DIY rainwater harvesting system – Grit

Learn more about gutter water harvesting by building a rainwater harvesting system to water your garden and fill storage tanks.

Lisa and I own 21.5 wooded acres in western Washington State. As retirees, we maintain a small hobby farm, grow vegetables in raised beds, and tend our 20,000 square foot garden specially designed for the benefit of native pollinators.

And although our 65-foot well tapped into a shared aquifer with ample storage, harvesting rainwater for non-potable use made sense for long-term sustainability. The rainwater harvesting kits sold by vendors for the DIY homeowner were too small or impractical for our needs, so we decided to start from scratch with our own design. A year after our initial installation, and with a few tweaks along the way, our system is working like a charm. The only maintenance required is clearing debris from gutters and emptying underground pipes and the pump in the fall.

DIY Rainwater Harvesting System Calculations

To build a rainwater harvesting system like ours, you will need to consider two calculations. The first and most important is the size of the system, and the second is its capacity. We estimate that our 300 square foot raised beds require 2 inches of water twice a week from June 15th to September 15th (12 weeks). This estimate is based on our experience, so you may need to adjust to your own situation. Knowing that there are 0.62 gallons of water in 1 square foot, 1 inch high, our calculation (0.62 gallons x 300 square feet x 2 inches per week x 12 weeks) equals 4,464 gallons. Therefore, we installed two 2,000 gallon tanks. The additional 464 gallons come from collected winter overflow and are routed to storage tanks and an occasional summer shower.

The second calculation is the number of inches of rain needed to fill the system. Our 36 foot by 64 foot barn with a 6/12 pitch roof and 2 foot eaves (about 2,400 square feet) fills the 4,000 gallon tank and underground pipes with about 3 inches of rain. Because we’re on the west side of the Cascade Range, the tanks fill up in no time, so our design includes a heavy-duty maintenance-free overflow bypass.

Component Considerations

Then we gathered the parts to put the system together. Fortunately, both tanks were available from a local supplier, so there was no delivery charge. The underground pipes, risers, fittings and valves are all PVC Schedule 40. We purchased the microfilter placed between our gutter and the barn riser from a vendor that specializes in rainwater harvesting. The importance of the microfilter cannot be overstated, especially with the amount of debris that lands on our barn roof. Without this filter, the tank would require frequent cleaning.

Our design, based on Hydrology 101, is simple. The top of each riser, one on each side of the barn, interfaces with its respective roof gutter through the microfilter. The bottom of each barn riser is connected to an underground pipe buried 18 inches deep. From there, the underground piping system is constructed with Y, T, and 90 degree fittings. We also needed pipe diameter reducers. The entire piping system must be sealed to prevent leaks and, more importantly, to maintain the vacuum needed to fill the tank from the bottom. In theory, the top of the barn riser should be at least the same elevation as the top of the tank. However, in practice we have kept the top of this riser 12 inches higher in elevation for every 50 feet of distance to the tank, allowing for real world conditions.

Storage tanks. We used two tanks rather than a 4,000 gallon tank to keep tank height to a minimum and avoid appearing like an industrial site. Unfortunately, this meant that the second tank had to be placed 17 inches below ground level to compensate for the slope between the two tanks, creating additional labor and expense. To avoid wasting storage capacity, we kept both tanks the same, their height at the same elevation, and the distance between them to a minimum.

The layout of the underground pipes does not matter in terms of slope, assuming that no part rises to an elevation above the bottom of the lowest reservoir. This became a problem when placing the second tank below ground level.

Unique to our system is the clear PVC riser between the upper and lower sills, located in the tank closest to the garden beds (our main tank). This transparent PVC acts as a site glass so that we know when the tank is almost empty, an aid to prevent damage to the pump. There is a section fitting at the top of the clear PVC riser with a removable plug, which allows for easy cleaning in case of algae buildup. Additionally, the top overflow pipes and fittings are unglued, for easy reconfiguration if needed. A 2 inch to 1 1/2 inch reducer is required to pass from the lower and upper sills of the tank.

Valves. After sizing the system, the second most important step is valve placement. It’s a bit more involved with the second tank, but not complicated. In our situation, we installed five inline 1/2 turn ball valves of varying sizes. The primary tank has three valves. The pump is isolated and when closed, the pump can be drained to prevent frost damage in winter. The sill of the pump is higher than the pump itself, so it can be primed by gravity in the spring. The second valve disconnects the primary reservoir from the underground pipes. The third is located in the drainage basin. This allows the entire underground pipe system and clear PVC riser to drain if a hard freeze is forecast.

a rainwater collector with a connection below ground level

For the second tank, there are two valves. One valve isolates the tank from the underground pipes, and the other allows this tank to empty without necessarily affecting the primary tank or the underground pipes. Both valves interface with the lower sill using a 2 inch PVC tee fitting.

To better understand the use of valves, consider our system as three components: the two reservoirs and the underground pipes. Essentially, the valve configuration gives flexibility and minimizes water loss if isolation of one or two components from each other becomes necessary.

Descent interface. We reconfigured the interface between the gutter and the downspout with a 1 1/2 inch PVC pipe that can swivel away from the microfilter. The redirected flow prevents unwanted debris and soap from entering the system when flushing needle gutters or cleaning solar panels on the south-facing barn roof. Consider placing a 1 1/2 inch to 1 1/4 inch high PVC pipe inside the gutter at the downspout to limit debris buildup on the microfilter. Of course, the downside of doing this means that water will still be present in the gutter. A 3 to 2 inch reducer, placed underground, is needed to go from the barn riser to the lower sill of the main tank. Placing the reducer closest to the barn minimized the water stored in the underground pipes.

The reconfigured gutter with a rotating downspout leading to the microfilter above the barn riser.

Tank interiors. Inside the primary tank is a float with an in-line strainer connected to the pump sill by a flexible suction hose long enough to reach the top of the tank. When the tank is nearly empty, the float and filter are raised slightly above the bottom of the tank to minimize churning of deposited debris. The barbs are used as the interface between the suction pipe, the sill of the tank and the pump. These internal tank parts are available from most vendors selling rainwater harvesting systems.

Storage tanks. The storage tanks are filled by overflowing the upper sill of the reservoirs. Excess water from these reservoirs simply runs sideways onto the ground away from buildings, as any drainage would. These tanks supplement our storage capacity and the water provided allows our chickens and goats to continue to have fresh water.

a tank overflow for a rainwater harvesting system

The pump. I recommend a pump of 1.0 horsepower or less to reduce damage to fragile plants in early spring; there is a lot of pressure to reach our furthest raised bed. If your pump is kept outdoors, consider a cover to protect it and increase its lifespan.

rainwater collection tank with on-demand pump

When selecting a pump, consider the “request” versus “delivery” option. The demand pump we purchased is powered by an internal switch, so when the pressure drops, the water flows and its pump is always ready. This benefits automated sprinkler systems, as manual activation, as on a return pump, is not required. However, a disadvantage of the demand pump is the risk of emptying the tank if a pipe leaks or is inadvertently left open.

We may one day install an automatic drip system, so we opted for the pump on demand. For now, however, the outdoor electrical outlet for the pump is wired through the light switch inside the greenhouse, so the pump only receives power when the greenhouse light is on. Seeing the light come on once I’m done watering, especially at night, reminds me to turn off the pump. It is quite easy to reconfigure the wiring if we are installing a drip system.

Rules and Regulations

Before embarking on rainwater harvesting with your own DIY system, check with county and state regulatory agencies to make sure water harvesting is permitted in your area. Rules and regulations vary, so it’s important to have information before you begin.


Jim Mahar is a retired electrical engineer and master gardener living in Washington State. Jim and Lisa practice conservation and, when not traveling, enjoy tending their extensive pollinator garden without using pesticides. Their home is 100% solar powered and Jim is frequently involved in community projects aimed at benefiting our natural world.

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