The first few millimetres of rain to wash off a roof carry the bulk of the dust, bird droppings, leaf tannin and general grime that's built up since the last downpour. A first flush diverter captures that initial dirty water and sends it to waste before the cleaner water behind it is allowed through to the tank. Blue Mountain Co's range covers five different ways of doing it, and which one suits a given install comes down to how much space is available and how much water needs diverting.
No chamber needed: the Carrot range
The First Flush Carrot 90mm ($84.99) and First Flush Carrot 100mm ($118.89) use a self-contained mechanism to measure and discard the first flush, then seal off, without needing a separate storage chamber underneath. That makes them the easiest option to retrofit into a tight spot — mounted straight onto the downpipe rather than needing room below for a chamber.
Classic chamber-style diverters
The traditional design uses a length of pipe as a storage chamber that fills with the first flush, then overflows cleanly to the tank once full. The Basic Tee 100mm ($65.95) is Blue Mountain Co's original, entry-level version. The Catch-All Tee 100mm ($76.95) is a step up — the catch-all tee stops water skipping across to the tank until the first flush is genuinely complete, rather than relying on timing alone. The Max Chamber with Catch-All Tee ($62.29) uses a larger-diameter chamber to divert more water in a shorter length of pipe, which matters where there's limited height under the gutter.
High-volume Delta diverters
For bigger roofs or commercial catchments, the Delta range trades a compact footprint for genuine volume: the listing states every 1 metre of Delta chamber diverts 73 litres. The Delta Post/Wall 100mm ($315.65) uses 100mm pipe for the chamber, which keeps the install straightforward even at this scale. The Delta Commercial 150mm ($338.99) and Delta Commercial 225mm ($707.19, includes the 225mm tee and reducer to 100mm) step up again for large commercial downpipes, while still using 100mm pipe to build the actual diversion chamber.
In-ground and post/wall, 300mm chamber
The In-Ground 90/100mm ($197.59) and Post/Wall 90/100mm ($208.00) diverters use large-diameter 300mm pipe (bought separately) to build a high-volume chamber either buried in-ground or mounted above ground. For the post/wall version, a galvanised steel stand ($111.79) supports the chamber without needing to fix it to a wall.
Choosing between them
A small roof with reasonably clean runoff (minimal overhanging trees, no bird roosts) can usually get away with a Carrot or a Basic Tee chamber. A larger roof, a catchment with more leaf litter or bird activity, or a commercial site is better served by the Delta or 300mm-chamber ranges, simply because they hold more of the dirty water before it has a chance to reach the tank. Every diverter in this range needs emptying or draining between first flushes — see our companion guide on manual vs electronic release valves for how that part works.
See the full range in our First Flush Diverters collection, part of the wider Blue Mountain Co rainwater harvesting range.
Frequently Asked Questions
Do I need a storage chamber for every first flush diverter?
No. The Carrot 90mm and 100mm both divert the first flush internally without needing a separate chamber underneath, which makes them the simplest option where space is tight.
How much water should a first flush diverter hold?
It depends on roof size and how dirty the catchment is. As a reference point, the Delta chamber range is rated at 73 litres of diversion per metre of chamber pipe, so a bigger or dirtier roof needs a longer chamber. Call our team if you're not sure which size suits your roof.
What's the difference between the Basic Tee and Catch-All Tee chamber diverters?
The Catch-All Tee stops water bypassing into the tank line until the first flush chamber has genuinely finished filling, which the Basic Tee doesn't guarantee on its own.
Can a first flush diverter be installed in the ground instead of on a wall?
Yes. The In-Ground 90/100mm diverter is built for that, using the same 300mm chamber pipe as the Post/Wall version above ground.