Roof Catchment Calculation Tool
Use this tool to calculate the gallons of rainfall you can harvest annually.
Nomenclature
Footprint of structure
To calculate the roof catchment area(footprint of the structure) simply multiply the length by the width of the structure. Roof pitch is irrelevant when calculating catchment areas.
*Average rainfall per year
The average rainfall per year in your area can be found through a google search or by going here and typing in your zip code.
Type of roof material
The type of roof is required to calculate the loss coefficient which is variable by roof material. The following coefficiencts are used in the selections above: Tin(5%), Asphalt(10%), Clay(8%), Ceramic(6%), Cement(7%), Rubber(9%), Unsure(0%).
Assumed Collection Efficiency
There is an additional loss calculated in the result which is a calculation of the annual average coefficient based several factors: Areas with hotter than average summers will have a higher percentage. Winds may play a factor in catchment efficiency. Roof pitch will also have an effect (e.g. a flatter roof will have more rain bouncing off). Typically, these losses equate to an 85% efficiency rate, do not go below 60%, and do not go higher than 90%.