Rainwater is collected from two buildings, my house and my workshop, both have metal roofs. The total collection area is approximately 3000 ft/sq. The house is a "wet" system where water from the downspouts is collected in an underground manifold and routed to the roof washer and storage tank. The house is 2 1/2 story so there is sufficient head pressure to gravity feed the 12' tall fiberglass tank. The shop building is a "dry" system where water is collected directly from the downspouts into a small tank and pumped into the main collection manifold.
Both buildings have Plastmo 5" half-round vinyl gutters installed. On the house, downspout are Plastmo 3" vinyl where they exit the gutter and transition into 3" Schedule 40 PVC where the downspout is lower than the 12' tank entry. On the shop, since most of the downspouts are horizontal, the gutters empty directly into 3" Schedule 40 PVC. Each downspout on the house is equipped with a self-cleaning leaf deflector of my own design (photo upper right). A wire mesh basket on the shop surge tank catches any debris that may have accumulated into the gutters.
First flush cleaning is accomplished with a 200 gallon Booley Eliminator. An additional 75 gallons of flush can be obtained if the underground manifold is empty prior to a rain event. This gives a maximum first flush of 11 gallons/square foot. This can be adjusted by retaining water in the Eliminator or manifold as dictated by rain event frequency and other environmental factors (pollen, leaves, dust, etc.).
Water is drawn from the storage tank through a WISY floating filter to a 2" outlet at the bottom of the tank. From there, water is routed through a 1 HP pressure pump, a triple-section filter bank (photo lower left), and a UV sterilizer. The tank was ordered with two additional 2" outlets on opposite sides of the tank in case I decide to add a sand filter at a later date.
As of this writing, February, 2017, the system has functioned with a minimum of maintenance. I have replaced the house pressure pump and change the filters and UV lamp yearly. The tank has never been dry, its gotten close a few times, but we've always gotten rain in the nick of time. More photos in the Gallery.