Gutter & Downpipe SizerEnter the roof, the rainfall intensity, the gutter and the downpipe. Get the peak flow, whether the gutter and the outlet can carry it, and how many downpipes the roof needs, on a live plan and gutter section.

SheetT-35
MethodRational + Manning
Unitsm, mm, L/s
By@smart_tools_every_week
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Plan area = gutter length × roof run, since rain falls straight down. One gutter run catches one roof slope. A gable roof has two gutters, so check each. Outlets share the flow equally.

Peak flow–
Gutter carries–
Downpipe takes–
Downpipes needed–

Roof plan

Gutter section

Checks

    Downpipe sizes for this roof and gutter

    DownpipeTakes L/sRoof per pipe m²Pipes for this roof