First Flush Water Diverter or roof washer takes the first flow of rainwater which may contain contaminants from the roof and gutters seals it off and then automatically diverts the flow of cleaner water to the tank. Supplied in kit form just add appropriate length of pipe based on the quantity of water you wish to divert.. Dimension - 11.6 x 9 x 5.7 in.. Item Weight - 3.3 pounds. Size - 4 in. Round