A first flush diverter is a capped pipe that fills with the dirty first runoff from a roof before any water goes to the tank, and its volume is set at 1 L per m² of roof. A 40 m² roof therefore needs 40 L, which a 100 mm pipe holds at about 8 L per metre, so 5 m of pipe; a slow drip hole empties it between storms.
Part of Cisterns and Rainwater Storage
A first flush diverter sends the first part of each rainfall, which carries the roof’s dust, droppings, leaves and insects, to a holding pipe instead of the cistern. It is the single cheapest protection for stored rainwater.
The First Millimetre Of Rain Carries Most Of The Dirt
The first 1–2 mm of rain off a roof is filthy. Rain depth in mm equals litres per m², so 1 L per m² of roof is the first 1 mm. The diverter pipe, typically 50–100 mm in diameter, is capped at the bottom and fills with that water before the flow is directed on to the tank. After the rain stops, a small drip hole in the cap slowly empties the pipe, which resets it for the next storm.
The Pipe Must Be Long Enough, Or Wide Enough, For The Roof
Volume per metre of pipe is the cross-section area times 1 m. For a 100 mm pipe: π × 0.05² = 0.00785 m², so one metre holds 0.00785 m³ = 7.85 L, about 8 L. For a 40 m² roof:
- Volume needed: 40 m² × 1 L/m² = 40 L
- Length of 100 mm pipe: 40 ÷ 7.85 = 5.1 m, so 5 m
- For a 150 mm pipe: π × 0.075² = 0.01767 m² = 17.7 L per metre, and 40 ÷ 17.7 = 2.3 m
A pipe of 1–2 m in a 100 mm diameter holds only 8–16 L, enough for 8–16 m² of roof. A large roof needs a wider pipe, several pipes, or a small drum or tank used as a diverter.
What You Need
- Pipe of 50–150 mm diameter, long enough for the roof
- A cap with a small drip hole at the bottom, or a screw cap that opens for cleaning
- A tee or branch in the downpipe, with the diverter pipe running vertically below it
- Support — a full 40 L diverter holds 40 kg of water
- A place for the discharge away from the tank and foundations
Method: Branch, Fill, Drip, Reset
Step 1: Work out the volume as above, with 1 L per m² of roof.
Step 2: Fit the diverter pipe vertically below a tee in the downpipe, capped at the bottom.
Step 3: Make the drip hole small, so that the pipe empties over a day or two and not during the storm. Too large a hole and clean water flows out of it instead of to the tank; too small and the pipe is still full when the next rain starts.
Step 4: Fit a clean-out cap so you can flush out the sludge that collects in the bottom.
Step 5: Check after the first rain that clean water reaches the tank, that the pipe has emptied by the next storm, and that the discharge soaks away from the tank.
Worked Example
A roof of 40 m² has a first-flush volume of 40 L (the example on the cisterns page). If most of your showers are 2 mm, the diverter takes 1 mm of each, half of the rain, which is why a light-shower climate loses more water to the diverter than a climate of heavy storms; see Roof Coefficient.
Common Mistakes
- No diverter. Contaminated water enters the tank every time it rains.
- A diverter too short for the roof. Dirty water spills on to the tank once it is full.
- A blocked drip hole. The pipe stays full and the next storm’s first flush goes straight to the tank.
- Using a diverter in place of treatment. It lowers contamination but does not make water safe; see Water Purification.
What’s Next
- Learn next: Leaf Screens — the stage before the diverter
- Combine with: Settling Chamber — the stage after it