What Is a Cistern?

Definition of a Cistern

A cistern is a watertight tank that collects and stores water for later use and is typically made of reinforced concrete, cinder block, precast concrete, fiberglass, and steel. Your home’s gutters and downspouts usually funnel rainwater into your cistern, and you can use that water as a supplemental supply, an emergency supply, or for irrigation. Cisterns can also hold groundwater, household greywater, spring water, well water, or treated city water.

Professionals can install cisterns above ground, underground, or into building structures. Burying cisterns underground might be the best option, though, as underground cisterns keep water cool during warm months and prevent it from freezing during colder weather.

Cisterns intended for domestic use typically hold at least 5,000 gallons, but sizes and setups vary according to homeowners’ needs.

When choosing a cistern material, keep in mind that not all tanks are created equal. According to TOH’s reporting on rainwater systems, stone and cement cisterns can leach minerals into stored water, and galvanized tanks can release zinc unless a PVC liner is used. Fiberglass tanks, though not the most attractive option, are “by far the cleanest and most durable.” If your cistern is above ground, keep it out of direct sunlight, which can heat the water and foster algae growth. Also, be sure any tank is covered or screened to prevent mosquitoes from breeding inside.

In one real-world example featured in This Old House magazine, a couple in the Florida Keys installed a 7,500-gallon concrete cistern to collect rain runoff. Though their home was still connected to municipal water, the system provided over half of their water for both potable and nonpotable uses. A diversion valve ensured that the first raindrops washed the roof clean of leaves, dirt, and insect droppings before the water flowed through copper gutters into the cistern, where three filters treated it for sediment and bacteria.

Pro Tip

Kevin O’Connor, host of Ask This Old House, explains: “A cistern is simply a large covered tank for collecting rainwater. They’ve been used all over the world, and archeologists have found ones that are at least 4,000 years old.” Historically, cisterns supplemented wells for drinking or bath water, irrigated gardens during dry spells, and even served as emergency water sources for fighting fires in remote areas.

Types of Cisterns

Most cisterns fall into one of three basic categories: rainwater cisterns, water storage cisterns, and greywater reuse cisterns.

Rainwater Cisterns

Rainwater cisterns are the most common type of cistern and connect to your home’s gutter system. Water that falls onto your roof flows into the gutters, through the downspouts, and into the cistern.

You can use the collected runoff to irrigate your lawn or garden, or if you treat the water, you can use it for indoor use and even for drinking. Roof catchment systems that collect rainwater typically include a roof washer and a filter box to minimize potential water contaminants that reach the underground cistern.

Expert Insight: Dr. Hari Krishna, president of the American Rainwater Catchment Systems Association (ARCSA), says: “Rainwater is the purest water you can find.” Unlike well water heavy with minerals or chlorine-treated municipal supplies, collected rainwater is naturally soft — making it ideal for everything from watering the lawn to drinking, provided it’s properly filtered.

Pro Tip

TOH plumbing and heating expert Richard Trethewey installed a rainwater collection system on the Winchester, Massachusetts, TV project and noted: “Everywhere else in the world, people collect rainwater. It makes sense in Massachusetts, too.” A diversion valve — sometimes called a roof washer — is essential: it ensures the first raindrops wash the roof clean of leaves, dirt, and insect droppings before cleaner water flows into the cistern.

Water Storage Cisterns

Water storage cisterns are common in rural areas where public water is unreliable, inadequate, or unavailable. Homeowners can use water storage cisterns to tap into a nearby spring or groundwater in these areas.

A water storage tank provides a buffer against drought or service interruptions and has the potential to collect enough water to cover your daily needs. Installing a residential well and water tank can also save you money and help you eliminate your monthly water bill.

Greywater Reuse Cisterns

You can recycle wastewater from domestic activities such as laundry, dishwashing, and bathing with a greywater reuse cistern. These tanks collect and treat greywater instead of using rainwater or water storage systems.

A greywater cistern will not provide potable water, but it can help reduce your overall water consumption. This cistern can help you collect water from your shower, sink, or washing machine and use it to flush toilets, wash your car, or irrigate your lawn.

How It Works: Not all greywater sources are equal. According to TOH’s plumbing coverage, source water from a bathroom sink—not a kitchen one, which is at high risk for bacteria such as E. coli—is recommended for reuse systems like sink-to-toilet hookups. The easiest greywater source to tap is a washing machine, since its pump is already pulling water out and discharging it into an accessible standpipe that you can extend outside the house. Just take care to strain the water through a sand filter or polyester felt mesh bag before reuse.

Pro Tip

Keep in mind that greywater reuse rules vary by state. Flushing toilets with water draining from showers, sinks, and tubs is allowed in International Plumbing Code states, but it’s prohibited by the Uniform Plumbing Code in 17 others, mostly in the West. Check your local codes before investing in a greywater cistern system.

Why Have a Cistern?

Cisterns help homeowners tap into additional water sources, reduce their water consumption, and prepare for emergencies.

Emergency Water Supply

Ground cisterns provide backup if disaster strikes or service disruptions affect your primary water supply. Having this water source can be lifesaving if you live in an area prone to flooding or hurricane damage, as you can fill your cistern with city water ahead of potential natural disasters.

Irrigation Systems

Cisterns make it possible to keep your lawn lush and green without wasting water. You can use your cistern’s rainwater or greywater to water crops and ornamental plants or offer it to livestock. Every drop of water you save is a step toward self-sufficiency and environmental conservation.

Supplemental Water Supply

Many homeowners use cisterns to reduce their dependence on municipal water. Having your own water supply can help you lower your water bills. This is especially advantageous if you live in an area with high water rates or one prone to drought, hurricanes, or service interruptions.

Fighting Fires

The water stored in your cistern can be a helpful resource for firefighters. This is especially true if you live in a remote area far from ponds or fire hydrants. Quick access to plenty of water can help protect your family and property.

On a recent TOH project in Ipswich, Massachusetts, the team faced this challenge firsthand. As TOH landscape contractor Jenn explained: “Over the last several years, rainfall has been below average here and the reservoirs have been dipping. The town of Ipswich has declared a severe drought and they’ve also placed a mandatory water ban. So you can’t irrigate your landscape.” The solution? Civil engineer John Morin designed a system to capture roof runoff and direct it to a cistern on the property, ensuring the landscape could be watered without using town water.

Cistern vs. Well

Cisterns and wells both help homeowners reduce dependence on municipal water systems, but there are key differences between them:

Depth: Underground cisterns are shallower than wells and typically go no deeper than 40 feet. Wells, meanwhile, are sometimes hundreds or thousands of feet deep so that they can extend past the water table and tap into abundant water sources.
Location: Cisterns can function above or below ground and are typically near your home. Wells should be further away from potential contamination sources, such as buildings or septic tanks.
Storage and capacity: Cisterns are essentially water storage tanks and can hold between a few hundred gallons and several thousand gallons. Wells draw water directly from aquifers, offering a potentially unlimited supply of water.
Water quality: The rainwater that cisterns collect typically requires filtration and treatment to ensure potability. The quality of well water varies and requires testing but may not require any treatment.
Water source: Most cisterns collect and store rainwater, while wells draw up groundwater from deep in the earth.

Pro Tip

Richard Trethewey, TOH plumbing and heating expert, points out a key drawback of well water: “That’s the thing about a well, you just don’t know what you’re gonna get out of the ground. When you have a municipal water supply, you have a certain assumption about water quality.” Well water can contain acidity, iron, radon, or other contaminants that require specialized treatment systems.

Water Quality

Quick Tip

Cistern water can harbor various contaminants, from harmless sediment to deadly bacteria. Test water from your cistern regularly to ensure it’s safe for drinking, bathing, and cooking.

Potential Contaminants

Leaves, debris, and animal droppings can wind up in your rainwater cistern, affecting taste and exposing you to harmful bacteria, such as E. coli. Naturally occurring minerals such as iron and manganese can also affect the taste and color of water, while heavy metals pose a more serious health risk.

Testing and Monitoring

Be sure to test your cistern water for bacteria, heavy metals, and other contaminants at least once a year. Monitor the acidity and hardness of your water to protect your home’s plumbing system. Local water authorities or private labs can provide testing services.

Pro Tip

Norm Abram, master carpenter on This Old House, suspected something was wrong with his own well water when he found “bright green stains in my bathroom fixtures.” Tests revealed the stains were from acidic water leaching copper from his pipes, plus high concentrations of calcium and magnesium. He installed a whole-house point-of-entry system to neutralize the acid and soften the water, plus a reverse-osmosis unit under his kitchen sink for “an extra margin of safety.” The lesson: visible signs like staining or discoloration in your fixtures are often the first clue that your water needs treatment.

Treatment Options

You can employ several cistern treatment options to remedy issues or eliminate contaminants. Filtration systems remove sediment and larger particles, while UV lights or bleach can help disinfect the water. A roof washer or flush-first diverter can prevent rooftop debris from reaching the cistern. Consider installing an acid neutralizer, water softener, or reverse osmosis system to address specific water quality concerns. Before considering a water softener system, you’ll want to familiarize yourself with water softener system pricing.

Our Conclusion

Cisterns can keep your garden green, help you to conserve water, and lower your monthly expenses. Rainwater requires treatment to be safe for drinking, but it can be a literal lifesaver during emergencies.

Cisterns are not a one-size-fits-all solution. Carefully weigh your specific needs and the initial investment against the potential long-term benefits to determine whether installing a cistern makes sense.

FAQ About Cisterns

What is the purpose of a cistern?

The primary purpose of a cistern is to store water for various uses, such as irrigation or emergency drinking water. Cisterns collect rainwater or water from other sources and store it until you need it.

How long does water last in a cistern?

Cistern water can last for months or even years depending on the tank size, water use, water quality, and maintenance required.

Is cistern water safe to drink?

Cistern water is not usually safe to drink due to potential contamination. Test and treat your water regularly to ensure safety.

What is the difference between a well and a cistern?

Cisterns typically capture rainwater or surface water, while wells tap into groundwater. Cisterns are also shallower than wells and offer limited storage capacity.

Can I install a cistern myself?

You may be able to install a cistern yourself, but this will depend on your skill level and the type of system you require. However, we recommend consulting a professional to ensure proper installation.