Harvested Rainwater Guidelines

Rainwater harvesting can be done with any roofing material if it is for non-drinking use only. For potable use of rainwater, the best roof materials are metal, clay, and cementitious although all roof material types have been used(except asbestos). Asbestos roof materials used in older homes should not be part of a system to provide drinking water. Asphalt shingles can contribute grit to the system and need a pre-filter for the water before it enters the cistern. Lead materials in any form should not be used in the system (i.e. lead flashing).

• Gutters are used to convey water from the roof to pipes to the cistern.

• If a straight run of gutter exceeds 60 feet, use an expansion joint.

• Keep the front of the gutter one half inch lower than the back.

• Provide a gutter slope of 1/16 inch per foot minimum.

• Provide gutter hangars at 3 feet O.C.(on center).

• Gutter should be a minimum of 26 gauge galvanized steel or 0.025 inch aluminum.

• Downspouts should provide 1 square inch of downspout opening for every 100 square feet of roof area.

• The maximum run of gutter for one downspout is 50 feet.

• The conveyance piping from the gutter system to the cistern or filter should be Schedule 40 PVC or comparable in a 4 inch diameter. Do not exceed 45 degree angle bends in horizontal pipe runs and provide 1/4 inch slope per foot minimum. Use one or two-way cleanouts in any horizontal pipe run exceeding 100 feet.

• The storage tank (cistern) must be sized properly to ensure that the rainwater potential is optimized. See the previous section regarding capacity for sizing information.

• Cisterns can be located above or below ground.

• The best materials for cisterns include concrete, steel, ferro-cement, and fiberglass.

• When ordering a cistern, specify whether the cistern will be placed above or below ground and if the cistern will be used to store potable water. (Fiberglass cisterns are constructed differently to meet the various criteria.)

• If using a manufactured tank designed to hold drinking water, the tank should conform to the published specifications of the American Waterworks Association. (See Resources.)

• Cistern characteristics
  • A cistern should be durable and watertight.

  • A smooth clean interior surface is needed.

  • Joints must be sealed with non-toxic waterproof material.

  • Manholes or risers should have a minimum opening of 24 inches and should extend at least 8 inches above grade with buried cisterns.

  • Fittings and couplings that extend through the cistern wall should be cast-in-place.

  • Dissipate the pressure from the incoming water to minimize the stirring of any settled solids in the bottom of the cistern. This can be accomplished in a concrete cistern by placing concrete blocks (cavities facing upward) surrounding the base of the inlet pipe. The blocks can be 8"x 8"x16" blocks with the pipe exiting one inch above the bottom of the cistern. Baffles to accomplish the same result can be made as part of fiberglass cisterns. This is not a concern for cisterns that always have a large reserve.

  • The use of two or more cisterns permits servicing one of the units without losing the operation of the system.

  • Have a fill pipe on the cistern for adding purchased water as a backup.

  • Have a cover to prevent mosquito breeding and algae growth from contact with sunlight.

• The rainwater may become contaminated by dirt, debris, and other materials from the roof surface. The best strategy is to filter and screen out the contaminants before they enter the cistern.

• A leaf screen over the gutter and at the top of the downspout is helpful.

• A primary strategy is to reject the first wash of water over the roof. The first rainfall will clean away any contaminants and is achieved by using a "roof washer."

• The main function of the roof washer is to isolate and reject the first water that has fallen on the roof after rain has begun and then direct the rest of the water to the cistern. Ten gallons of rainfall per thousand square feet of roof area is considered an acceptable amount for washing. Roof washers are commercially available and afford reliability, durability, and minimal maintenance to this function.

• Roof washing is not needed for water used for irrigation purposes. However, prefiltering to keep out debris will reduce sediment buildup. A sand filter can also be used.

• Removing the water from the cistern can be achieved through gravity, if the cistern is sufficiently high enough, or by pumping.

• Most cases will require pumping the water into a pressure vessel similar to the method used to withdraw and pressurize water from a well (except a smaller pump can be used to pump from a cistern).

• A screened 1.25 inch foot valve inside the tank connected to an 1.25 inch outlet from the cistern approximately one foot above the bottom (to avoid any settled particles) will help maintain the prime on the pump. A float switch should be used to turn off the pump if the water level is too low.

• Another alternative is the use of a floating filter inside the cistern connected to a flexible water line. This approach withdraws the water from approximately one foot below the surface which is considered to be the most clear water in any body of water.

• The water that will be used for potable purposes can pass through an inline purification system or point of use water purification system. Other uses for the water do not need additional purification. (Water purification options are not discussed in the Sourcebook.)