Rainwater harvesting is nothing new—people have been collecting water from rooftops for centuries, especially in areas with limited water supply. In our region, rooftop water is harvested and used for private residences as well as large commercial buildings.

You might be surprised by how much rainwater collects on your roof. A half-inch of rain falling on a 1,000 square-foot roof generates about 300 gallons of water. When you consider how many inches of rain you receive every rainy season, you quickly recognize that it would take a lot of storage capacity to capture all the rain that hits your roof.

Regulatory Considerations: Technically, the Washington State Department of Ecology has authority over any water in the state—including the runoff from your roof. In April 2007, the Department of Ecology asserted that in would not require a water-rights permit for residential rainwater harvesting: “Ecology recognizes that rainwater collection has many benefits and that regulating the use of small amounts of rainwater was probably not the intention of existing laws. Therefore, Ecology is currently not requiring a permit for small (de minimus) uses of rainwater. ... Ecology is directed by law to protect existing water rights and water resources. Ecology could respond to any complaints of harmful effects from your rainwater use.”

Options for harvesting rainwater include:

• Rain barrels (usually 40 to 80 gallons of capacity) to collect water for outdoor use.
• Cisterns to collect water for outdoor use and non-drinking uses (e.g., flushing toilets).
• Cisterns combined with treatment systems to collect water for all household uses, including drinking. These systems are becoming more widely used in areas that have limited drinking water available, such as on the San Juan Islands. This guide will not address filtration, but offers information for further investigation in the resources section.

Managing stormwater: Rain barrels and cisterns only help slow stormwater flow if they are empty at the start of a winter storm. You can accomplish this, and still save water in the summer, by attaching a garden hose in winter and opening the valve so that water rushing into the barrel from your gutters is slowly dispersed into your lawn or landscape. In late spring, close the valve to store water for summer.

Once rain barrels and cisterns are full, the roof water needs to be redirected to prevent overflowing. The best option is to use a downspout diverter that returns the water to downspouts when the storage tank is full (see resources). Downspouts should then be directed into a stormwater feature or fully dispersed in the landscape.

Rain barrels:

Rain barrels are available in a variety of sizes and are made from a variety of materials, including recycled oak whiskey barrels and recycled plastic storage drums. You can make your own or buy one ready made. Some water utilities sell them at a subsidized price to their customers. Use only barrels that you know were used for food or food-grade materials—never use drums that stored chemicals or petroleum products.

Rain barrels should have a tight-fitting lid to protect children and prevent mosquitoes from breeding, as well as a built-in, easy-to-clean screen on top to filter large debris. Rain barrels are sold in a range of colors. You can paint yours to blend in to your home’s exterior.

Rain barrels are heavy when full, so they need to sit on level, packed earth or gravel, or elevated on leveled concrete blocks. Don’t place them where they would block doorways if they fell over in an earthquake. Some municipalities require them to be strapped to the house for this reason.

Capacity: Given their limited size, rain barrels do not offer the capacity of cisterns. However, you can increase the storage capacity to harvest enough water to meet many summer watering needs, especially if you also use drought-tolerant plants.

• Place rain barrels at each downspout.
• Link two or more rain barrels together to increase the storage at each downspout.
• You can buy ready-made linking kits or make your own (see resources).
• For a house with four downspouts, linking two 75-gallon barrels at each downspout could store 600 gallons of water for summer use.
• The simplest and least expensive step up from a rain barrel to a larger (300-500-gallon) storage capacity may be to buy poly tanks sold by agricultural suppliers. For safety, choose one with a broad base. Provide a perfectly level foundation of cement or packed gravel.

Capturing the roof runoff:

• Position your first rain barrel directly below your downspout, and any linking barrels immediately adjacent to the first.
• Elevate the rain barrels by placing them on stable concrete blocks. This will help with water flow when you harvest your water in the summer.
• Install a downspout diverter into your downspout. Some diverters automatically close when the rain barrel is full so that your roof runoff once again flows to the bottom of the downspout. Others require you to monitor fullness and close the diverter yourself.
• Once the rain barrels are full, the water should flow to the bottom of your downspout and ideally be diverted to an on-site stormwater feature.
• Keep your gutters and downspouts clean to prevent leaves from clogging your downspout diverter.

Cisterns: 

An investment in a cistern system to collect and store rainwater will take your harvesting options to a whole new level over rain barrels. While the concepts and materials for cistern systems are simple, designing a system for your household’s use can be complex. When planning a cistern system, hire an experienced, well-qualified rainwater-harvest designer or engineer.

Cisterns are made from a variety of materials, including plastic, fiberglass, concrete, metal and wood. Each tank can hold hundreds or even thousands of gallons of water. Most systems rely on several tanks to meet their water needs. Tanks can be above ground or buried. They can be artfully designed to add to your home’s aesthetics or hidden under a deck or by vegetation.

Untreated cistern water can be used for a variety of household tasks, including laundry and toilet flushing, as well as landscape irrigation. Harvesting water for these needs requires installing pumps as part of the overall design.

Cost: Cistern systems can be expensive, but can pay for themselves over time with water savings. Costs will vary tremendously depending on system components and design. In new construction, cistern systems can save money by decreasing some stormwater infrastructure costs. In one example, supplying a home with a 10,000-gallon storage capacity for laundry and toilets added about $8,000 to construction costs. 

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