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How to Choose and Use a Home Rainwater Catchment System

How big is your H2O footprint? With water shortages a growing concern, it makes sense to calculate and then try to reduce it. Once you do, a rainwater catchment system will guarantee that the water you do use comes directly from the sky without the environmental burden caused by relying on your local water treatment plant.

How to Choose and Use a Home Rainwater Catchment System

The simplest rainwater catchment systems provide water for irrigation and washing cars, patio furniture, etc. The next level allows for non-potable indoor uses like toilets and laundry. The most complex can provide water suitable for any use—including drinking. Here’s what you need to know to choose and use one.

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How They Work

First, the roof captures the rainwater. If the water is going to be used for drinking, certain roofing materials such as asbestos and tar are not recommended. Next, a conveyance system using slightly modified gutters and downspouts directs water into a storage tank or cistern. Tanks should have an overflow system consisting of a hole near the top and a hose or pipe directing excess water away from the house.

Rainwater Catchment System
Rainwater Catchment System

The final components deliver water for use. Simple setups have a hose or faucet attached to a collection tank; more complex ones direct water into a pump and then possibly through filters and a sterilization system. Roof washing devices are available that divert the first 10-15 gallons of rain away from the collection tank, keeping the majority of contaminants such as animal droppings, pollen, and soot out of the system.

Rain Barrels

A rain barrel is the simplest way to harvest rainwater. It consists of a moderate-size container, usually between 40-100 gallons tied directly into a downspout. These can be purchased for around $100 or made for $20-$50. Making your own is fairly simple once you locate a container. Detailed instructions can be found on the WikiHow website.

No clear regulations are available for using untreated collected water on food plants. It may be best, however, to avoid using the water on vegetable gardens and fruit trees, or at a minimum to use water only at the base of the plant to avoid getting contaminants on any edible parts.

Non-Potable Indoor Use

A system with a 500- to 4,000-gallon cistern could provide both outdoor irrigation and the water for laundry and flushing toilets. While the water is filtered for particulates, it is not treated for drinking.

After water is collected from the roof into the cistern, it moves into a pump that generates the pressure to move the water through a separate plumbing system. Filtration can occur in the downspouts, inside the tank, or after the water leaves the cistern. Plastic and metal cisterns are available at a cost of $500-$2,000, depending on size.

Potable Indoor Use

These elaborate systems are intended to provide all the water for a household. However, it is recommended that another water supply also be available in case of sustained dry periods.

Rainwater Harvesting System Diagram
Rainwater Harvesting System Diagram

After water is collected in the cistern, a pump moves it to an indoor storage tank. Then the water is treated using filters, ozone or UV irradiation to make it safe to drink. Finally, it is piped throughout the house. Local plumbing codes vary, but most require a backflow device that prevents the water from flowing back into a public water supply.


Maintenance for water catchment systems includes keeping the gutters and downspouts clear, cleaning or replacing filters, and periodically cleaning the storage tank.  Potable water systems also require additional maintenance on the sterilization system and testing of the water for contaminants and bacteria.

Choosing a System

To decide on a system, evaluate how much water is needed and for what it will be used.  For watering gardens and flower beds, one or several rain barrels should suffice. A 500- to 2000-gallon cistern would be needed for larger-scale irrigation and indoor non-potable uses.

For a potable system, determine the total water needed using the following calculation. Assume 40 gallons of water used per day per person. Next, determine the square footage of the roof. On average, every 1000 square feet can store 550 gallons per inch of rainfall.

For example, a household with two people would require 29,200 gallons per year. With a 1000-square-foot roof, it would have to rain 53 inches a year to provide enough water. Of course, a larger roof would require less rain.

For most, rain harvesting cannot fulfill all water needs. However, diverting rainwater for outdoor and indoor non-potable uses will lower your water bill while helping reduce demand on public water supplies as they become increasingly stressed. It also limits the amount of runoff into streams and storm-drain systems—minimizing your environmental impact. All in all, a water catchment system is an easy and low-cost way for you to live a more sustainable existence.

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