What Is Rainwater Collecting – Rainwater Collection System Basics

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Rainwater collection is the process of capturing, storing and reusing rainwater. There’s a lot to know about rainwater collecting and it’s uses, so let’s go over the basics to help you start collecting rainwater too!

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More people are beginning to collect rainwater as the climate crisis accelerates and more extreme weather creates longer droughts and depletes groundwater. A rainwater collection system is a great way to offset irrigation water use for your landscape or garden, reducing the need to use city-provided fresh water and paying for it. Rainwater harvesting is another interchangeable term you’ll hear and see used for rainwater collection.

Whether you choose a large custom-designed system or a simple rain barrel collection system, rainwater collecting is a great sustainable choice with many benefits.

This is a long post to cover the basics of rainwater collecting, its uses, and how to calculate the amount of potential rainwater. It also covers rainwater collection system options (tanks, cisterns, barrels), supplies, typical install steps, rainwater harvesting laws, costs, and a ton more common FAQs. Hop around if needed!

Rainwater Collecting For Beginners

Woman looking at her rainwater collecting system - DIY stacked rain barrels
Overjoyed when the system filled up with rainwater for the first time!

How Rainwater Collecting Works

Rainwater collection systems include a way to collect rain, direct the water, and a container to store the rainwater. A typical basic system will be using a roof to collect the water, a gutter and downspout to direct the water, and a barrel to store the water.

So basically, it’s just capture, store, and reuse!

Captured Rainwater Uses

Collected rainwater has many uses such as landscape watering, outdoor chores, grey water uses, and fire suppression/emergency water.

Hand Watering Your Landscape or Garden

Rainwater is a great way to water your landscape or garden without needing to use potable municipal water. It can save you some money on your water bill too. Many rain barrels or systems won’t always have enough water pressure to hook up a hose to it. Therefore, it typically means you’re hand watering your landscape or garden.

We use rainwater to care for our container gardens on our deck and backyard planting beds, and use a simple irrigation system and irrigation timer to water our vegetable garden. It’s a balance of finding what works best for your lifestyle and home setup.

Household Outdoor Chores

Beyond watering plants, captured rainwater can be used for other household outdoor chores. It can be used to wash a car or wash a puppy’s paws or people’s feet. Even refilling a bird bath or fountain is another captured rainwater use. Anything outdoors that requires water usage and isn’t being directly consumed, can use rainwater!

Watering the garden using water from a rainwater collecting system

Part of A Grey Water Recycling System

I’ll be honest, most of us will only start rainwater collecting to help water our yard and gardens or use for outdoor chores. However, rainwater harvesting can be part of a larger grey water recycling system. Grey water is any building’s water that’s not coming from a toilet (aka black water), like sinks, showers, and dishwashers. It’s typically filtered and stored in a tank and then reused for similar uses. Rainwater can be collected and connected to a grey water system for added water intake.

Grey water systems can be used for running dishwashers, flushing toilets, washing machines, and more. However, the house has to be built to utilize grey water (separate piping, metering, etc.) or has to be renovated and converted. It can take a lot of effort to create a grey water system if a house is not originally set up to use it.

Fire Suppression or Emergency Water

Lastly, another use for rainwater after collecting it can be for emergency situations. It can be used for fire suppression or emergency water. If somethings on fire and it’s the closest water source, use it. If for some reason the municipal water becomes contaminated (it happens!), your water needs to be turned off or some other case, you can boil the rainwater and use it for emergency potable water. Also, it doesn’t have to be a fire emergency, and could totally just be to put out your outdoor firepit!

Rainwater Harvesting Laws

Is rainwater collecting illegal?

It seems crazy, but the answer is yes, in some places you could be illegally collecting rainwater. While no state outright outlaws it, some put some very specific restrictions on how and how much rainwater you can collect. Thankfully, it’s completely legal and encouraged in a majority of states. World Population Review has a full list of rainwater laws by state in the U.S. If located somewhere else, then look up your local government’s laws.

Rainwater Collection System Types

Rainwater collection systems can vary greatly and seriously come in so many forms! Sizes can range from one 55 gallon barrel to over 5,000 gallon cisterns and more. The systems can be a basic DIY or more complex.. Here’s a couple of the typical collection systems.

Rain Barrel

Rain barrels are probably what most people picture when they think of rainwater collection or harvesting. A rain barrel is simply a container that collects and stores your rainwater with an inlet and an outlet. You can purchase one or DIY one (see where and how later on!).

Daisy-Chained System

A daisy-chained system is simply multiple smaller containers/tanks connected together through pipes or overflowing into the next tank. It allows you to expand on your system and store more with potentially cheaper containers. This is how we created our DIY stacked rain barrel system to add capacity vertically in a small space.

Above-ground Cisterns or Tanks

Above-ground cisterns and tanks are pretty standard, but do require space and access.

There are standard cylindrical above-ground tanks with wide diameters, and then there are more slimline tanks that are tall and rectangular.

Below Ground Cisterns or Tanks

Below ground cisterns or tanks are also an option and fairly similar to the above-ground tanks. However, these below ground tanks take a lot of effort to install properly and obviously digging out all the soil. Also, if not installed properly, they could “float” up to the surface and cause issues. Usually it’s cheaper and easier to just use one of the above ground rainwater collection systems.

Required Elements To Start Rainwater Collecting

Every rainwater collection system has a few key components.

  • Inlet – typically a downspout directing rainwater into the barrel/tank
  • Outlet – typically a spigot or hose bib connection to allow rainwater to be dispensed.
  • Overflow – all systems need an overflow point if the tank gets to full capacity. This can simply be an opening at the top or a set of additional piping to redirect water to the downspout.
  • Debris Collection/Mesh Screens – Screens over all openings are important to reduce any debris in your tank (clogging the system), and to prevent insects or pests from getting in.
  • Made of Non-transparent Material – Whatever container used needs to be opaque. This is important as sunlight and water can mean algae growth, which you don’t want. Ensure your barrel or tank material completely blocks sunlight.

Where Do I Get A Rainwater Collection System?

There are a multitude of options for rainwater collection systems as evidenced by the number of different types of barrels and tanks that exist.

Home Improvement Stores & Online

You can get basic rain barrel systems at home improvement stores like Home Depot or Lowe’s. It’s typically a one barrel system that holds between 50-100 gallons. You can also purchase rain barrels online through Amazon, Overstock, or Wayfair.

If you’re trying to reduce costs or not buy new, check out Ebay or Facebook Marketplace for people selling rain barrels secondhand.

DIY Rain Barrel Systems

If you’d rather make your own system and maybe save a little money, then building a DIY rain barrel is pretty common. All you need is an opaque barrel, spigot, and overflow valve. There’s a ton of DIY videos and DIY kits out there.

Being big DIY-ers, we actually designed and built our own Stacked Rain Barrel System. It’s a little beyond the basic rain barrel DIY’s you find online, but we love it. Having a stacked system can increase water pressure and take up less space because it’s vertical. We always make DIY plans for our projects too, so check them out.

How Much Water Can You Collect Through Rainwater Harvesting?

If you know the roof square footage and look up monthly rainfall in your area, then calculating the total amount of water collected is straightforward. 

Catchment Area (roof square footage) x Monthly Rainfall (inches) x 0.62 (conversion factor) x Collection Factor (75%-90%, Some loss dependent on the surface).
Catchment Area

This just refers to the surface area of where the water is caught and flows down to the lowest point. Most roofs are divided up into several catchment areas due to the pitch/slope of the roof. This is why you have multiple downspouts capturing and draining the rainwater from your roof.

Considering that the roof has multiple downspouts, not the entire roof area will drain to one area. It’s important to understand this as you’re determining where to connect your rainwater collection system. If you choose a small catchment area (roof drainage area), then you’ll collect fewer gallons of rainwater. If you choose a larger catchment area, then you’ll collect more gallons. Determine what best fits your needs.

Monthly Rainfall

You can find your monthly rainfall with a quick internet search or look at NOAA’s precipitation frequency data tables. Typically, you’ll look at a 24-hour event and a recurrence interval (look at the lower end of 1, 2 or 5 year storms). If you’ve never seen rainfall tables, then it might be a little confusing. But seriously, just get an idea of how much monthly rainfall there is in your area. It doesn’t have to be perfect!

Collection Factor

This is all about the surface that the water is running over. Different types of roofs and surfaces will have a loss factor meaning you aren’t going to collect 100% of the rainfall off your roof. Unless you have a really smooth roof (like a steel roof), then I’d assume a lower percentage collection factor like 75% or 80%.

Example Rainfall Calculation

Here’s a quick example calculation for a 1,000 square foot roof with a monthly rainfall of 3.28 inches and using a 80% collection factor.

1,000 x 3.28 x 0.62 x 0.8 (80%) = 1,627 gallons per month

I did these calculations in college, and then watched many civil engineers I worked with in the past run calculations to ensure enough stormwater (aka rainwater) was collected on the design project site.

I’ll be honest, we could have done the calculations to determine how much we could collect, but we didn’t. We built ourselves a DIY stacked rain barrel system with three 55 gallon barrels and were satisfied with whatever amount of water we collected. FYI our barrels were full after one decent rain!

Installing A Rainwater Collection System

Depending on the type of system you choose, the specific install steps will be different. However, the following steps are common and need to be considered for most systems.

Determine Location

You’ll want to choose a location near a gutter downspout with convenient access for filling watering buckets or near the garden/planting beds. You’ll also want to ensure the downspout you choose collects a good amount of roof water runoff per the calculations mentioned previously.

Create Strong, Sturdy Level Foundation

Another key step is ensuring the foundation the rainwater collection system sits on is sturdy and level. Some sit on gravel beds or concrete pads, while others on an elevated platform. It’s best practice to read the manufacturer’s instructions and follow their recommendations. If you’re DIY-ing it, then do a little research on what others did or go ahead and pour your own concrete slab like we did.

The barrels and tanks are pretty hard to topple over when full of water, but definitely better to be safe than sorry!

Showing a DIY rainwater collecting system sitting on a concrete slab
We poured a concrete slab and created a strong frame to elevate our rain barrels

Gravity & Head Pressure Are Important

Typically, many of the smaller rainwater collection systems are elevated somehow. This is important because elevated containers take advantage of gravity. Water creates head pressure due to its natural heaviness exerting weight and pressing down on itself. So the larger volume of water and/or higher off the ground, the more head pressure will come out of the outlet point.

Very large, tall tanks can sometimes exert enough pressure to connect a hose to and get water flow downhill from the system. It’ll never be like an irrigation system unless a booster pump is used.

Secondly, elevating a water container allows for enough space below the spigot or hose bib to fill a bucket or watering can. Make sure to plan to have your system at least slightly elevated, so you can fill up a decent sized bucket. Otherwise, you might be making a lot more trips!

Clean Systems Using Screens & First Flush Diverters

Lastly, another key element during the install is providing ways to keep the system clean. Keeping the system clean and free from clogging debris is important. If you’ve ever cleaned a gutter, you know rooftops collect a ton of leaves, twigs, sediments and insects. You’ll want to keep this debris out of your rainwater storage.

This can be done in a few ways. Using an initial fine-mesh screen on the gutters or downspout openings can catch all that initial large debris washing off the roof. Most rainwater tanks have additional screens built-in to help prevent the secondary debris and insects from getting in. 

Additionally, you can add what’s called a “first flush diverter” to the system. It’s pretty much an extra set of piping that redirects the first rush of water off the roof, and captures any debris and sediments in that piping. Once that piping is full, it blocks the chamber and allows “fresh” water to flow into the rainwater storage tank.

It’s worked best for us to have both screens and a first flush diverter to keep our DIY rain barrel system clean!

Is Rainwater Harvesting Worth The Cost?

Depending on the size and cost of the system, the payback period (aka time until you’ve saved the same amount as it cost to buy) can vary greatly. It could theoretically be “years” before you pay it back or only a few months with a small system.

The average price per gallon of city water varies by state and/or country and utility companies can charge by different rate types, so it can be a little difficult to breakdown. 

The EPA claims that 30% of the average water used by families per day (106 gallons) is devoted to outdoor uses and landscape irrigation accounts for nearly one-third of all residential water use (~9 billion gallons per day in the US!). If you’re watering a garden or landscape, you can easily use100 gallons or more on watering based on my experience. We have an irrigation timer with a water sensor that records how many gallons of water is used each time.

Benefits Beyond Saving Money

Beyond the potential water cost savings, there are environmental reasons for rainwater collection. It can offset a portion of your household’s water usage. In some areas, water scarcity occurs especially in droughts and prevents people from using city water.

Additionally, storing water onsite helps to slow down rainwater in large storms. Many city infrastructure systems are struggling with stormwater (aka rainwater) management as the piping systems become overwhelmed in big storms and lead to flooding and damages nearby or further downstream.

Lastly, it can add to people’s feeling of self-sufficiency by having water stored at their home in case of an emergency.
These reasons are why we chose to create a DIY stacked rain barrel system, rather than the potential cost savings. Whatever your reason, it’s great!

Jumping for joy the first time the system is full! Will be adding a decorative wood screen around it soon.

Can You Drink Rainwater

Although it wouldn’t be my first choice to drink rainwater, there are a few ways to treat rainwater. These will more than likely keep you from getting sick, but not guaranteed.

  • Boil The Water – boiling the rainwater for several minutes will help kill most pathogens
  • Add Bleach – adding some NSF-approved household bleach to the tank will also help kill pathogens. Ensure you have the proper ratio of bleach to water.
  • Add A Filtration System – running rainwater through a high-end filtration system that guarantees to remove parasites, bacteria, and contaminants can work.

If you’re looking for emergency water, then you might consider IBC tanks filled with drinking-safe water separate from your rainwater collection system. Again, you can buy IBC tanks new online or look for secondhand options on Ebay or Facebook Marketplace.

More Sustainable Living Ideas

Now you know a ton about rainwater collecting and are hopefully thinking of getting started. Make sure to check out some of our other sustainable living articles like our sustainable living journey or our going solar series. There’s several DIY projects and plans in our DIY archives to help you be sustainable!


Hope this guide on rainwater collecting and rainwater systems helps you get started on your rainwater harvesting journey! Let us know if you have questions or any suggestions in the comments below!

Disclaimer: This post includes affiliate links, and I will earn a commission if you purchase through these links. Please note that I’ve linked to these products purely because I recommend them and they are from companies I trust. There is no additional cost to you.

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