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rainwater collection

How to Collect Rainwater

By Stephen ColletteRise Writer
Sep 16, 2019

We all can collect rainwater at our homes. Unless regulated by your jurisdiction, collecting rainwater can help save precious clean water—and reduce your water bill. The approach does not have to be all-or-nothing; you can collect rainwater in many ways. So, let’s jump in and splash around and see how we can all save some water!

How Much Rainwater Can I Collect? 

The answer to this question depends on two variables: your average yearly rainfall for your region and the area of your roof. For those living in the U.S., you can find your annual rainfall data here.

When calculating your roof area, you will want to calculate the horizontal plane since water falls from the sky vertically. If you are capturing rainwater from only one downspout, you will need to know your roof's area that pitches to that downspout. The key metric? Every inch of rain produces 0.62 gallons of water per square foot. So for a 10 x 10 area, one inch of rainfall will give you 62 gallons. To figure this out for your home, use this handy online calculator (here is the metric version). But before you begin, make sure your jurisdiction allows you to collect rainwater and find out if there are any usage limitations.

How Can You Use Rainwater Around Your Home?

Homeowners can use rainwater for all household needs. Historically, and in many farming communities around North America, homes had whole-house cisterns. Families would locate their cisterns in the farmhouse basement, and all the downspouts around the house drained into them. Some homes still operate 100-year-old open storage tanks. We don’t recommend open containers for safety and water quality reasons. When set up and sized correctly, closed cisterns can supply water to the whole house as needed.

Outdoor Gardening

The most common use of collected rainwater is for watering your flowers and gardens. Rain is good for plants, as it is naturally soft and has not been treated with chlorine, fluoride, etc. The amount of rainwater you need to collect depends on your garden's size and water needs. Typically, though, you would not need a whole house water collection system just for your gardens.

Indoor Plumbing Fixtures

Homeowners can use rainwater for laundry and flushing toilets, but there are complications. First, not all jurisdictions allow this. Second, you cannot connect the rainwater plumbing system to the rest of the water lines in your home. It should be a separate water system with its own water lines that are correctly and uniquely labeled. Some cities require different colored pipes, while others require labels every several feet stating it is rainwater.

If you want to use the water inside for flushing toilets and laundry, you will need to understand how much water those appliances use in an average week. This allows you to size the storage tank appropriately. You may want to use it for one or numerous fixtures, so you will have to be specific in your calculations. Choosing low-flow toilets and efficient ENERGY STAR washing machines will help you reduce your need for water in the first place.


Before using rainwater for drinking, you will want to know whether the water collected is safe. That answer depends on where you live as well as the material of your roof. Asphalt roof shingles are not ideal for drinking water because some of the bituminous materials abrade off and into the water, making it unhealthy. Wood shingles may be okay for drinking water, but some tannins can stain the water, change the taste, and affect water quality. Metal roofs are an excellent option for drinking water collection; make sure the roof’s finishes do not contain chemicals. Terracotta or cement tile, standard in many hotter regions, is ideal for collecting rainwater for drinking.

Components of a Rainwater Harvesting System

The main parts of a rainwater harvesting system include the collection area, the pre-filter, the storage tank, more filtration, and then supply lines.

  • The roof is the collection area. The water runs off the roof, down the downspouts, and into a storage tank. 
  • The pre-filter captures the leaves and debris coming off the roof. Typically most of the debris is in the first few gallons (liters) of water, so you can use a “first flush” approach to divert the first few gallons (liters). The rest goes into a storage tank with a bug screen-sized filter added to capture leaves and larger bits from the roof.
fitzwater rain system
Fitzwater raintank. Photo Credit: Gardenista
  • For a storage tank, you can start small with a rain barrel. Rain barrels can be architecturally beautiful and blend into your style. Accessible to all homes, you can install easily and for relatively low costs. For more storage, you can install rain cisterns under your deck or porch, like this Aquabarrel cistern.
under deck rain storage
Photo Credit: Aquabarrel
  • For even more storage, you can install above or below-ground cisterns, which come in a wide range of materials. Indoor tanks are typically for large-scale buildings but can be done on a residential scale and stored in a basement
  • If you have an indoor system, you need overflow protectors and flush-out systems. These systems can get costly and complicated, depending on your usage. There are numerous rainwater harvesting system manufacturers. It does not have to be a DIY approach, which can lead to water leaks and problems. 
  • If using the rainwater indoors, you need stepped-up filtration, including micron filters and a chlorination component. Whatever your usage, you will need to filter the rainwater you collect. The level of filtration is a function of your roof and your final use. If you are using it to water the plants, you only need to keep the leaves and mosquitoes out. A simple screen can be used to filter out debris. If you plan on drinking it, you will need to filter it for solids—with a micron filter. Typically, a chlorine-based approach will remove any potential bacteria before hitting the drinking tap. Using it for toilets and laundry requires some filtration level and will depend on the building code in your state. 
  • Finally, from the post-filter, the water goes to its end-use. Keeping the plumbing lines visible helps keep an eye on how well the filter is working and whether there are any issues.

How Much Are Rainwater Harvesting Systems?

A simple rain barrel collection system is relatively inexpensive, running between $50-$150 depending on size and add-ons (like hose attachments). Large rainwater harvesting systems can cost as much as $12,000 with a national average of $2,500.

rainwater collection
Photo Credit: Fixr

The more you understand your actual usage of water, how much and where you are using it, the easier it is to determine the value of any size system. Knowing where your water goes is critical, as a 10,000-litre system to water your balcony plants is overkill, and your financial payback will be centuries. Knowing how much your toilets or gardens use helps you purchase a system that suits your needs. The payback—how much savings you realize on your reduced water bill to pay for your investment—will always be more reasonable when sized appropriately.

Bottom Line

Harvesting rainwater is an excellent way to reduce water usage and save money. Our plants and gardens do better with rainwater instead of chlorinated water. It also helps you manage your stormwater on-site. Rain barrels can look like architectural details, making them accessible to all. If you want to take the step, remember to size it appropriately. Knowing where you consume the water, the amount of water you need, and how much rain falls in your area can help you design and install a system that truly fits your needs. 

Disclaimer: This article does not constitute a product endorsement however Rise does reserve the right to recommend relevant products based on the articles content to provide a more comprehensive experience for the reader.Last Modified: 2021-01-22T00:09:58+0000
Stephen Collette

Article by:

Stephen Collette

Stephen Collette is a Building Biologist, Building Science Consultant, LEED Accredited Professional, and a Heritage Professional. Stephen is the owner of Your Healthy House and lives in Lakefield, ON with his wife and 2 daughters.