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Cast Iron Outlet · Technical Tool

Rainwater Flow Calculator

Answer three quick questions about your roof and we'll size your cast iron gutter and downpipe system to UK rainfall standards, then show you exactly which products fit.

m²
The plan-view footprint of the roof draining into this gutter: length x width on the ground, not the sloped surface.
How steep the roof is. Steeper roofs shed water faster, so pitch adjusts the effective area. Most modern pitched roofs sit between 30° and 45°.
mm
The longest straight length draining to a single outlet. Measure from the far end to the downpipe. If there's an outlet at each end, measure to the midpoint.

We combine these against UK rainfall standards and recommend the smallest cast iron gutter and downpipe that meets the required flow.

How the Rainwater Flow Calculator works

Cast Iron Outlet's Rainwater Flow Calculator sizes a gutter and downpipe system using the UK standard drainage method. It takes three inputs: your roof's catchment area (the floor-plan footprint in square metres), the roof pitch angle, and the longest gutter run to the nearest outlet, and returns the minimum required flow rate in litres per second, then matches it to compatible cast iron profiles.

The calculation

  • Effective catchment (Answer A) = catchment area (m²) × roof pitch factor × UK rainfall intensity (0.02).
  • Flow ratio (Answer B) = longest gutter run (mm) ÷ standard gutter depth (50 mm). This selects a reduction factor that accounts for how efficiently a long gutter drains.
  • Required flow rate (l/s) = Answer A ÷ reduction factor. We then list every gutter profile whose rated capacity meets or exceeds this figure, and recommend the smallest suitable downpipe (65 mm, 75 mm or 100 mm).

The roof pitch factor and rainfall intensity follow the UK Rainwater Specifier's standards, so the result reflects real drainage performance rather than a rough guess.

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