter Fall Calculator Australia

The Roof Gutter Fall Calculator Australia exists specifically for the eaves gutter — the channel that runs along the roof edge to collect and carry rainwater to downpipes or rainwater heads. Unlike the box gutter calculator (which targets concealed internal gutters), this tool focuses on the external roof gutter that most Australian homes and commercial buildings have along every fascia. It computes the fall in millimetres for any run under AS/NZS 3500.3 guidance, plans rainwater head positions along long roof lines, and gives the fall each segment needs when a continuous eaves gutter drains to multiple outlets.

Roof gutter fall is calculated using the standard Australian ratio convention:

Fall (mm) = Run (m) × Rate (mm/m), where Rate (mm/m) = 1000 ÷ X for a 1:X ratio

For eaves gutters the common starting point is 1:500 (2 mm/m). A 9 m roof gutter at 1:500 needs 9 × 2 = 18 mm of fall to the downpipe. For longer runs split to multiple outlets, each segment is treated separately: Segment fall = Segment length × Rate. The outlet planner calculates how many rainwater heads are needed and the fall drop per segment, so long roof gutters can be set out in one pass.

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.

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.