How to Calculate Roof Catchments

The area over which rain falls is called a catchment area -- literally the area in which rain is caught. The roof of a building is a catchment area and design details such as guttering, water storage and drainage are based on the anticipated rainfall figures and the size of the roof catchment area. Because the catchment area is the area covered by the roof, rather that the surface area of the roof itself, the slope, shape and design of the roof are irrelevant to the calculations.

Tape measure

Instructions

- 1
  Measure the external length and width of the building beneath the roof. If the building is not rectangular, divide it into several individual rectangles. For example, an L-shaped building or a rectangular house with an extension can both be regarded as two rectangles. Measure the length and width of each rectangle.
- 2
  Multiply the length by the width for every rectangle. The result is the area within each shape. This is also the area covered by the roof and thus its catchment area.
- 3
  Combine the areas for the separate rectangles to obtain an overall value for the building. The result is the total roof catchment area. For example, an L-shaped building consisting of two rectangles with areas of 500 square feet and 300 square feet has a total catchment area of 800 square feet.
- 4
  Multiply the total catchment area by the anticipated rainfall over the desired time period. For every 1 inch of rain falling on each square foot of roof, you will catch 0.62 gallons of water. For example, an 800 square foot roof receiving 5 inches of rain results in a water catchment of 2,480 gallons -- 800 x 5 x 0.62 = 2,480.

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