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A first flush diverter is a simple part of a rainwater harvesting system that sends the first dirty roof runoff from a storm away from your storage tank. After that first portion is diverted, cleaner roof runoff continues into the rain barrel, IBC tote, or cistern.
The first water running off a roof can carry dust, pollen, leaves, bird droppings, insects, and other material that built up during dry weather. A first flush diverter helps keep some of that material out of stored rainwater. The CDC recommends considering a first flush diverter as one way to improve collected rainwater quality.
It is useful, but it is only one pretreatment step. It does not make rainwater safe to drink.
How a First Flush Diverter Works
A basic rainwater system may follow this path:
Maintenance needs and roof conditions help determine whether first flush diverters are worthwhile for a particular system.
Roof → gutter → leaf screen → first flush diverter → storage tank
When rain begins, water flows down the gutter and enters the diverter before reaching the tank.
Many diverters use a vertical pipe or chamber:
- The first roof runoff enters the chamber.
- The chamber fills with this first portion of water.
- A floating ball or similar device rises as the chamber fills.
- Once the chamber reaches its designed capacity, the main flow is directed toward the rainwater tank.
- The captured first-flush water slowly drains away so the diverter can reset before another storm.
Designs vary. Some do not use a floating ball. The important point is that a measured portion of the first runoff is kept out of the main storage tank.
The U.S. EPA describes first flush diverters as devices that retain or redirect the initial roof runoff, helping remove smaller contaminants such as dust, pollen, and animal droppings before water enters storage.
Why the First Water From a Roof Is Usually Dirtier
A roof acts as a catchment surface between storms. Anything that settles on it can be washed toward the gutter when rain returns.
That may include:
- Dust and soil
- Pollen
- Leaves and plant material
- Bird or rodent droppings
- Insects
- Ash and smoke particles
- Material from roofing and gutters
- Airborne pollutants
This buildup can be greater after a long dry period.
The CDC notes that dirt, germs, and chemicals can enter collected rainwater as it moves through the air, across the roof, and through gutters and storage equipment. Bird droppings are one example of material that can wash into a tank during a storm.
A first flush diverter cannot remove everything, but it reduces the amount of concentrated early runoff entering the tank.
Is a First Flush Diverter the Same as a Leaf Screen?
No. They handle different types of debris.
A leaf screen catches larger material before it travels farther through the system. It can stop leaves, twigs, and similar debris.
A first flush diverter deals with the first portion of water flowing off the roof. That water can contain much smaller particles and dissolved contaminants that would pass through a coarse leaf screen.
A useful system may therefore have both:
Gutter screen → leaf or debris filter → first flush diverter → tank
One does not replace the other.
Keeping leaves and other organic material out of a tank also reduces the amount of material that can settle and break down inside storage.
How Large Should a First Flush Diverter Be?
There is no single first-flush volume that is correct for every roof.
The needed volume can depend on:
- Roof catchment area
- How long the weather has been dry
- Nearby trees
- Bird and animal activity
- Dust and air pollution
- Roof material and condition
- Intended use of the stored water
- Local guidance
- The design of the diverter
A larger roof collects more water from the same amount of rainfall, so it normally requires a larger diversion volume than a small shed roof.
If a sizing method specifies a runoff depth, you can calculate the corresponding volume from your roof area.
For U.S. units:
First-flush volume in gallons = roof area in square feet × flush depth in inches × 0.623
For example, the formula can tell you how many gallons a specified runoff depth represents for your particular roof. The important part is choosing an appropriate flush depth rather than assuming the same value works everywhere.
The CDC specifically notes that the amount diverted depends on the size of the roof feeding the collection system.
Follow the diverter manufacturer's sizing method and any applicable local rainwater guidance when choosing the actual capacity.
Where Should the Diverter Be Installed?
A first flush diverter is normally installed after basic roof debris screening but before the water enters storage.
The exact arrangement depends on the system.
For a rain barrel, the diverter may be installed on the downspout before the barrel inlet.
For a larger cistern, several downspouts may feed a common collection pipe. The system may use individual diverters or a larger diversion arrangement before water reaches the tank.
Whatever the layout, check that:
- Pipes are large enough for expected flow.
- The diverter is accessible for cleaning.
- The drain can empty and reset.
- Diverted water drains somewhere appropriate.
- Overflow and discharge water are carried away from foundations and other areas where concentrated runoff could cause problems.
- Screens and fittings do not create major restrictions that make gutters overflow.
A complicated system with several roof sections or large underground storage may be worth having designed or checked by an experienced rainwater installer.
Does a First Flush Diverter Waste Rainwater?
Explore the role of a first-flush diverter to evaluate service access for sediment removal and odor prevention.
It intentionally gives up a small portion of each suitable collection event.
That is the tradeoff.
Without diversion, you can capture more water, but the dirtiest early runoff may enter your tank. With diversion, you lose some collection volume in exchange for better control of sediment and contaminants.
For a small barrel in an area with little rainfall, that loss may be noticeable. For a large roof and tank, it may represent a relatively small part of the total storm runoff.
This is another reason not to make a diverter much larger than the system requires.
Does a First Flush Diverter Make Rainwater Drinkable?
No.
A first flush diverter can improve incoming water quality, but it cannot establish that roof runoff is potable.
Potable means suitable for drinking.
Rainwater that looks clear can still contain germs or chemicals. Contamination can come from the atmosphere, roofing materials, animal waste, gutters, pipes, or the tank itself.
The CDC states that collected rainwater is not necessarily safe to drink and recommends appropriate testing when rainwater is used for drinking, cooking, or bathing.
If rainwater is intended for drinking, treat it as a complete water-supply system. That can involve suitable catchment materials, debris control, first-flush diversion, appropriate treatment stages, protected storage, regular maintenance, and current laboratory testing. Local health and plumbing requirements also matter.
A first flush diverter should not be treated as a substitute for those steps.
First Flush Diverters Still Need Maintenance
A diverter only works properly if it can empty and reset.
A common problem is a small drain orifice becoming blocked by dirt. If the chamber remains full between storms, the next storm may bypass the intended first-flush process.
Inspect the system regularly and look for:
- Blocked drain holes
- Sediment inside the chamber
- Stuck floating balls
- Damaged seals
- Insect or mosquito access
- Loose pipe connections
- Cracked fittings
- Leaks
- Overflow during heavy rain
Also keep gutters and upstream screens clean.
The amount of maintenance depends on the property. A roof surrounded by trees may need attention much more often than an open roof with little debris.
What Happens If You Do Not Use One?
A rainwater harvesting system can work without a first flush diverter, especially when the stored water has a low-risk non-potable use. But more of the material washed from the roof may enter storage.
Over time, this can contribute to:
- More sediment in the tank
- Dirtier inlet screens and filters
- More organic material in storage
- More frequent cleaning
- Poorer water appearance or odor
- Greater contaminant loading
NSW Health recommends first flush devices because they reduce the amount of roof debris and contamination washed into rainwater tanks.
Whether one is necessary for your particular installation also depends on intended water use and local requirements.
First Flush Diversion Is Part of a Bigger System
A well-planned rainwater system controls contamination at several points rather than expecting one device to handle everything.
That may mean:
- Keeping the roof and gutters in reasonable condition.
- Screening out leaves and large debris.
- Diverting the first roof runoff.
- Using a screened and protected tank inlet.
- Keeping insects, animals, and light out of storage where practical.
- Providing a safe tank overflow.
- Cleaning and maintaining the system.
- Adding appropriate treatment when the intended use requires it.
For garden irrigation, the main goals may be keeping debris out of the tank, pump, and emitters.
For indoor or drinking-water use, water quality and plumbing separation become much more important. Rainwater should not be allowed to contaminate a treated drinking-water supply through an improper plumbing connection.
A first flush diverter is useful because it tackles contamination before it reaches the tank, where removing settled material becomes much harder.
Frequently Asked Questions
Is a first flush diverter worth installing?
It often is useful for roof-based rainwater harvesting. It keeps the first and generally dirtier portion of runoff out of storage, which can reduce sediment and contaminant loading. Its value increases when the roof collects noticeable dust, pollen, animal droppings, or other debris.
Does every rain barrel need a first flush diverter?
Not necessarily. The decision depends on the roof, intended water use, system size, maintenance needs, and local requirements. Even a simple garden-water system should have suitable debris screening and regular cleaning.
How much water should a first flush diverter remove?
There is no universal volume for every system. The required amount depends partly on the catchment area and site conditions. Use a sizing method suitable for your system rather than choosing an arbitrary number of gallons.
Does a first flush diverter remove bacteria?
It can divert some contaminated roof runoff before it reaches storage, but it is not a disinfectant and cannot be relied on to remove bacteria or other disease-causing organisms. It does not make rainwater potable.
Should the diverter empty after every storm?
Most automatic first flush designs need to drain after rainfall so they are ready for the next event. Check the design instructions. A blocked drain can prevent the device from resetting correctly.
Do I still need a filter if I have a first flush diverter?
Possibly. A first flush diverter and a filter perform different jobs. Screens or filters can remove debris and particles, while the diverter rejects an initial volume of roof runoff. The equipment needed depends on how the water will be used.
Can I add a first flush diverter to an existing rainwater tank?
Often, yes. It can usually be added somewhere in the pipework between the roof drainage system and tank inlet. Make sure the new pipework can handle the expected flow and that the diverter remains accessible for draining, inspection, and cleaning.
