Rain falls. In rainwater catchment, our job is to catch that rain and use it with purpose. The catchment area is the area which intercepts the rain. Knowing the catchment area is the first step in calculating the volume of water that can be caught from rainfall. A flat roof is a great example of a catchment area. Another example is an open barrel sitting alone out in the rain, in which the catchment area would just be the circular open top of the barrel.
To determine a catchment area, assume that the rain falls straight down. That means that no matter what the pitch of your roof is, it is just the footprint (i.e. the vertical projection) that determines how much rain you can catch (i.e. catchment volume).
The two buildings in Figure 4-1 have identical floor plans and different roofs. The flat roof (left building) is equal to both the footprint and the collection area. The pitched roof (right building) has a larger roof area than the flat roof. Imagine laying the pitched roof flat; its area would be much greater. If the roof area on the left is 1200 square feet, the roof area on the right is 1600 square feet. However, since the rain is falling straight down, it only sees the 1200-square-foot collection area created by the identical 1200-square-foot footprint of either building.
For a building with a roof of 1400 \(ft^2\), and a footprint of 60 ft x 20 ft, the catchment area is calculated to be 1200 \(ft^2\), using the area of a rectangle represented by the following equation:
\[Area\; of \;Rectangle = length * width \]
\[A = 60\;ft*20\;ft = 1200\;ft^2\]