Given:
- 36 inches annual rainfall
- a collecting roof 8 x 8, raised 2 feet on one side/end for a moderate slope (total area 61.9 sq ft)
- 22.32 gallons of water per foot of rained-on area
That all adds up to 1,383.11981 gallons of water annually. That seems like an awful lot of irrigation-quality water from a pretty small system (a simple shed roof, with or without a shed under it).
Check your area here.