Introductory Guide and Overview on Rainwater System Design
The steps to designing a rainwater harvesting system will depend on a few key points that are unique to the practice of harvesting rainwater. In general, rain collection systems follow a similar and fairly basic blueprint that makes all systems alike.
The result of a well-designed rainwater harvesting system is a setup that is simple in operation, low in maintenance, yet highly effective in providing large volumes of useable water.
This post is written for people new to rainwater harvesting and collection systems as well as those looking to improve their current setup and ensure they are not missing anything and being as effective as they can.
Guide Contents
Rainwater Harvesting Blueprint
For a rainwater harvesting system, the basic blueprint has four parts: (1) a catchment area, (2) a channeling system, (3) a collection container, and (4) accessories.
Each of the four above parts are recommended to be made from certain materials, properly maintained, and can include additional parts to improve how well the system works and the cleanliness of the water.
At its most basic, a brief overview and definition for rainwater harvesting: (1) rain falls on a surface, (2) then runs off into gutters, downspouts or pipes, (3) and into something like a container to store the harvested rainwater for use when needed; (4) rainwater harvesting accessories are added onto a setup to improve water quality and reduce maintenance.
The simplest of rain harvesting setups may not use rain harvesting accessories, while some rain collection containers may have a few built into the collection container. Which rain harvesting accessories are needed can and will likely vary based on your unique application, where you live, your environment, and how you will use the rainwater.
Pre-Design Considerations
Before expanding on the basic blueprint for rainwater harvesting, we will look at eight important pre-design considerations. These are part of the key points mentioned at the beginning and help in the overall design and planning process for a rainwater harvesting system.
1) What Will You Use the Rainwater For?
Knowing what you will use the rainwater for will help to plan your overall setup and choose the best materials, accessories, container size, container type, and placement. Using rainwater outdoors to water plants or the lawn may only require a simple setup, while using rainwater for fresh, drinkable water or in-home fixtures and appliances will require a more complete, complex system. Basically, the cleaner you want the rainwater, the more top-quality materials and accessory components you will need.
For more on rainwater uses, see our post on What Can I Use Rainwater For.
2) How Much Rainwater Can You Collect?
How much rainwater you can collect directly relates to the size of your rain catching area, often a roof, your rain collection container, often a rain barrel or rain tank, and expected rainfall1. Unless you will build a dedicated structure, the size of your rain catching area is often fixed, limiting the amount of rain you can capture to the area size and how much rain you get. The amount of expected rainfall is also limited to where you live.
This leaves the collection container as the main part you can adjust to increase how much rainwater you can collect. If you use a 55 gallon rain barrel, you will only be able to harvest 55 gallons of rain at a time. If you increase the size of your rain container, you can collect more rainwater.
About 623 gallons of rainwater can be collected from every 1 inch of rain that falls on a 1,000 square foot catchment area.
Here is the equation to do the math for your own situation:
Rain Catching Area (in Square Feet) x Volume of Rainfall (in Inches) x 0.623 = Rainwater Harvest (in Gallons).
To find out how much rain you can collect, know what your average rainfall is per month, season, or year and know the square footage size of your catchment area. Using governmental data records2 or reputable websites to approximate regular rainfall is good for estimating how much rainwater you can collect.
3) How Much Water Do You Need?
How much water you need to collect from rainwater harvesting will depend on what you plan to use the water for. Estimating rainwater needs can be done by reviewing past water bills or by referring to online resources for average water use in household applications. Research suggests U.S. household water use3 averages anywhere from 25 gallons to 100 gallons of water a day per person.
Using rainwater to water lawns and gardens is one of the most common uses and can be a large volume of water. In town and city settings, as much as 70% of regular water use can go to watering lawns and gardens during the growing season. If you have a water bill, you can look at how much water you used during the growing season and plan your rainwater container around this volume. Using rainwater to replace this can help with public water supply, restrictions, and lower your water bill.
4) What Size Rain Container Do You Need?
Choosing a rain container size directly depends on what you will be using the water for, how much water you need, and how much rain you can collect. Almost any size is available. Modern rain containers range in size volumes from small, few gallon containers to 100,000 gallon containers and more.
Choose a rain container that will store enough rainwater so you won’t run out and will have water when you need it, including during dry times when it does not rain. Choose a rain tank that will be big enough, but not too big, and do not choose one that will be too small.
Often in rainwater harvesting, a bigger tank is a better tank. Just make sure you have the right location and amount of space to fit the container.
5) Where Will You Place Your Rain Tank?
Consider the layout of your property and whether you have space for the rain tank size you have chosen. In rainwater harvesting, rain tanks are often placed as close as possible to the structure they will be collecting rainwater from. This improves convenience, reduces the amount of installation work and plumbing materials, and therefore lowers costs.
Review size measurements for available rain tank products. Unless choosing a rain barrel, a rain tank can have a substantial size that can make moving it to where you want it difficult or problematic if there are fences, gates, or other property structures in the way. The slimline rainwater harvesting tank was made for fitting through doorways and tight spaces to make installation easier, especially in residential backyards, while still offering more volume storage than a rain barrel.
6) What is Your Environment Like?
The environment plays a role in what contaminants and debris may commonly be in your rainwater and can therefore provide insight into what rain harvesting accessories you may need. Contaminants make it into the rainwater mostly from the catchment area4.
Environmental factors include nearby vegetation, animal life and activity. If there are trees and frequent animal activity, then you should expect and prepare for related debris. If the environment is more urban with limited trees and animals, or none, then you should expect rainwater that is mostly free of debris.
Details such as human-caused air pollution, pollen, animal droppings, and harmful microorganisms can also be a factor in the environment but are typically only important when using rainwater for potable water uses.
7) Will You Need Pressurized Water and Therefore a Water Pump?
A water pump is used to provide pressurized water from the rain container. Pressurized water is needed to run irrigation systems, such as sprinklers, and to distribute water to faucets and appliance taps for use. Without a water pump, water flow may be very slow and fairly inefficient for use. A pump is not needed when simply using rainwater for watering gardens or landscapes with a watering can or hose.
8) Does Your State or City Offer a Rainwater Rebate or Grant?
With increasing awareness and promotion of rainwater harvesting as an alternative water source, many U.S. states and cities are beginning to offer incentives in rebates, tax deductions, and even grants for new rainwater harvesting setups. These are most often provided on the city level. State-supported incentives are often for larger setups, businesses, and/or agriculture. What is offered can vary extensively based on where you live. There is often a pre-approval process and forms that need to be filled out prior to purchasing and installing a barrel, tank, or other parts.
Doing a little research and making some phone calls to your local state or city government departments could save you money when putting in a new rainwater harvesting system. The top US states with rainwater harvesting incentives are Texas, Arizona, California, Iowa, Illinois, Kentucky, and New Mexico. For more on this, see Top 5 States for Rainwater Harvesting with Government Supported Incentives.
Rainwater Catchment Areas
A rainwater catchment area is a term used for the surface where rain will fall and be collected from5. Most often this is the roof of a structure such as a home, business, barn, shed, etc. They can be called collection surfaces and capture areas. A rainwater catchment area can be anything that provides a spot for rain to fall on then flow from into gutters and/or plumbing then channel the rainwater into a container.
Ideally, the catchment area will be large enough to collect enough rainwater based on where you live and for your use when and how the water is needed and for however long, such as year-round or just during dry months or for the peak watering season.
Rainwater catchment areas are best made from materials that will not hold water, meaning impervious materials and that are sloped to drain the rainwater. The best construction materials for rainwater harvesting are sealed, protected, high quality seam metal, corrugated sheet metal, and polymer coated roofs.
Catchment areas should not include any parts, pieces, or materials made from or that feature chemicals or ingredients known to be harmful to human health.
Rainwater Channeling Systems
Rainwater channeling systems, also known as a conveyance system6, include the gutters, downspouts and pipes used to take rainwater as it flows from the catchment area and direct it to a container for harvest, storage and later use. Metal and vinyl gutters are good choices. Gutters with mesh screen gutter guards are best.
PVC pipe is okay for rainwater harvesting and is commonly used. Choose ANSI/NSF 61 certified PVC, even if the rainwater will not be used for drinking. Size 3” or 4” PVC pipe is often used and recommended. The larger sized plumbing will help limit water loss during heavy rain and ensure rain and any debris flows unhindered through the plumbing.
If your catchment area has multiple sides with gutters and downspouts that are not connected and you want to collect rainwater from the whole catchment area, you can always add gutters or piping to connect them all together ending at your rain container.
Rainwater Storage Tanks
There are many products available today that can be used for collecting and storing rainwater. Common choices include rain barrels, repurposed shipping drums, IBC totes, and larger volume rain tanks. As rainwater harvesting practice, know-how7, and products increase, the ability and ease of harvesting larger volumes is also increasing. This is leading more homeowners, farms, ranches, vineyards, educational institutions, and businesses to choose the larger rain tanks for their rainwater harvesting applications.
Rainwater storage tanks are available in many sizes, styles, and build materials to provide the best fit for a property, use, or project. Rain tank sizes are available in almost any volume needed depending on what the container is manufactured from. Rain tanks are available made from plastic, stainless steel, galvanized steel, or fiberglass.
Plastic rainwater tanks made from polyethylene are perhaps the most common type, especially in residential applications. Popular styles for rainwater harvesting include aboveground vertical rainwater tanks, the thin-profile slimline tank, and underground water tanks.
Rainwater Storage Tank Options
- Plastic Rainwater Tanks: Plastic rainwater harvesting tanks are made from FDA approved polyethylene resin and are ANSI/NSF Standard 61 certified for drinking water uses. Plastic tank types include cylindrical storage tanks, slimline water tanks, IBC totes, rain barrels, and underground cisterns. These rain collection containers range in volume sizes from 100 gallons to 20,000 gallons.
- Galvanized Steel Rainwater Tanks: Galvanized steel rainwater harvesting tanks are metal tanks with a galvanized finish. They are typically available in either one piece, two piece, or build-onsite models. One piece or two piece galvanized rain tanks can easily be set up and installed DIY. These models range in volumes from 90 gallons to 3750 gallons. On the other hand, build-onsite galvanized rain tanks usually require professional installation provided by the manufacturer. These models provide high storage capacity ranging from 20,000 gallons to 100,000 gallons.
- Stainless Steel Rainwater Tanks: Stainless steel rainwater tanks are metal containers made entirely from 304 grade stainless steel. They are available in either one piece or two piece models. Stainless steel rain tanks have a volume range from 90 gallons to 3750 gallons.
- Fiberglass Rainwater Tanks: Fiberglass rainwater harvesting tanks are manufactured from glass fiber reinforced plastic. They are often selected for use in commercial, industrial, and agricultural applications. Fiberglass rain tanks are often made to order with a volume range customized to the use.
Rainwater Storage Tanks Should Include:
- A lid or be a closed system. Containers used to hold rainwater are recommended to not be open to the air. Closed containers will prevent debris from entering the water supply, limit growth of algae, and prevent pests such as mosquitoes from being able to breed or contaminate the rainwater. Closed head barrels, IBC totes, and bulk rainwater tanks are all designed this way.
- A rainwater inlet. An inlet allows rainwater to be collected by the container. The inlet can stop at the tank manway and pour water into the tank (common) or be plumbed directly into the tank through a fitting.
- A rainwater outlet. An outlet allows the collected rainwater to be used. Consider a ¾” or 1” bulkhead fitting and hose bib if a garden hose will be used with the container.
- An overflow pipe. Rain tanks should have an overflow kit or pipe added to the container. An overflow pipe gives an outlet for incoming rainwater when the container is full. Overflows prevent excess rainwater from flowing out of the manway or backing up inside the inlet. The overflow pipe should be at least the size of the tank inlet pipe, if directly plumbed, should not be positioned above the tank outlet, and should be plumbed to discard excess water away from the bottom of the tank and away from nearby structures.
- An air vent. For completely closed containers, an air vent will prevent pressure buildup during rain events and improve water flow. Most plastic tanks include a vent built into the lid. The overflow can also function as an air vent. If the container uses an open manway for the rainwater inlet, a vent is not needed.
- A foundation. For larger volume rain tanks especially, a prepared foundation base to install the tank on is recommended and in some cases required. Common tank foundations include concrete slabs, pea gravel or crushed rock. See our article on How to Install a Rainwater Tank for steps, options, and recommendations.
Additional Recommendations for Plastic Rainwater Storage Tanks
Particularly when choosing plastic built containers for collecting rainwater, there are several additional recommendations that can help provide a healthier storage tank and cleaner rainwater for use. These guidelines center around limiting or preventing sunlight from reaching the stored rainwater. Limiting sunlight will help maintain water temperature and avoid unwanted algae and microbial growth resulting in less cleaning and better water quality. Sunlight reaching the water in a rain tank is what allows algal growth. Algae cannot grow if sunlight is blocked.
- Buy an opaque tank. Opaque water tanks are non-translucent by design. Opaque tanks limit the amount of sunlight that can pass through the tank walls and reach the water.
- Buy a dark colored tank. Dark colored water tanks restrict even more sunlight from entering the tank. However, they also absorb more heat energy and can warm the rainwater which may be undesirable for some uses.
- Paint a dark colored tank with a reflective paint that is light in color. Lighter colors absorb less heat energy. Painting a dark colored tank with a reflective, light-colored paint will maximize blocking both sunlight and heating.
- Consider an underground water tank. What an underground water tank requires in extra work and cost in installation, they make up for in the benefits they provide. Underground water tanks provide a nearly stable water temperature and are not affected by sunlight, which eliminates heating and algae concerns.
Rainwater Harvesting Accessories
Accessories for rainwater harvesting are specially made for rain collection systems, and most are made from materials that are drinking water quality. There are various accessories available, and the types needed will depend on how clean you need the rainwater to be, how much debris is common in your rainwater, how much you want to improve the system, and your unique system setup.
At a minimum, rain system designs are recommended to include a downspout filter or a tank screen. Both these accessories use steel mesh screens to catch and remove debris carried from the roof. Examples of debris are vegetation material such as leaves, sticks, seeds, and berries, and remnants from animals and insects.
For systems with open plumbing from an inlet and/or overflow, flap valves are also a highly recommended accessory designed to keep insects and other pests out of your system.
A list of available rainwater harvesting accessories includes:
- Tank Screen
- Solar Shield
- Tank Screen Cover
- Downspout Screen
- First Flush Diverter
- Flap Valve
- Air Gap
- Water Level Gauge
- Pre-Tank Filter
- Water Pump
- Post-Tank Rainwater Treatment System
We offer all of the above listed accessories. You can find all our available rainwater accessories for sale here.
For an extensive look into the details, use, and how to install common rain harvesting accessories, read our comprehensive Rainwater Harvesting Accessory Guide.
Going into the details on designing a rainwater treatment system is beyond the scope of this post. For more on post-tank rainwater treatment, see Rainwater Harvesting for Potable Water as well as How to Turn Rainwater into Drinking Water.
Takeaway | How to Design a Rainwater Harvesting System
Designing a rainwater harvesting system should follow a blueprint that includes four basic building blocks. A successful system will have a catchment area for the rain to fall on, a channeling system to control water flow from the catchment, a rainwater storage container to hold the water for use, and rainwater harvesting accessories to improve water quality and how the system works. When planning and designing a rainwater system, it helps to know ahead of time what you will use the rainwater for, how much rain you can collect, how much water you will need, and what size rain container is best.
If looking to implement your own rainwater harvesting system, we several styles and many volumes for poly rainwater containers. We also offer metal rainwater tanks and the most recommended and used rain harvesting accessories.
For information on our available rainwater harvesting products or help in ordering, contact us, our professional support staff is always ready to assist.