A first-flush diverter is a small device that throws away the first portion of rain that runs off your roof, before the rest of the water is allowed into your tank. It is the cheapest and most effective way to keep the bulk of the debris that lands on a roof out of your stored water.
Why the first rain is the dirtiest
When it starts to rain, the first water to run off the roof is not clean rain. It washes over the surface and picks up everything that has been sitting there since the last rain:
- Dust and pollen that settled on the roof between storms.
- Bird droppings and insects that accumulated on the surface.
- Leaves, needles and small debris that landed on the roof.
- Algae, moss and lichen growing on the roof, which the first water dislodges.
This initial wash-off is concentrated in the first few minutes of a storm. Once the roof surface has been rinsed, the water running off is much cleaner. A first-flush diverter captures that dirty first portion and diverts it away, so the tank only receives the cleaner water that follows.
How it works
A first-flush diverter is typically a vertical pipe or tube that sits between the downpipe and the tank inlet. The basic idea:
- When rain starts, the first water fills the diverter tube from the bottom up.
- The water in the tube is the dirty first flush. It is held in the tube and does not enter the tank.
- Once the tube is full, the water level rises above the top of the tube and overflows into the tank inlet. From that point on, the cleaner water goes into the tank.
- After the storm, the dirty water in the tube is drained away, so the diverter is ready for the next storm.
Some diverters are simple gravity-fed tubes. Others have a float or a baffle that controls when water is allowed through. The exact mechanism varies, but the principle is the same: hold back the first portion, let the rest through.
Sizing the diverter
The size of the diverter determines how much water is thrown away. A larger diverter holds more water and removes more of the dirty first flush, but it also wastes more water. A smaller diverter wastes less water but may not remove as much of the initial contamination.
The right size depends on your roof area and how dirty the roof tends to be. A larger roof with more debris will benefit from a larger diverter. A smaller roof in a cleaner environment may get by with a smaller one.
VERIFY: the typical recommended diverter volume per square metre of roof area, and whether any standard or council guidance specifies a minimum size — check a plumbing supplier’s sizing guide or a council’s rainwater harvesting guidance before stating a specific figure.
As a general rule, the diverter should be sized so that it can hold the first flush from your roof area without overflowing into the tank during a typical storm. If you are unsure, ask your supplier for a recommendation based on your roof area.
Maintenance
A first-flush diverter is low-maintenance, but it does need occasional attention:
- Empty the diverter after a storm if it does not drain automatically. Some diverters have a drain valve at the bottom that you open to empty the dirty water.
- Check the diverter for blockages. Leaves and debris can clog the inlet or the drain, which will prevent it from working correctly.
- Inspect the diverter for damage or corrosion, especially if it is made of metal. A cracked or corroded diverter will not hold the first flush properly.
If you notice that the water in your tank is consistently dirty or has a strong smell, check the diverter first. A blocked or undersized diverter is a common cause of poor water quality.
Where it fits in the system
The first-flush diverter sits between the downpipe and the tank inlet. It is one of several components that work together to keep your stored water clean:
- Leaf guards on the gutters keep large debris out before it reaches the downpipe.
- The first-flush diverter removes the dirty first portion of the rain.
- A tank inlet filter (sometimes called a mesh screen) catches any remaining debris before the water enters the tank.
- A tank lid keeps out insects, leaves and other contaminants.
Each layer removes a different type of contamination. Together, they make a significant difference to the quality of the water in your tank.
Is a first-flush diverter required?
A first-flush diverter is not always a legal requirement, but it is strongly recommended for any rainwater system that will be used for drinking or cooking. Many councils and water safety guidelines recommend or require a first-flush diverter as part of a rainwater harvesting system, especially for rural or off-grid properties.
VERIFY: whether your council requires a first-flush diverter as a condition of consent for a rainwater tank, and whether any standard specifies a minimum diverter size — check your local council’s building consent requirements or a plumbing supplier’s guidance before stating a specific rule.
Even where it is not required, a first-flush diverter is a low-cost improvement that makes a real difference to water quality. If you are installing a new tank, it is worth including one. If you have an existing tank without a diverter, adding one is a simple upgrade that can improve the quality of your stored water.