Urban Affairs and New Nontraditional Programs

Components of a Rainwater Harvesting System

 The catchment systemThe system is made up of components that catch, convey, clean, store, and distribute water (Figure 1). The actual materials and methods used will vary with the catchment surface, the location and size of storage containers, and how the water will be used.


Surfaces that can be used include roofs, greenhouses, clean and tight pool covers, and some patios and decks. Most roofing materials are acceptable for water collection. Roofing materials that contain asphalt or that have lead flashing (galvanized metal or sheet metal) should not be used if you plan to purify your water for in-home tap water. Aluminum is the best for rooftop rainwater harvesting.


Gutters typically collect the water from your roof or elevated deck and move it to downspouts. Downspouts can be used to deliver water to a storage tank. If your tank is above ground, you may need to remove you original downspouts. There should be a least one downspout for every 50 feet of gutter run and 1 square inch of downspout diameter per 100 square feet of roof area (2 X 3-inch downspout will support 600 square feet of roof area).

If you only have one storage tank, all of the downspouts will need to be connected to a 4-inch line that runs to the tank. Sharp bends in the line that lead to the storage tank should be avoided. You may just prefer to put a rain barrel or other container under each downspout.

Other types of catchment, such as pool covers or other surfaces, can be used but will require construction of a collection/conveyance system or a small sump pump that can be used to transfer the water to a tank. Structures, such as a quonset-shaped greenhouse, will require a gutter system at the hip board or at the lower roofline if the greenhouse has a peaked or rounded roof with vertical sides. You can purchase flexible downspouts, gutter extensions, drainage pipe, and connectors at most stores carrying plumbing and landscaping materials.

Cleaning the Water

Dust, bird droppings, and tree droppings such as acorns, samaras, branches, or leaves can accumulate on the roof and other catchment surfaces between rain events. A first flush diverter (Figure 2) is a pipe that tees off the main line, catches the first flush of water, and has a plug or trickle drain on the bottom. Once this pipe fills up with the initial dirty water, the remaining water bypasses the pipe and runs directly to the storage container. The plug at the end of the first flush diverter should be removed and the pipe drained after each rainfall event or, if there is a trickle tube, the water will slowly clear from the tube.

Figure 2. A first flush diverter.

Figure 2. A first flush diverter removes the initial flush of water from the catchment surface so it does not end up in the storage tank. The size of the diverter tube should be proportionate to the catchment surface.

Photo Permission from Rain Harvesting Pty Ltd. www.rainharvesting.com for both figures.

Diverter tubes should redirect 0.1 to 0.5 gallons of water per 10 square feet of roof surface. For example, if you have 100 square feet of catchment surface and want to redirect 0.1 gallons of water per 10 square feet of roof, you will need to divert 10 gallons of flush water. This would require a 6-inch PVC line 6.8 feet long. The calculation is as shown in the box below.

Volume = p x r² X length of tube

Length of tube = volume/(p X r²)

10 gallons = 2,310 cubic inches of water (1 cubic inch = .004329 gallons). You must convert gallons into cubic inches to put all units in inches.

p = 3.142

r² = the radius of the pipe squared (a 6-inch PVC pipe has a radius of 3 or 9 inches squared)

2,310 cubic inches = 3.142 X 9 square inches X length of tube (inches)

2,310/(3.142 X 9) = 81.7 inches or 6.8 feet of 6-inch PVC pipe

Other downspout filters called a "leaf beater," "leaf eater," or "roof washer," depending on the source, can be placed in the downspout (Figure 3) above the first flush diverter. The purpose is to remove large objects like leaves and acorns from the water source prior to entering the first flush diverter. If your gutters are covered, a roof washer will probably be unnecessary. If you use trickle or drip irrigation, you should put in an inline filter after the pump (Figure 4). This will remove sediment that clogs trickle tubes and drip lines. The filter's screen size should be at least a 200 mesh. Remember, the larger the mesh size, the finer the screen.

Figure 3. The leaf beater and first flush diverter.
Figure 3. The leaf beater and first flush diverter (left) as a unit and installed into a home catchment system (center). The leaf beater removes leaves and large objects from the water flow before the water enters the first flush diverter (right). Photo Permission from Rain Harvesting Pty Ltd. www.rainharvesting.com for above photos.
Figure        4. An inline filter.
Figure 4. An inline filter can reduce sediment that will clog trickle and drip irrigation lines. The valve at the bottom allows you to drain sediment and the plastic casing screws off to give you access to the screen filter.

If rainwater enters the tank at the manway or top hole, the end of the downspout should be covered with 1/4-inch or smaller hardware cloth or a screened basket (Figure 5) should be placed in the manway entrance to reduce the amount of organic matter that enters the tank. The hardware cloth should be connected in a way that allows for easy removal for cleaning out debris. Other options include roof washers that screen the water prior to it entering the collection tank.

Figure        5. A screened basket.
Figure 5. A screened basket in the manway hole will prevent a lot of organic matter and mosquitoes from entering the tank.

(Adapted with permission from Rain Harvesting Pty Ltd. www.rainharvesting.com)