Demystifying the Downspout Diverter: Rain diverters for barrels, tanks & cisterns

As a rainwater catchment company, we answer lots of questions about the rain diverter for barrels, tanks and cisterns. Just how do you get water from your roof downspout into a rainwater storage system, anyway?

The BlueBarrel Rainwater Catchment System™ provides a top-to-bottom (roof to barrel to garden, that is) solution for harvesting and storing rainwater, and then irrigating with it.

This article covers the standard downspout rain diverter (included with our DIY RainKits™ and sold separately for use with other kinds of rain barrels and tanks), demystifying how it works. We'll also cover some alternatives, for those who are thinking:

"What if the standard solution won't work for me?”

Gutter vs downspout
A quick breakdown:

On the average building, rainwater cascades down the roof and into gutters, which run horizontally to capture water from the roofline. Then, water courses through a number of downspouts, the vertical elements that carry stormwater to the ground.

Without rain barrels, this water usually makes its way into a storm drain, or is piped away from the building onto the landscape.

Rain barrels allow you to divert this water and store it as a free source of the highest quality irrigation water. This has environmental benefits both in terms of water conservation and stormwater mitigation.

BlueBarrel's Standard Downspout Rain Diverter

The BlueBarrel System™ uses a flexible-fit, rubberized downspout diverter. This nifty piece intercepts rainwater flowing through a downspout and redirects it into a linked system of barrels. Once the barrels reach full capacity, the diverter redirects excess flow down the existing downspout automatically.

When you customize your BlueBarrel System in our online store, you get to choose from three different diverter sizes for an easy-to-install and well-fitted downspout solution.

After 10+ years in business (and longer for R & D!), this is the downspout rain diverter we like best, especially for a multi-barrel system like the BlueBarrel System. Here’s why: 

It’s easy to install:

EarthMinded Flexi Fit Downspout Diverter for Rain BarrelsThe rubberized diverter head is flexible. It inserts into standard downspouts with a simple hole, and no need to cut your downspout. 

We carry versions fitted for standard 2" x 3” and 3" x 4” rectangular downspouts, and also for round downspouts between 3” and 4” in diameter. (Click here for details on how to measure your downspouts to select the right diverter size.) The rubber diverter head seals inside the downspout. Water collects in an interior reservoir and flows swiftly through the inlet hose, into your tank or barrel(s). 

It optimizes the inflow rate and handles overflow automatically:

“But what about that big hole in the middle?”

Ah, there’s another common question. The most brilliant thing about this diverter is that it optimizes inflow to fill rate, and handles system overflow automatically. Because of that hole, this is all possible with no on-and-off switch. It’s a true set-it-and-forget-it solution.

If your barrels have any capacity at all, water will flow in. When they are full, it will flow down as if they weren't there. This is an easy and automatic way to keep barrels topping up even as you use the water throughout the seasons.

When installed correctly (with the inlet hose completely level), a full barrel will cause excess water to back up in the hose. Once the water column starts rising in the downspout, excess flow falls down that hole in the middle to exit your downspout as it normally would.

This adds major convenience—you don't even need to think about system overflow! While there are plenty of ecological overflow solutions you can pursue when you have time, rest assured that you will not be creating any new drainage problems when you install a BlueBarrel Rainwater Catchment System™.

a downspout diverter
Our standard diverter connected to a downspout (with a Leaf Eater installed above it). Photo courtesy of BlueBarrel customer Jan K.

How does the downspout rain diverter capture enough water with the hole in the middle?

First, it's important to understand how water flows through a downspout to begin with. Many people imagine stormwater coursing through the gutters, and then free-falling inside the downspout. But that's not how it happens. Surface tension causes water to hug the interior walls of the downspout as it exits.

(Have you ever tried to pour water from one container to another, only for it to run down your arm and drip from your elbow? That's the same effect. The water hugs the surface, whether it's your arm, or a downspout!).

So most water will be delivered directly to the reservoir at the edge of the diverter head. Only when rainfall volumes exceed the inflow rate of the inlet hose will you begin to see simultaneous overflow: another important concept that we'll help you understand.

Understanding Simultaneous Overflow

In heavy rains, you may notice water is entering your system and exiting the bottom of the downspout at the same time. You can double check to make sure the diverter head is installed correctly (with a level inlet hose and not crimped inside the downspout), but otherwise, this is how the piece is designed to work.

If water enters the system faster than it can move through the pipes that connect all the barrels, you will end up with backup. This can lead to uncontrolled overflow, which can be a big problem.

So simultaneous overflow could also be thought of as balanced inflow. That sounds better, doesn't it?

How hard does it have to rain before we see simultaneous overflow?

That depends on the size of your rooftop, paired with the rainfall intensity rate, which can change from minute-to-minute. The smaller your collection surface is, the harder it has to rain before you'll see simultaneous overflow. If you’re collecting from a very large surface, you may see simultaneous overflow with just moderate rains.

This (beige-painted) BlueBarrel System taps into two downspouts for a faster fill. Photo courtesy of BlueBarrel customer Erik D.

Is Simultaneous Overflow Bad?

We don't think so! Simultaneous overflow (a.k.a. balanced inflow) allows inflow and overflow to be more balanced throughout the season.

Your storage tanks have the ability to collect a certain amount of water. Excess water will overflow regardless, so by optimizing the inflow rate, overflow exits at lower rates over a longer period of time. This increases your ability to infiltrate this water onsite, which is what you really want for the best ecological design. It is also your best bet for minimizing the chances of damage caused by uncontrolled overflow. 

How to Increase Inflow:

That said, we do have a few suggestions for how to increase inflow for faster fill. First, just remember a multi-barrel system like the BlueBarrel System can't fill any faster than water can get through the connection lines. While there are some creative ways to increase the rate of transfer between barrels, in a standard setup, the most sound approach is to match the inflow rate to the rate at which the system can fill. Otherwise you will have issues with backup.

But there are many ways to increase inflow without creating overflow issues. For example, the system above collects from two separate downspouts: one at each end of the system. The BlueBarrel System is very flexible in this way. Our interactive online store allows you to choose all kinds of customizations, including extra diverters, so you can feed a single system from multiple downspouts.

Selector Valves:

The picture below shows another creative approach to increasing inflow. With two diverters collecting the same downspout, one is installed higher than the other. The lower diverter will catch overflow from the higher one in heavy rains. But again, for a small system, there is such a thing as too-much/too-fast, so the RainRouter Selector Valve on the bottom diverter allows the user to send excess water through a hose to direct it elsewhere. (RainRouters are available in our online store as well).

double diverter set up
This is another system installed by Erik D. He used a second diverter to capture overflow from the first.

Full Diversion:

Still, many folks contact us about cutting the downspout to divert all flow into the rain barrel. While it’s common for beginners to want to fill the barrel as quickly as possible, unless you have large tanks sized to capture full-season rainfall volumes, this is usually not a good design decision. Diverting all water into a barrel requires a separate overflow (we’ll go over that below), and your system will fill so quickly that all excess water will overflow at sometimes-higher-than-manageable volumes.

Again, our professional recommendation is to allow some simultaneous overflow when rainfall volumes are excessive. Overflow is much easier to manage at lower and more consistent rates. 

In most cases, a more balanced inflow solution with automatic overflow handling makes the most sense. Review the double-diverter strategies above for balanced ways to increase inflow while still benefitting from the automatic overflow function.

What if My Downspouts are Not Compatible?

We carry diverter kits for 2" x 3" and 3" x 4" rectangular downspouts, and round downspouts between 3" and 4" in diameter. This covers most standard residential and industrial sizes.

Do you have a different size, or maybe a completely different roof drainage system like rain chains or scuppers? No need to worry. We have a surprisingly simple solution. 

Leaf Eater as Adapter

A Leaf Eater is a standard upgrade to any rainwater system, and is highly recommended for keeping leaves, mosquitoes, and other debris out of your rain barrels or tanks in any case. Also known as a debris excluder, it is one of our most popular accessories

The great thing about the leaf eater is it adapts to any of the standard sizes listed above—round or rectangular! Installation, while simple, does require a clean cut to the downspout. This is a good thing if you have non-compatible downspouts, or no downspouts at all.

Water free-falls onto the leaf eater, so it doesn't matter what dimension you have above it. Downspout material is inexpensive and you can replace your lower segment with 2" x 3" downspout material for a standard diverter connection, as shown in the video. 

While we don't love the idea of plastic in general (except for re-used of course!), plastic downspout material is a low-hanging fruit in this case. It is inexpensive, easy to cut, and seals very well against the rubberized diverter head. 

In fact, the "leaf eater-as-adapter" solution is so effective, it really matters not what's going on "upstream" of the leaf eater, as long as you can funnel water onto the screen in one way or another. 

Rain chains can be cut to release onto a leaf eater. Leaf eaters can be installed to catch water from a scupper, or a makeshift drainage system. The options are limitless!

leaf eater as adpater options
Non-standard downspouts? Simply direct your roofwater onto a Leaf Eater to adapt!

What if I Still Want Full-Diversion?

In general, this is not a proportional approach for smaller systems like single rain barrels, or even linked rain barrel systems, for the reasons described above. But it may be a good idea for larger tanks. 

Whether your tanks are large or small, for proper ventilation and overflow handling, you need to have an overflow equal to or greater in size than the sum of all inlets. To make that simple, if you have a 3" inlet, you need a 3" overflow on the same vessel.

Overflow Issues and Capacity Loss:

Imagining this on a 55-gallon rain barrel, you can see how the overflow port takes a significant bite out of your storage capacity. Multiply that loss for a multi-barrel system. In addition to uncontrolled overflow, loss of storage capacity is another reason full diversion is not ideal for smaller systems.

While our prefab diverter handles overflow automatically by design, full-diversion will need to be paired with a carefully-designed overflow management system.

Consider that overflow rates will be irregular and sometimes very high. Direct overflow to an infiltration basin at least 15' from any structures, where it can safely infiltrate. 

Custom Downspout Diverter
"I commend you on your how-to videos. I've watched many videos for home projects and yours is head and shoulders above most on YouTube. Jesse is precise and easy to understand. Being a plumbing professional, I thought I'd watch your videos and improve upon the design, but now I can't say I'll do this any other way than following your instructions."
- Robert | San Luis Obispo, CA

This BlueBarrel customer created his own inlet. A separate overflow of at least the same size will be required, to route excess water to a safe drainage spot. Photo courtesy of BlueBarrel customer Robert E.

Custom downspout for rain barrels
"Success! My husband installed our 10-barrel system in a day. He added a gutter over the path to feed into the downspout. Genius!"
- Georgann | Montara, CA

In this submission from Georgann C., although BlueBarrel's standard diverter is used, it enters from the top of the barrel. Because her inlet hose is not level, Georgann employs a separate overflow that exits behind the barrels, directed to a drain that feeds a small pond.

What if I Don't Have Gutters or Downspouts at All?

The simple answer is you would need to install them. But not necessarily. What you really need to do is gather your water into a single location so that it can be collected. Really, as described above, all you need to do is get the water onto a leaf eater in one way or another.

Check out Occidental Arts & Ecology Center's (OAEC) Wonder Gutter solution for one inexpensive way to create your own gutter system!

What if I Don't Have a Roof?

In comes the rainwater funnel. Rainwater harvesting works because the roof catches a large volume of water and (usually) funnels it through a series of gutters and downspouts. This concentrates large volumes of water for easy collection. 

Can't I just put an open barrel in a field?

Sure, but if you get one inch of rain, you'll have exactly one inch in the bottom of your barrel... until it starts evaporating!

This is the beauty of the collection surface: That same inch of rain falling on a 1,000 square foot rooftop will generate over 600 gallons of high-quality rainwater. That's enough to fill 11 standard 55-gallon rain barrels, to put things in perspective. 

Even a very small surface (like an 8' x 8' garden shed) will fill a 55-gallon barrel with less than 1.5" of rain. If you don’t have outbuildings to collect from, you can create a modest increase to your collection surface with a rainwater funnel.

While it won't give you the kind of surface area that even a small rooftop would, the funnel can increase your surface area two times or more. This may be a useful solution for open-field watering needs like livestock, birdbaths, establishing orchard plantings, or habitat restoration

rain barrel with rain saucers
No roof? No problem! You can outfit your barrels with a funnel, a.k.a. "rain saucer"

Outbuildings and Solar Panels:

Consider installing a small outbuilding, or even a shelter to shade your storage tanks. Some folks have installed free-standing solar panels in the middle of a field and collected from those. Solar panels have high collection efficiency, and provide other ecological benefits as well. Wouldn't it be useful to have some shade or storage (or energy production!) out next to your stored water anyway?

rain barrels on shed

Look how much storage Peggy K. fit along the side of her 10' x 12' garden shed.


With no other rooftops nearby, the Windsor Garden Club also utilized a garden shed to harvest water for their planted beds. Those solar panels make for a nice collection surface!

With this in-depth primer, we hope you're ready to start harvesting, no matter what kind of downspouts you're starting with. Step through our online planning tools to customize a rainwater system for your place, and feel free to contact us with questions. We're here to help!


Article by Jesse Savou, MA Ecological Design, ARCSA/ACCE Certified Rainwater Harvesting Professional. Graphics by Olivia Loughrey, MS Ecological Design. Photos contributed by BlueBarrel customers all over the USA!


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