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Most rainwater diverters do not divert all the water flowing through a downspout.
Many common downspout diverters send part of the rainwater to a barrel or tank while letting the rest continue down the original downspout. This is often intentional. It helps the system handle heavy rain and gives excess water somewhere to go when the storage tank is full.
The exact amount diverted depends on the diverter design, rainfall rate, pipe size, hose size, installation height, and how freely water can enter the tank.
How a Rainwater Diverter Works
A rainwater diverter is installed in or connected to a downspout. As roof runoff moves through the downspout, the diverter directs some or all of that water toward storage.
A typical setup looks like this:
Roof → gutter → downspout → diverter → rain barrel or tank
Water that is not diverted usually continues through the downspout toward the normal drainage point.
This arrangement is useful because your roof can produce water much faster than a small barrel can accept or store it.
Some Diverters Only Capture Part of the Flow
Many rain barrel diverters are designed to collect only part of the water moving down the pipe.
They may use:
- An opening inside the downspout
- A small collection chamber
- A hose leading to the barrel
- A screened outlet
- A fitting that takes water from the side of the downspout
During light rain, a large share of the available water may enter the diverter.
During heavy rain, water may move through the downspout faster than the diverter hose can carry it. The extra water continues down the downspout.
That does not necessarily mean the diverter is working poorly. It may simply be operating as designed.
Full-Flow Diverters Work Differently
Some systems are designed to redirect the full downspout flow.
Instead of taking water through a small side outlet, the downspout itself may be routed toward the storage system.
For example:
Downspout → large pipe → cistern
This type of setup can capture much more runoff because the water does not have to pass through a small diverter opening or hose.
However, even a full-flow system needs a safe path for excess water.
Once the tank fills, incoming water must be able to leave through an overflow or bypass without backing up into the gutter system.
Why a Diverter May Not Capture All Available Rainwater
Several parts of the system can limit how much water reaches your tank.
Rainfall can exceed the diverter's flow capacity
A small hose can only move so much water.
During a hard storm, the downspout may carry water faster than the diverter outlet can move it toward storage.
The remaining water stays in the downspout.
The tank may already be full
Many rain barrel diverters are installed so that water stops entering the barrel when the water level reaches approximately the height of the diverter connection.
The excess then continues through the downspout.
This can reduce the need for a separate barrel overflow on some systems, but the exact behavior depends on the diverter design and installation.
Small fittings restrict flow
A large downspout feeding a narrow hose creates a bottleneck.
The downspout might be capable of moving far more water than the hose leading to the barrel.
More storage capacity will not fix that restriction unless the conveyance path can also handle more flow.
Screens and debris can slow the water
Leaves, roof grit, seeds, insects, and other debris can collect around screens or small openings.
As the opening becomes restricted, less water reaches storage.
Regular inspection and cleaning help keep the diverter working properly.
Poor installation can reduce collection
The diverter may depend on being installed at a certain height or orientation.
A sagging hose, uphill section, blocked fitting, or incorrect tank connection can reduce flow.
Follow the diverter manufacturer's installation requirements rather than assuming every model works the same way.
Does a Downspout Diverter Stop Collecting When the Barrel Is Full?
Some do.
Review the performance of rain barrel diverters to assess capacity against realistic demand and refill rates.
A common rain barrel setup places the diverter opening and the barrel inlet at similar heights.
As the barrel fills, water in the connection reaches the diverter level. New runoff then tends to continue down the downspout instead of flowing into the full barrel.
This is sometimes called an automatic bypass arrangement.
Other systems do not work this way. They require a separate overflow pipe from the barrel or tank.
A storage container should always have a reliable way to handle excess water. Do not depend on an assumed automatic shutoff unless the diverter is specifically designed for it.
A Rainwater Diverter Is Not the Same as a First-Flush Diverter
The names can be confusing.
A downspout or rain barrel diverter sends roof runoff toward storage.
A first-flush diverter does almost the opposite for the first part of a storm. It sends an initial amount of roof runoff away from the main storage tank.
That first runoff can carry more dust, leaves, droppings, and other material that collected on the roof between rains.
After the first-flush chamber fills or completes its diversion cycle, later runoff continues toward storage.
A first-flush diverter does not make collected rainwater safe to drink. Drinking-water use requires a suitable collection and treatment system, ongoing maintenance, appropriate laboratory testing, and compliance with applicable local requirements.
Can You Capture Nearly All the Water From a Downspout?
You can design a system to capture a much larger share of the runoff, but storage capacity alone is not enough.
The complete water path has to be able to handle the expected flow:
Roof → gutter → downspout → prefilter → pipe → tank → overflow
For high collection rates, pay attention to:
- Downspout size
- Pipe and fitting size
- Restrictions from screens and filters
- Number of downspouts being collected
- Tank inlet capacity
- Tank venting
- Overflow capacity
- Expected rainfall intensity
A large cistern connected with properly sized pipe will generally accept more flow than a rain barrel connected through a small hose.
Even then, some water may be lost through first-flush devices, screens, splash, overflow, leaks, evaporation, or intentional bypasses.
How Much Rainwater Is Available?
Before trying to capture more flow, it helps to know how much water the roof can produce.
In U.S. units, a useful theoretical estimate is:
Gallons = roof area in square feet × rainfall in inches × 0.623
For example, 500 square feet of roof receiving 1 inch of rain has a theoretical yield of:
500 × 1 × 0.623 = about 312 gallons
You should not assume all 312 gallons will reach the tank.
Real systems have losses from wetting the roof, first-flush diversion, debris removal, overflow, and other factors.
This also shows why a small barrel can fill quickly. A 50-gallon barrel represents only a small part of the runoff from many roofs during a moderate storm.
Should You Try to Divert All the Water?
Not necessarily.
For a small garden barrel, partial diversion may be exactly what you want. It provides useful irrigation water without requiring the barrel to handle the entire roof flow.
Capturing a larger share makes more sense when you have:
- A larger storage tank or cistern
- Enough usable demand for the stored water
- Properly sized inlet piping
- A reliable tank overflow
- Safe drainage for overflow water
Trying to force all roof runoff through undersized hoses or fittings can create backups and drainage problems.
The goal should be a balanced system rather than simply capturing every drop.
Frequently Asked Questions
Does a rain barrel diverter collect 100% of the rainwater?
Usually not. Many rain barrel diverters intentionally collect only part of the downspout flow. Excess water continues through the original downspout.
Why is water still coming out of my downspout after I install a diverter?
That can be normal. The diverter may only take part of the flow, especially during heavy rain or when the barrel is full.
Will a bigger rain barrel make my diverter collect more water?
Not always. If the diverter opening or connecting hose is the limiting part of the system, increasing tank capacity will not increase the flow entering it.
Can I send the entire downspout into a rain barrel?
A small rain barrel is generally not suitable for receiving unrestricted downspout flow unless the system has been designed with adequate inlet and overflow capacity. A full barrel still needs somewhere safe to send incoming water.
What happens when a rainwater tank is full?
Incoming water should move through a properly designed overflow or bypass. Overflow should be directed to a safe drainage area rather than allowed to collect around a foundation or back up into the gutter.
Is a first-flush diverter the same as a rain barrel diverter?
No. A rain barrel diverter sends runoff toward storage. A first-flush diverter temporarily sends the first portion of roof runoff away from storage to reduce some of the debris and contamination entering the tank.
How can I make my rainwater system collect more water?
Look for restrictions throughout the system. Diverter openings, hoses, filters, pipes, tank inlets, and overflow arrangements all affect how much water can move through the system. Increasing one part may not help if another part remains the bottleneck.
