Rainwater Catchment Systems — From Contributor Eric McGlynn

In southwestern Wisconsin, this spring was wet and cool, while midsummer was hot and dry. What this meant to the backyard gardener or microfarmer was a slow start to the growing season. Once fruits and vegetables started to grow, recent dry weather quickly wilted plants. When rain did come, storms dropped a large amount of rain in a short period, and water largely ran off the hard, dry ground. There is a simple way to moderate these wet and dry periods with some type of rain catchment system that stores runoff from a roof.

For those who are dependent on treated municipal water, irrigating through dry periods poses several challenges, primarily lack of infrastructure. For people drawing water off of a municipal service, the cost of using water can be prohibitive and local government may limit watering during dry periods. With some planning and a minimal investment, rain catchment systems that capture water when rain is abundant offer renewable and consistent irrigation through hot and dry weather.

Safety Considerations

Before detailing storage options, safety associated with water collection must be reviewed. Common concerns relate to contaminants like poly-cyclic aromatic hydrocarbons (PAH) or heavy metals ending up in rainwater catchment systems. While contamination is unavoidable, rainwater catchment is generally safe when certain precautions are taken.

A review of studies indicates rainwater will predictably pick up contamination present in the environment. According to DeBusk and Hunt (2012), emissions, pesticides, and industrial waste in the atmosphere settle on roof surfaces and are washed into rainwater catchment systems. The authors of this study found that if you live in an urban area, even higher levels of industrial contaminants are probable, yet concentrations are likely low. A study by Haberland, Backacs, and Yergeau (2013) in New Jersey found no detectable levels of PAH, but bacteriological contamination from animal droppings was found in each sample. This result was consistent with findings from Philadelphia where E. coli and Enterococcus were present in 62 and 77percent of rain barrels in the sample (Hamilton, et al. 2018). Bacteriological contamination is probable, so diverted rainwater should be limited to irrigation.

Homeowners may also question how roofing construction affects contamination. In a guide developed by the State of Texas, asphalt shingles, cedar shakes, tar, and aggregate roofs, reportedly leech some chemicals into rain runoff, but water captured is reportedly “suitable” for watering (Texas Water Development Board, 2005). This wording raises more questions than the guide suitably answers; however, the previously cited studies also measured the presence of heavy metals. The results found the levels of leeched metals was minimal. When compared to rainwater control samples, lead concentrations averaged 3.7 ug/L (micrograms per liter) and zinc averaged 78.7 ug/L, which was well below the standard set by the Environmental Protection Agency of 5000 and 2000 ug/L respectively (Hamilton et al., 2018). Similarly, the New Jersey sample also found that heavy metals detected in rain barrels tested well below the safety standard set for irrigation (Haberland et al., 2013).

With these study results in mind, simple practices to avoid bacteriological illness would apply to heavy metals. Watering roots would limit how much contamination comes into direct contact with consumables. Although, it should be noted that root crops may concentrate heavy metals. After harvesting, thoroughly wash produce with clean water before eating as an additional precaution. Additionally, a small amount of chlorine could be added to storage containers to sanitize bacteriological contamination in water.

Types of Collection and Storage

A system for collecting rainwater can range from small to large and simple to elaborate. Burying a cistern or setting up a large tank capable of holding tens of thousands of gallons requires heavy equipment and technical expertise. This article intends to illustrate examples that are well within the budget and capabilities of anyone with basic hand tools.

1. Stock tanks

One of the simplest options is to redirect a downspout to a stock tank and dip buckets or watering cans into the tank as needed. The stock tank can be placed on level ground or buried when constructed from plastic. If aesthetics are consideration, stock tanks can be planted and house a few fish to give the appearance of a water feature. The downsides of stock tanks are large footprints and the open tanks collect leaves and debris, which requires periodic cleaning. It is my opinion that stock tanks should be fenced, secured, or placed only in areas where children and animals are unlikely to fall into them.

2. The 55-gallon drum rain barrel

There have been a proliferation of rain barrels showing up in residential yards. Single rain barrels are functional, but the capacity is limited. Common watering cans range from two to three gallon capacities. Dipping a watering can in a rain barrel results in a noticeable drop in the water level. To increase storage capacity, while maintaining a small footprint, a hole-saw, Uniseal gasket,* and some PVC pipe can simply increase storage capacity by connecting barrels.

For this project, a barrel was placed under the valley of a roof to catch rainwater. A downspout could also be redirected into one of the barrels. A free pallet offered a sturdy base and was easily cut down and leveled. A 2-in Uniseal gasket was selected because the size is large enough for goldfish fish to swim between the barrels to eat mosquito larva.

To install the Uniseal gasket, a 3-in hole-saw bit makes the appropriate sized hole. Burs that might interfere with a tight seal were cleaned up with a razor blade and the gasket was installed as per manufacturer’s instructions. It was critical to bevel the edge of the PVC to allow the pipe to be pushed through the Uniseal. A food-grade lubricant like cooking oil was applied to the gasket and pipe end. The pipe was seated into the gasket and pushed through. To make the system modular and easily serviceable, rubber couplings, fittings, or caps are used to plumb the rain barrels. A step-bit or small hole-saw bit can be used to install a hose bib.

3. The 275-gallon food grade tote

The advantage of the tote is that the closed container keeps evaporation, insects, and leaf litter to a minimum. Additionally, totes can be stacked and piped in series so water storage can be expanded as needed. Totes are also easily located for free or at minimal cost. Totes from suppliers that carry documentation, like material data safety sheets, are preferable so only food-grade containers are used.

Filling the totes with a downspout can be accomplished in various ways. In this instance the tote is filled through the top and water is gravity fed from a pressure cap with a hose bib attached. A piece of window screen is inserted into the top opening to prevent debris and insects from fouling the water. To prevent the tote from overflowing, a simple pipe redirects water to the yard once the tote is full. The downside of a single gravity-fed tank is that drawing water can be slow, particularly as the water level drops. This system works well for low pressure applications.

The second example is gravity fed from the bottom of the tanks. Top caps are left in place, but loose on the threads to allow venting. A downspout is routed to a 4-in PVC pipe reduced to 2 in to fit the ball valves on the bottom of the tanks. The reason for using a bottom-fill system is it allows multiple tanks to be easily filled from one downspout. Furthermore, the ball valve that comes on each tote allows individual tanks in the series to be shut off and isolated from the system. If a reserve tank is desired, shut off the valve on one tote when it is at capacity, and open the valve when the need for reserve water arises. Once again, a simple overflow is piped in to allow excess water to run off once the tanks are full. This system uses a repurposed pool pump to increase water pressure in the line. The pump is not necessary. The pressure of the water in two totes is sufficient for basic irrigation; however, inclusion of the pump makes watering much more efficient when time is limited.

Planning Considerations and Best Practices

To determine how much water can be directed to a planned rain catchment system, determine the area of the roof. This is accomplished by measuring the footprint of the roof. Multiply the length by the width of the roof to determine square footage, multiply the square footage by the amount of precipitation in inches, and then multiply this number by .623 to determine the number of gallons of water the roof will collect. For example, one inch of rain on a common 10-×-12-ft steel garden shed with a roof that measures 10.5 × 12.5 ft (131 ft²) could collect 81.6 gallons of water ((131 × 1) × 0.623). These calculations show how 4–5 inches of spring rain on a small roof could predictably collect 300–400 gallons of water.

To limit the amount of contamination entering your system, consider using a gutter guard or some type of sturdy mesh to catch debris and insects. Remember to check this barrier regularly to ensure the flow of water does not become obstructed. When open containers like stock tanks, rain barrels, or 55-gallon drums are used, mosquito larva will undoubtedly be present in your containers. Consider using a cover, bacterial mosquito control tablets, a thin layer of cooking oil, or inexpensive goldfish. These containers use goldfish for insect control (one has been at it for over a decade), or window screen as a physical barrier.

A final consideration is to build the system so it can be easily winterized. The use of rubber couplings and fittings cannot be emphasized enough, because these unions are temporary. Piping can be removed and stored as cold weather approaches, which allows rain barrels to be turned over and totes to be completely drained. When spring rains return, the rubber fittings allow plumbing to be slipped back on, tightened, and quickly returned to service.


The water storage options covered by this article only scratch the surface as there are numerous open or closed containers that can be repurposed or purpose built for your project. With some ingenuity and basic hand tools, water from a roof can be redirected and preserved. An appropriately sized rain catchment system can provide a steady supply of water for gardens and summer plantings through dry periods, without relying on treated municipal water or adding expensive infrastructure.

*Link is for example purposes, not an endorsement of any website. Shop around.


DeBusk, K. M., & Hunt, W. F. (2012). Water quality benefits of harvesting rooftop runoff. World Environmental and Water Resources Congress 2012. doi:10.1061/9780784412312.063

Haberland, M., Backacs, M., & Yergeau, S. (2013). An investigation of the water quality of rainwater harvesting systems. Journal of the NACAA, 6(1). Retrieved from https://www.nacaa.com/journal/index.php?jid=205

Hamilton, K., Parrish, K., Ahmed, W., & Haas, C. (2018). Assessment of water quality in roof-harvested rainwater barrels in greater Philadelphia. Water, 10(2), 92. doi:10.3390/w10020092

Texas Water Development Board. (2005). Texas manual on rainwater harvesting. Retrieved from Texas Water Development Board website: http://www.twdb.texas.gov/innovativewater/rainwater/doc/RainwaterHarvestingManual_3rdedition.pdf


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