Construction/ CALCULATOR

Roof outlet and downpipe count

Convert roof catchment and design rainfall into a preliminary number of rated outlets or downpipes.

Connected projected roof aream²Use the horizontal area that drains to this gutter or outlet group.

Local design rainfall intensitymm/hUse the required local short-duration intensity, not annual rainfall.

Runoff factor%Fraction of rainfall assumed to reach the drainage path.

Planning capacity margin%An explicit preliminary margin; do not substitute it for local design rules.

Rated capacity per outlet or downpipeL/sUse tested manufacturer data for the complete outlet arrangement and applicable head.

Estimated peak runoff

3.00 L/s

Flow including margin

3.60 L/s

Preliminary whole outlets

3 outlets

Combined rated capacity

4.50 L/s

Average connected area per outlet

40.00 m²

Uniform rainfall and equal flow sharing between clear, identical outlets. The entered capacity must come from applicable product data. Gutter geometry, outlet position, valleys, bends, surcharge, blockage, wind, snow, ice, overflow paths and local rules require separate checks.

FROM ESTIMATE TO ACTION

Use the result, not just the number.

Verify the inputs

Measure or document each input. If a value is only an estimate, record its source and test a conservative alternative.

Keep the calculation traceable

Save the input link or copied result together with the date, units and assumptions. A number without its inputs cannot be checked later.

Confirm the decision

Compare the estimate with the related guide, current product data and requirements that apply at your location before buying or building.

Inputs to verify

Connected projected roof area (m²): Use the horizontal area that drains to this gutter or outlet group.
Local design rainfall intensity (mm/h): Use the required local short-duration intensity, not annual rainfall.
Runoff factor (%): Fraction of rainfall assumed to reach the drainage path.
Planning capacity margin (%): An explicit preliminary margin; do not substitute it for local design rules.
Rated capacity per outlet or downpipe (L/s): Use tested manufacturer data for the complete outlet arrangement and applicable head.

Continue your calculation

Construction

Gutter fall and support set-out

Turn gutter length, intended fall and maximum support spacing into end level and evenly spaced support points.