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Box Gutter Fall Calculator Australia

Box gutters carry concentrated flow inside the roof, so they start at a strict 1:200 minimum fall. Work out the drop in millimetres for any run.

Box gutter fall (min 1:200)
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Set the box gutter run and fall — the tool returns the drop and checks it against the 1:200 minimum.

Live preview — box gutter fall to the sump
level reference drop run HIGH END LOW END →
Gutter (sloped) Water flow Level reference
Set the run and fall to see the drop and compliance check.
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A box gutter sits within the roof structure — in a valley, behind a parapet, or between roof planes — and concentrates large volumes of water, so Australian practice treats it more strictly than an eaves gutter. Under AS/NZS 3500.3 a box gutter has a minimum fall of 1:200 (5 mm per metre) toward the sump or rainwater head, and this calculator starts there. Pick a run and the minimum (or a steeper) fall to read the drop in millimetres; the in-range check flags anything shallower than 1:200.

Box gutter reference

Fall from the 1:200 minimum

Ratiomm per mFall over 12 mNote
1:2005.0 mm/m60 mmMinimum (AS/NZS 3500.3)
1:1506.7 mm/m80 mmPreferred where possible
1:10010 mm/m120 mmFaster, more headroom needed
1:7513.3 mm/m160 mmSteep
1:5020 mm/m240 mmVery steep
Interactive — from the 1:200 minimum up, drag the run
level reference fall HIGH
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The 1:200 figure is the standard minimum; it is not a complete design. A compliant box gutter also needs correct width and depth, sumps, downpipe sizing and overflow provisions per AS/NZS 3500.3 and the NCC. Always have concealed roof drainage checked by a qualified hydraulic designer.
Frequently asked

Questions answered

What is the minimum fall for a box gutter in Australia?
AS/NZS 3500.3 sets a minimum fall of 1:200 for box gutters — that is 5 mm for every metre of run, falling toward the sump or rainwater head. Many designers use a steeper fall where headroom allows, because box gutters carry concentrated flow and any sag or back-fall risks overflow inside the roof.
How do I calculate box gutter fall?
Multiply the run by the fall rate. A 12 m box gutter at the 1:200 minimum falls 12 × 5 = 60 mm. Enter the run and choose 1:200 (or steeper) above to get the fall in millimetres, with the ratio, percentage and angle alongside.
Why do box gutters need more fall than eaves gutters?
A box gutter usually sits within the roof and concentrates large flows, so an overflow drains into the building rather than over the edge. The minimum 1:200 fall, generous sizing, sumps and overflow provisions all guard against that, which is why box gutters are treated more strictly than eaves gutters.
Does a box gutter need an overflow?
Yes. AS/NZS 3500.3 requires box and internal gutters to have overflow measures — typically an overflow device or a sized overflow outlet — so a blockage sheds water safely outside the building. Fall is only one part of a compliant box gutter design.
Can a box gutter be laid level?
No. A level or back-falling box gutter ponds and is a common cause of internal leaks. The standard requires a continuous fall of at least 1:200 the whole way to the sump or outlet.

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