How do I calculate how much rain my roof will collect?
Here is a site which helps us calcuate just that.
http://www.vacd.org/winooski/winooski_raingarden_water.shtml
Measure the perimeter of your roof then calculate the area as if the roof were a flat surface (like a floor
    plan) - no need to  take pitch and slope into account because using the perimeter to calculate the area
    will yield an answer that gives you the surface area the rain falls upon.  Please see the figure below for
    clarification.

Measure the roof from the plan view(bird’s eye view), ignoring the angle of the roof.  If our house is 20ft x 50ft, we have 1,000 square feet.We will expect an 80% efficiency in catching rain from the roof.  This varies depending on the surface, but 80% is typical for a shingled roof.


1) annual rainfall is 41 inches/yr (or 3.42 ft)
2) Lets say the perimeter of the roof is 20 X 50 = 1000 sq. ft.
3) assume 80 % efficiency for collection (typical for shingled roofs)
4) 7.5 gal./cubic ft.
3.42 ft of rain/yr * 1000 sq ft * .80 = 2736 cubic ft.
2736 cubic ft * 7.5 gal./cubic ft = 20,520 gallons of water collected per year