We have about 2000 square feet of roof on the outbuilding. Most of this is metal roof -- ideal for water gathering. I installed a 3000 gallon above ground cistern beside the barn to catch and hold rain water next spring for summer use in the garden. I figure that with careful mulching and drip watering that 3000 gallons can last at most one month so we will still need to draw water from the well, but this can be done slowly and in off hours to refill the cistern during late August and September. The tank is situated in altitude below the roof but about 3 feet above the garden, so we should hopefully be able to use gravity to get the water from the tank to the garden. Unfortunately, there is not enough drop between the largest roof and the tank to use gravity, so I will need to install a small sump pump in a rain barrel to transfer water from the roof to the cistern.
I'm concerned about freezing. The tank itself won't freeze in this climate but the drain plumbing could freeze. For that reason I won't start filling it until March and when I install the drain plumbing and sight tube (to see how full the tank is) I will make it so that the piles can be drained next winter, even if there is water left in the tank.