Divert the First Flush | Rain Harvesting Steps | BMCo

Divert the
first flush

Having water that isn’t fit-for-purpose is the same as having no water at all. So it’s important to stop dirty or hazardous water getting into your tank and contaminating your rainwater supply.    

Hazardous fine particles that are blown onto your roof and washed off by the first few millimetres of rain pose a particular threat to your rainwater quality. Diverting this first “flush” of high-risk water keeps these hazardous particles out of your tank to protect the quality of the rainwater you harvest. That’s why incorporating first flush diversion is vital if you want to enjoy a reliable and fit-for-purpose supply of cleaner rainwater.  

Divert Flush 1

In particular, we recommend doing all you can to account for challenges posed by:


Vegetation           Unwelcome Intruders

Fine Particles      Hidden Activity


Incorporate first flush diversion

First flush diversion protects your rainwater quality by minimising the volume of suspended and dissolved fine particles that end up in the water you harvest.

A vital part in any Rain Harvesting system, first flush diverters channel, capture and isolate the most contaminated rainwater from your roof in special diversion chambers.  You can then manually empty this water or automatically release it into stormwater or your garden.

First Flush How It Works Line Drawing
Divert Flush 2(1)

Calculate your diversion needs 

Generally speaking, the more water you divert, the better the quality of the rainwater that ends up in your tank. However, diverting more rainwater than necessary can lead to volume shortages, so it’s important to calculate your diversion needs, and divert water accordingly.  

The following formulas offer general guidelines for calculating how much water you should divert: 

  • Areas with low pollution (open fields, no trees, no bird droppings or animal matter, clean environment) = divert 0.5 litres of water per square metre of roof 
  • Areas with higher pollution (leaves and debris, bird droppings, animal matter/carcasses, pollution) = divert at 2 litres of water per square metre of roof.  

This calculation will allow you to determine what length and size pipe or pipes you need for your diversion chambers.

Products that can help

First Flush Diverters
First Flush Diverters

Find out more about applying the 8 Rain Harvesting Steps


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Rain Harvesting Handbook


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