Roof Drainage Capacity

The capacity of a roof profile to drain water is determined by its geometry and the roof pitch. The catchment area is the distance between rib centres times the length, and the effective cross-section area and the wetted perimeter is taken to the height of the overlap on corrugate, or capillary bead on trapezoidal and trough section profiles.

The Maximum Run Calculator gives the maximum length that a roof can drain at a given pitch and rainfall intensity. The manufacturer’s data can be accessed from the drop-down box; for other profiles, the data can be entered manually into the worksheet.

Trough Capacity

Trough capacity

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Where the flow of water is concentrated by penetrations or spreaders, go to the Maximum Area above Penetration Calculator.

Penetration Gutters

RD09 Penetration gutter

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