Building a rainwater collection system off of a roof for rainwater harvesting is a great way to supply water to your garden. It also can help with runoff and soil erosion.
Depending on where you’re located, you may not need to use the water collected after each rain, but it is helpful to have the extra storage during the warmest parts of the year.
The amount of water you can store varies depending on the size of the rainwater storage tank you choose. This article covers the key components of a rainwater collection system, rainwater storage containers, rainwater system design, and other details.
Components in a Water Collection System
Rainwater Catchment Surface
Gutters & Downspouts
1. Rainwater Catchment Surface
The most common rainwater catchment surface in a system is the roof of a home or other building. The large surface area directs the water into gutters that feed into downspouts.
If you don’t have access to a roof, it’s easy to get creative with the catchment system. You can use a tarp pulled taught or recycled metal roofing or siding mounted at an angle to mimic a roof (similar to a lean-to shed). The larger the roof area, the quicker your storage tanks will fill.
Properly sloped gutters will direct water into a downspout (vertical gutter pipe), which directs the water to the ground or into your water collection system.
Preventing debris from entering your rainwater collection system is vital to the longevity of the system and the cleanliness of the water.
Integrate a filter into the downspout. This is your first line of defense from any large debris that may enter your system.
Installing a first flush diverter after the filter is a secondary way to improve the quality of the rainwater collected.
What is a first flush diverter?
A first flush diverter is a device designed to divert and capture the initial runoff of rainwater during a rainfall event, which is often the dirtiest water. This initial runoff can contain dust, debris, bird droppings, pollutants, and other contaminants that have accumulated on the roof's surface since the last rain. By diverting and discarding this "first flush" of water, the subsequent rainwater collected in a storage tank is typically cleaner and safer for use.
Consider installing an additional fine sediment filter at the point the water enters the water collection tank. The fine sediment filter will capture small particles and prevent it from contaminating or clogging downstream parts of the system.
4. First Flush
A first flush is not a necessity in a water collection system, but it will help with disposing of the dirtiest water.
There are several different ways to include a first flush into your system. The most efficient method is purchasing a downspout filter that has the first flush diverter integrated. Keep in mind that a larger roof area will need a larger first flush capacity - the exact sizing can be determined with the help of the manufacturer.
A first flush diverter consists of a chamber or pipe that temporarily holds the initial runoff from the roof. The pipe contains a ball or float that rises with the water level. When the pipe is full, the ball or float closes the chamber and directs future incoming water into the rainwater system. The water in the first flush chamber is held until the end of the rain event when a slow-release valve drains it into the downspout to reset for the next rain.
As rainwater storage containers fill and empty, air must also be able to enter and exit to maintain equalized pressure. This is accomplished with a two-way venting device or a simple hole in the top cap of the tank. Purpose-built rainwater storage containers typically include an integrated solution for venting. Avoid obstructing this vent to prevent pressure build-up.
Including an overflow pipe in your system will prevent water from leaking through vents or backing up into the supply pipe.
An overflow is commonly connected to the outlet of the rainwater storage system. The height of the overflow pipe must be equal to or just below the height of the container(s). As the water level rises in the container, the water level will also rise in the overflow pipe due to hydrostatic pressure and eventually flow out once the tanks are full.
There are several different options for storing rainwater, but the two most common are 55-gallon drums and IBC totes (intermediate bulk containers). You can increase the amount of storage by linking the tanks together.
Determining the Size of Rainwater Storage Tanks
Determine the amount of rainwater you need to store by evaluating monthly water usage and average monthly rainfall.
The recommended amount of water for a garden is 1 inch of water per week. This varies depending on time of year, climate, sun exposure, and soil type. In the cooler months, a garden may require less and in the warm months, a garden will likely require more.
An inch of water is equivalent to 0.62 gallons per sq. ft. (reference: What is an Inch of Water) Using this as a starting point, you can determine the required storage capacity.
For example, if you have four 3 feet x 10 feet beds, the total garden area is 120 sq ft.
The garden would need approximately 74 gallons of water per week.
This information can help you size storage for your needs, but ultimately, the system's size may be limited by budget or space constraints.
Rainwater Storage Tank Options
1. Decorative Rain Barrels
Commonly available at home improvement stores, a decorative barrel will typically store 50-100 gallons of rainwater. This option is the easiest and most aesthetically pleasing choice, but also is more expensive. If purchased new, a decorative barrel is usually over $100, with higher quality and larger sized options extending into the $300-$500 range. A major benefit of ready-made rain barrels is the accessories such as a filter screen and hose connection come pre-installed, saving time and labor compared to other options.
2. Food Grade 55 Gallon Drum
An option for a budget DIYer is to collect rainwater in 55-gallon food-grade drums. These drums were previously used for safe storage of consumables and are often available at local food processing facilities. If you aren’t able to find one for free, the average price hovers around $20 to $30, making them an economical alternative to purpose-built tanks. However, there is some due diligence required when sourcing these barrels. Ensure they're labeled as "food-grade" to guarantee they previously stored non-toxic substances. Some may have held items like pickles or olives, leaving behind strong odors that could impact water quality, so a thorough cleaning (preferably by the previous user) is important. To save yourself this step, you can find them brand new, but this may erase the savings potential compared to buying a ready-made rain barrel. Regardless if you find one used or buy new, you will need to modify the barrel to fit your system with features like a screened inlet, hose connection, and overflow tube. If the barrel allows light in, it will need an opaque cover to prevent algae growth. With a bit of effort, these versatile and budget-friendly containers can become the core of your rainwater harvesting system.
3. Food Grade IBC Tote
Similar to the 55-gallon drum, IBCs (intermediate bulk containers) are lightweight, cube-shaped tanks that are ideal for rainwater collection. The main advantage of IBCs compared to 55-gallon drums is their size - the most common capacities are 275-gallon (1041-liter) and 330-gallon (1250-liter).
Additionally, their cubic shape and integrated pallet makes them easier to transport, stack, and build a decorative disguise around once placed near your home or garden.
Similar to the 55-gallon drum, you will need to ensure they are rated for food use, and exercise caution when buying used to ensure no toxic substances were stored previously.
IBC totes do have drain valves at the bottom, but there is no standardized connection or thread for these outlets. To connect them with your rainwater system, you will need to find the appropriate plumbing fitting to make the connection. Most IBC totes are a translucent plastic which lets light into the tank, so an opaque cover is necessary to prevent algae growth.
4. Large Volume Rainwater Collection Tanks
If you have major water needs or are designing a system for potable use, a purpose-built, large-volume water storage tank may be the best option. Capacities start at about 500 gallons, and there are options into the tens of thousands of gallons.
This capacity comes at a cost, which will be in the several thousand dollar range depending on tank size. Typically, these large tanks come rated for potable water use, meaning with a proper collection and filtration system, you can satisfy household and drinking water needs with rainwater.
Rainwater Harvesting System Design
Building a rainwater collection system for rainwater harvesting from a roof or other surface is a great way to supply water to your garden.
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Increasing Water Collection Capacity
The most economical and practical method of increasing your rainwater collection capacity is to link multiple tanks together. This is a relatively simple process, but there are few things to keep in mind:
Use a common size and type of storage tank (barrel, IBC tote, etc.) across the system to simplify the foundation design.
Set the tanks on a level foundation to ensure that the water level does not reach the overflow point before all tanks are completely filled - any tanks higher than the rest will result in unused air space at the top of the tank.
Plumb the tanks together at the bottom, which allows hydrostatic pressure to fill all tanks simultaneously, as opposed to filling one tank until it overflows into the next.
Ensuring all tanks are level from the start allows a common overflow pipe to be used, which prevents the need for additional holes in tank walls.
This modular approach of linking tanks allows you to expand or contract your rainwater storage capacity without any major design limitations.
Collecting Rainwater for Drinking Versus Gardening
If you're considering rainwater collection for drinking, installing a high-quality filtration and disinfection system is critical for your health. In addition to collecting sediment and debris from roofs and gutters, rainwater collects harmful physical and chemical contaminants from the air while falling. To establish a baseline of what contaminants are present in your rainwater, send a sample for laboratory testing. This will help determine the starting point for the wide array of filtration system options available.
While a simple filter screen is sufficient for garden use, filtering rainwater for potable use should start with one or more sediment pre-filters to remove fine particles, protecting more advanced downstream filtration components. Once the water is free of light-blocking sediment, a UV light filter should be used to kill bacteria or viruses that may be present. To achieve the cleanest drinking water possible, a reverse osmosis filtration system is one of the most effective and popular options. These systems will typically incorporate an activated carbon filter to remove a wide range of chemical contaminants before the reverse osmosis process. There are a host of other filtration options such as acid neutralization, iron removal, deionization, and distillation - the best way to determine the right setup is a lab test and consultation with a reputable water quality professional.
Rainwater filtration can be limited to a basic filter screen for garden use to prevent clogs and protect pumps. After all, unfiltered rainwater falls onto the garden all year.
Transporting Water to the Garden
There are two options for transporting harvested rainwater to the point of use - gravity or a pump.
1. Gravity Fed Irrigation
Gravity has some great advantages - no moving parts, no electricity, and reliability. However, the main limitation is slope. If your garden is slightly uphill from your rain barrels, water won’t flow. Elevating your rainwater storage tanks can help, but there is also a limitation to how high you can go. If your point of use lies lower than your storage system, hydrostatic pressure will be enough to feed a low-flow drip irrigation system or supply low pressure to a garden hose.
2. Irrigation with a Pump
Fortunately, small pumps have become fairly reasonably priced, especially if you can plug them into an outdoor outlet and don’t need to include a solar setup in the cost. Pump sizing for an irrigation system can be a somewhat complex task that depends on variables such as elevation gain, pressure and volume output needed, and the type of irrigation emitters. However, the lowest cost pump will be a small pond pump (3-5 gallons/minute), which should be able to feed most drip irrigation setups at a slow rate. You could also opt for a larger (slightly more expensive) pump that is capable of supplying higher pressure and flow, and use a normal garden hose at the point of use like you would if the source was a well pump or city water supply.
Rainwater Collection System Maintenance
An outstanding feature of rainwater collection is how passive and low-maintenance the system is after installation. However, there are a few key maintenance tasks to be aware of to keep the system healthy:
Clean filter screens monthly
Drain the system before freezing weather
Use an opaque cover over the tanks to prevent algae growth
Clean storage tanks yearly to remove sediment or algae growth
Inspect piping and storage tanks yearly to assess for cracks or leaks
Test valves and pumps yearly for proper operation
More advanced systems will require more maintenance and care, but these basic maintenance guidelines will keep most rainwater collection systems ready for use.
Collecting Water for Your Garden
Rainwater collection is a great way to water your garden in the warmer months! Building your own rainwater collection system requires planning but once you build the system, you’ll be able to use it for years to come!