An exploration of the first flush diverter by Jesse Savou, ARCSA A.P., Founder of BlueBarrel Rainwater Catchment Systems
Do you need a first flush diverter for your rainwater harvesting system?
A first flush diverter, sometimes called a roof washer, redirects the first flow of water from a storm away from your rain barrels or tanks.
That initial runoff washes dust, pollen, leaves, bird droppings, and other accumulated material from the roof.
The theory is straightforward: divert the dirtiest water so that cleaner rainwater enters your storage system.
It sounds like a good idea. However, many rainwater harvesting professionals do not recommend first flush diverters for every application.
At the American Rainwater Catchment Systems Association’s 2013 national conference, all five members of one professional panel said they preferred not to use them. In another session, keynote speaker Barnabas Kane of TBK Design noted that he had been hired to remove more first flush diverters than to install them.
In this photo, the straight-pipe to the right of the tank is a first flush diverter. Water enters the straight-pipe first, and as sediments sink to the bottom, additional water spills over into the rain tank.
Why the hesitation with first flush diverters?
For systems that collect rainwater primarily for garden irrigation, a first flush diverter can add water loss, maintenance, and complexity without providing much practical benefit.
Here are six reasons to think carefully before installing one:
1. First flush diverters are difficult to size correctly
A first flush diverter must divert the right amount of water for optimal performance, and this is difficult if not impossible to do. There are many variables that go into determining the optimal size for a first flush diverter.
Variables include:
- Roof size, slope, and material
- Rainfall intensity and duration
- Time since the last rain
- Gutter and downspout capacity
- Wind conditions
- Local air quality
- The amount of debris on the roof
Since most of these factors vary significantly even in a single location, there may be no single “correct” size for every storm.
If your first flush is too big, you limit your ability to fill your collection tanks. An oversized first flush diverter sends more water away from your storage system than necessary. This reduces the amount of rainwater you collect. Use a rainfall calculator to estimate how much water your roof can generate. However, you must subtract the water discarded by the first flush diverter—every time it rains.
If your first flush is too small, the unit will be overwhelmed and sediments will enter your primary storage anyway. In fact, if you have accumulated sediments in your first flush diverter from prior storms, you may even introduce extra particulates to your rain collection system. That brings us to the next concern: maintenance.
The organic matter that accumulates on your roof between rains is good for plants and soils. Why divert the light application of fertilizer that nature offers for free? (See #5.)
2. First flush diverters require regular attention
Most well-designed rainwater catchment systems are relatively low-maintenance. A first flush diverter can become one of the most maintenance-intensive parts of the system.
To work properly, the chamber must drain or be emptied between storms. It also needs periodic cleaning so that sediment does not accumulate inside it.
When maintenance is neglected or falls behind, accumulated debris can worsen the problem the device aims to solve.
Some first flush diverters use a slow-release drain. This allows the chamber to empty automatically after each storm. However, these drains create a controlled leak. In areas with intermittent drizzle, light rainfall, or long dry periods, it may continue drawing water away from the storage tanks when every gallon matters.
3. A first flush diverter adds a potential failure point
First flush diverters are often made from exposed vertical pipe. This makes them more vulnerable to accidental impacts, sun and ultraviolet damage, and freeze-cracking than other parts of the rainwater catchment system.
First flush diverters are installed upstream of the rain barrels or tanks by design. This means a damaged or poorly functioning unit may divert too much water, or prevent water from reaching storage altogether.
This will result in slow fill rates, or even empty tanks.
4. Your rainwater system still needs debris screening
A first flush diverter is not a replacement for proper screening.
Many states have adopted code to establish simple standards for building safe and effective rain catchment systems.
One key requirement is that all system openings, including inlets, open tops, and vents, must be screened with 16th-inch mesh. This prevents leaves and debris from entering the storage vessels, and protects against mosquitoes and other small creatures.
A debris excluder, such as a Leaf Eater, screens rainwater before it reaches the downspout diverter and storage system. This keeps larger material out while allowing water to continue flowing toward the rain barrels.
This photo features a BlueBarrel Rainwater Catchment System® with a Leaf Eater (a.k.a. "debris excluder") installed upstream of the standard downspout diverter. The white screened box prevents debris from entering the rain barrel system with durable, stainless steel mesh.
The black plastic inlet hose can be manually detached if the user wants to divert the first rain of the season.
The question, then, is whether a first flush diverter provides enough additional benefit to justify its water loss and maintenance requirements.
For non-potable systems used for irrigation, the required debris screening may provide the simpler and more practical solution.
BlueBarrel’s professionally-designed DIY BlueBarrel Rainwater Catchment System® uses closed, UV-resistant barrels with screened vents and inlets, and a downspout diverter that automatically manages inflow and overflow. A Leaf Eater can be added upstream where additional debris screening is desired.
5. Plants love the organic matter in the first rainfall
The season's first rainfall can contain pollen, dust, and decomposed organic matter.
For a non-potable system used primarily for garden irrigation, plants can benefit from the organic accumulation that the first flush delivers. Why go to the effort to divert what amounts to a nice fertilizer mix?
This does not mean that all roof runoff is suitable for every use. Roofing material, nearby trees, environmental contaminants, and the intended use of the water all matter.
However, garden irrigation systems do not always need to discard the beginning of every rainfall simply because it contains some organic matter.
6. Manually bypassing the first storm may be easier
In climates with a long dry season, the first rain of the year may wash an unusually large amount of dust and debris from the roof. You may decide that this particular runoff is worth diverting. That does not necessarily require a permanent first flush device.
With a well-designed downspout diverter (not to be confused with a "first flush diverter"), you can temporarily disconnect or remove the inlet hose. The first rainfall can then wash the roof while the runoff continues through the downspout.
Reconnect the system afterward, and it will capture subsequent storms without automatically discarding water each time.
This may be one of the world's greatest ironies: if an automatic first flush needs to be checked, cleared or emptied between storms, manually bypassing as described here requires less input than the "automatic."
So, do you need a first flush diverter?
Probably not.
You may decide against one when:
- The water will be used primarily for garden irrigation
- Your system already includes effective debris and insect screening
- Maximizing catchment is a priority
- You prefer a simpler, lower-maintenance design
At BlueBarrel, we focus on practical, non-potable rainwater harvesting for outdoor use. Our pre-designed multi-barrel system emphasizes screened storage, automatic overflow management, expandability, and simple maintenance rather than adding a first flush diverter by default.
Before adding one, consider what you plan to do with the water and whether the potential water-quality benefit outweighs the water loss, maintenance, and added complexity.
The most important goal is to build a safe, effective system that captures and stores as much useful rainwater as possible, and doesn't cause headaches that discourage the user from enjoying the benefits of rainwater harvesting.
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