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Weathermatic First Flush Rainwater Diverter

R 819.99 Vat Incl.
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Protect your rainwater quality by diverting the most contaminated water that washes off your roof with the first few millimetres of rainfall with a First Flush Downpipe Diverter.

Customise and install with ease

Supplied in a kit with T junction, ball and drip-drainer included, this first flush diverter can be installed on walls, posts, stands or even under your eaves. Simply add a customised length of pipe to divert your desired volume of dirty, contaminated water from each downpipe.

Improve your rainwater quality

By diverting and discarding the most contaminated water from your roof, the First Flush Downpipe helps keep hazardous bacteria and fine particles out of your tank. This improves the quality of the rainwater you collect and is a key step in effective Rain Harvesting practices.

Irrigate your garden

The first flush diverter’s slow release valves automatically drains the dirty water you capture so you can enjoy the benefits of first flush diversion again and again. The draining water can feed your garden irrigation system or be discarded into stormwater.

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.

The length of pipe required to form your first flush diversion chamber will depend upon how much water you need to isolate and capture. The following measurements are given as a guide only.

1 metre of 90mm pipe = 5.7 litres
1 metre of 150mm pipe = 18.8 litres
1 metre of 300mm pipe = 72 litres
3 feet of 4″ pipe = 2 gallons
1 metre of 100mm pipe = 8.8 litres
1 metre of 225mm pipe = 45 litres
3 feet of 3″ pipe = 1.5 gallons
3 feet of 12″ pipe =19 gallons

First Flush diverters help you plan for volume by allowing you to choose your catchment volume with custom catchment size and/ or an advanced release valve.

First Flush Diverters help divert the first flus, through their design. They are installed using a T junction or high flow inlet to which the diversion chamber is fitted. As the chamber fills with the initial dirty water from your roof, a ball rises until it seals the inlet, allowing the rest of your rainwater, which is much cleaner, to flow directly to your tank.


  1. Determine the length of 100mm pipe required for your first flush diversion chamber. As a rough guide: 1 metre of 100mm pipe holds approximately 8.8 litres of water. Ensure all cut edges are clean and smooth.
  2. To attach the threated coupling to your diversion chamber pipe, insert a length of male 100mm pipe inside the threaded coupling and glue in place using an appropriate sealant. Glue the other end inside your diversion chamber pipe. Attach the threaded coupling to your diversion chamber pipe applying solvent weld glue to the socket of the coupling and pipe, then push together and hold until the glue sets.
  3. Select the appropriate flow control washers and fit it into the hose connector with the side marked “TOP” showing. Save the remaining washers for possible future use. Start by using the Control Washer with the smallest gauge hole (lowest number). Try a larger gauge Washer if experiencing blockages. Save the remaining washers for possible future use.
  4. Insert the plastic filter screen in through the base of the screw cap with O-ring seal and secure by attaching the hose connector and flow control washer.
  5. Attach the screw cap with O-ring seal (and assembled components) to the threaded coupling.
  6. Measure your existing downpipe and cut to create space for the T-junction. Your diverter must sit at least 150mm from the ground when fully assembled, so select your installation point and measure and cut accordingly. Ensure all cut edges are clean and smooth.
  7. Connect the T-junction to the existing downpipe and install with solvent weld glue in the same manner as step 2.
  8. Insert the ball seat into the T-junction, with the narrow end of the seat facing down.
  9. Place the ball inside your first flush unit through the upper end of the diversion chamber pipe.
  10. Apply glue to the T-junction and upper end of the diversion chamber pipe and insert the pipe into the junction and hard up against the ball seat, holding until the glue sets.
  11. Position your assembled first flush diverter in place and attach to the wall using the supplied brackets, supporting the unit until it is fully secured. The upper bracket must sit directly under the T-junction, where it will hold the weight of the unit.

Download the installation guide here.


It is important to ensure that your first flush diverter outlet remains clear of any debris. If your outlet becomes blocked, the chamber will not empty and the first flush of water will not be diverted when it rains.

To ensure the flow of water through your diverter’s outlet, periodically unscrew the outlet to allow debris to fall out. If the diversion chamber is full of water, take care as it empties. Remove the hose connector, flow control washer, and filter screen and hose or wash the screen with clean water. Check the flow control washer for any blockages and remove and clean as necessary.

For best results and minimal maintenance, we recommend installing rain heads such as our Leaf Eater rain heads on all your downpipes to limit the volume and number of leaves and debris that reach your first flush diverter.