How to Install a Rainwater Tank

The winter rains are fast approaching. Conserve water and protect our environment by installing a rainwater harvesting system!

A rainwater harvesting system that involves a rain tank or cistern to store the rain for later use is referred to as an Active Catchment System. In this article, you will learn about the installation of rainwater tanks. Keep in mind that this type of installation is only one type of active catchment system. There are more complex systems that involve underground cisterns, series of tanks, or different end uses, just to name a few examples.


The water collected in an active rainwater catchment system can be used in a variety of ways. The most common in our area is outdoor irrigation. This can be as simple as putting a spigot on the tank and filling your watering can from it, or you can go a little further and connect the tank to your irrigation valves to allow for automatic watering through your drip irrigation system.

Other uses for rain tanks could include cleaning the car, filling a fountain, agricultural uses, and potable water uses.

 Site Planning

To get the most out of your rainwater harvesting system, careful planning needs to happen in order to decide on the placement and design of the system. This part of the planning stage is critical to your success and will help avoid issues that can arise during or after the installation.

To begin, draw a simple overhead sketch of your property. Note down the square footage of your roof or any other impermeable surface that you would like to catch rainwater from. These are your catchment surfaces. From this, you can determine how much rainwater you can collect, and thus how to correctly size your catchment system.

[themify_box style=”orange info” ] To calculate square footage (area), simply multiply length x width. For a roof, you can take your measurements at ground level and you can disregard the pitch of the roof. Remember, each downspout should have its own corresponding roof area calculated for your plan. [/themify_box]

Next, take your roof or other surface area calculations and multiply them by the average rainfall for your area. In San Luis Obispo, this is around 19 inches of rain.

After you have that calculation, you need to convert it to gallons. For this, simply multiply by 0.62.

[themify_box ]Example: Roof area = 1,000 square feet x 19 inches of rain x 0.62 = 11,780 gallons[/themify_box]

In the example above, you can see that even when using a relatively small catchment surface of 1,000 square feet, you can collect almost 12,000 gallons of water!

The amount of rain that can be collected is often a shock to people, but it is a testament to the exciting possibilities that rainwater harvesting can provide. The limiting factor is usually budget or space, rather than the maximum amount that can be collected.

Rainwater house

On your site drawing, mark out any obstructions, structures, and existing landscaped areas. Include driveways, sidewalks, and determine your property lines.

Now that you have your calculations for how much rain you can harvest and all other notable site elements are accounted for, you can use this info as a tool to refer to as you design your system.

Site Selection

A rain tank will typically be located underneath or near a downspout. Look at the space available under your downspouts and research tank sizes and options in order to figure out which tank will work best. Take special note of the tank dimensions and use a measuring tape to mark out where it will be located to make sure it will fit properly.

On your site plan, mark down where you would like to place the rain tank. Does this placement interfere with any other aspects of your property? Does it obstruct walkways or views? Does it conform to your local building code? Is it positioned in a place that is convenient for your uses? Can a suitable base be built for the tank?

Play around with your different placement options until you find the most acceptable spot. Sometimes placement is easy and you already have the perfect spot, other times you may have to compromise or use some ingenuity to find a site that will work for your needs.


Tank Sizing

It’s almost time to install your tank, but first you need to decide on which one you will use. When deciding which size tank to install, there are several factors to consider. By finding the balance between these factors, you will be able to narrow down your tank options and make an informed decision on which one to use.

Catchment Capability

This is the total amount of rainfall you are capable of collecting in an average rainy season off of your catchment surface. By using the calculations discussed earlier in this guide, you know the amount of rain you can collect off of your roof for every inch of rain. If you multiply that by the average rainfall for your area, then you will know the maximum amount of rain you can expect to harvest in a year.

This means that there is most likely no need to choose a tank that holds more than that. Since such a large amount of rain can be collected from even a small roof, more often than not the catchment capability is not the deciding factor for which tank size to get.


Space is much more likely to be a deciding factor for tank size. Most residential homes don’t have room for a very large tank underneath the downspouts. In the site selection section above, you identified possible tank placement options and measured out the available space. Refer to these measurements as you look at the different tanks you are considering.

Remember, you will likely need a little extra room around the tank for a base, piping, and a pump. Look at tank height as well. You will most likely need several feet of extra room above the tank to install a First Flush Diverter or other pre filter.


Once you know what tank sizes would work for your available space, you can further narrow down your options by looking at your budget. A larger tank costs more money, so if you are on a tight budget for the project, you may want to look at the smaller options.

[themify_box ]A word of caution – If you need a larger tank and are able to stretch your budget a little bit to get one, you might thank yourself later on. It is easier to install a larger tank now than to replace a smaller one later.[/themify_box]


How much rainwater do you need? This question will help you further narrow down your tank size. If you just want to have some extra water for the potted plants on your patio, there is no need to install huge rain tank, even if you have the catchment capability, space, and budget. However, if you want to irrigate your entire landscape on rainwater alone, you may want to get the largest tank you can. By evaluating your rainwater needs, you can match your tank size with your intended usage


Now that you have selected your tank, it’s time to get it set up. For a simple rainwater harvesting system, installation can be quite easy. However, if you are unsure of anything, please consult a professional for assistance.

Base Construction

Your rainwater tank will need some type of base to rest on. A rainwater tank gets very heavy when full of water, so a foundation is needed to provide support and stability.

If you happen to have concrete under your downspout, you can skip this step and place your tank directly on the concrete, assuming it is level and structurally sound.

Otherwise, a simple base can be built using pressure treated 2”x6” lumber and Float Gravel.

Use metal stakes to secure your 2×6’s to the ground. Make sure the base is slightly larger than the tank and that it is level. After the frame is constructed, fill it with Float Rock. That’s it!

Metal TankTank Placement

After your base has been constructed, it’s time to set up the tank. Get help with this and avoid injury or damage to property as you move the tank and carefully place it onto the base. Ensure the tank is level and resting securely on the base.

Down Spout Connection

With the tank in place, now it’s time to hook it up to the downspout. Mark your downspout at the appropriate height to cut, making sure that it is high enough to accommodate a downspout elbow.

Using a hacksaw, cut the downspout. Attach the elbow making sure it is directly above the rain tank inlet. This means the water will now flow through the downspout and into the tank.

First Flush

A First Flush Diverter prevents the initial flush of water after it starts raining from entering the tank. This water is typically rather dirty with built up sediment, pollutants, bird droppings, and leaves. The water is diverted preferably into your landscape to allow the soil to naturally filter any pollutants, but sometimes it is diverted into the storm drain.

First Flush Diverters are most commonly installed in-line on the downspout before the rain tank. They can help keep the water in your tank cleaner, reduce maintenance, and protect pumps from sediment and debris.

With that said, using a First Flush Diverter is not always needed, and critics of First Flush Diverters even criticize their effectiveness. For example, if you have a simple system without a pump just for outdoor watering and don’t have any trees overhanging the house, it might not be necessary to install a First Flush Diverter.


Every rain tank should have an overflow port that allows water to exit the tank once it is full. The overflow can be directed to wherever the downspout was formerly sending the rainwater, such as a drain. Alternatively, (and preferably), you can direct the overflow towards your yard or landscape to allow the excess water to permeate into the soil instead of flowing down the storm drain. Flex pipe can be attached to the overflow port and used to help direct the extra rainwater.


In some cases, it may be wise or even required to secure the rain tank. Some tanks are more unstable than others, and in California, earthquakes are always a possibility. Check with your local building department and/or consult a professional to decide whether or not it is necessary to restrain your tank. This is usually accomplished by strapping it to the house or another secure structure.


The outlet port is located near the bottom of the tank. From this outlet port, you can attach a spigot, a hose, or run a PVC pipe to an irrigation valve and irrigate your landscaping with the rainwater.


pumpA rainwater catchment system can use gravity or a pump to pressurize the system. Gravity can sometimes work well on taller tanks, raised tanks, when a tank is on a hill, or if you only are going to hook up a simple spigot to the tank.

In some systems, though, gravity alone might not provide enough pressure for you to be able to use your rainwater effectively. If this is the case, a pump can be used. Pumps are either submersible or they are in-line. A submersible pump is placed inside the rain tank in order to pressurize the water as it exits the outlet port. An in-line pump is placed outside the tank between the outlet port and wherever the water is being sent, such as a hose or irrigation valve.

If you are unsure whether you need a pump or if you need help choosing a pump, contact a professional installer or a rainwater system distributor for more information.


Now that you have your system set up, you can begin collecting rainwater. You’ll find that it becomes exciting to watch your tank fill and it feels good to be able to use the water you’ve collected.


Your rainwater catchment system will require periodic maintenance to prevent blockages, contamination, and to keep it operating properly. Thankfully, system maintenance is not usually very demanding.

Here are some guidelines for maintenance:

Every 3-6 months:

  • Check and clean pre-filtration devices such as the First Flush Diverter, tank screen, and gutter guards.
  • Clear debris from roof and trim any overhanging branches.
  • Check tank and system for insect or animal entry.
  • Check tank for algae. If algae is present, seal off any points where light is entering the tank.
  • Check tank and fittings for leaks.

Every year

  • Have a professional check any backflow prevention devices that were installed.
  • Check pump to ensure it is operating correctly.
  • Check roof condition.
  • If any post tank filtration devices were installed, check for correct operation and replace components according to the manufacturer’s instructions.
  • Inspect any caution signage or markings.


Rainwater harvesting systems vary in complexity, but if you want to install a simple system, the information above will help you get started. Collecting rainwater is a rewarding endeavor and is an important step towards sustainability in our dry climate. If you live in San Luis Obispo County and need help with your rainwater catchment project, please feel free to contact us.