Method 2: Gutter and Downpipe Capacity Determined by Calculations

After the catchment area has been determined by Catchment Area Calculation, the capacity of the gutters and downpipes can be determined by using the tables below.

An internal gutter normally requires 20 mm freeboard but using this simplified method to calculate catchment for in areas with a rainfall of more than100 mm/hr, and having a catchment basis of 200 mm/hr, an internal gutter would be over-designed if additional freeboard was added.

Similarly, internal downpipes, having no overflow to the exterior of the building, could be under-designed and their capacity should be increased by 25%.

N.B. Flooding is usually the result of a faulty drain, rather than the downpipe.

Gutter Capacity

Gutter Capacity

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As can be seen Gutter Capacity in , when the catchment area is identical, the gutter capacity at A can be reduced by half at B and to one quarter at C. The downpipe capacity required at both B and C is half that of A.

The preferred proportion for an internal gutter is 2:1, i.e., the sole of the gutter should be twice the height. The minimum height of an internal gutter should be 70mm, but the recommended height is 1/60 of the length.

The recommended maximum length of a coated steel gutter, without an expansion provision, should be 12 m. However, 6 m can be the maximum length if the outlets are spaced at 12 m as shown in Gutter Capacity. Non-ferrous metal gutters have length restrictions on their length based on their thickness.

When an external spouting has a dropper outlet or an external angle, the capacity of the spouting should be lowered by 10% for each outlet or angle. Outlets should be placed within 2 m of an angle.