An overflow pipe leaves the cistern at the same level as the inlet and carries any surplus away to a soakaway, and it must be at least as large as the inlet pipe or the tank backs up. For a 40 m² metal roof in heavy rain of 50 mm/h the peak inflow is about 0.5 L/s, which a 50 mm pipe on a 1% fall can pass; fit a fine mesh or flap on the outlet end.
Part of Cisterns and Rainwater Storage
The overflow pipe is the fixed exit for water above the tank’s full level. It sets the maximum water level and must pass the full inflow of the heaviest storm without surcharging the inlet.
Where The Pipe Sits Sets How Much The Tank Holds
The cisterns page puts the overflow at the same level as the inlet, with the outlet pipe 10 cm above the floor. The overflow’s invert (bottom inside of the pipe) is the highest water level the tank will reach. Put it at or just below the inlet pipe so a full tank cannot back up into the inlet and the gutter, and note that a lower overflow means less storage: every centimetre between the overflow and the roof is air.
The Pipe Must Pass The Inflow
If the overflow is smaller than what arrives, the level rises above it until pressure forces the flow through, or until water goes out of the inlet. Estimate peak inflow from roof area, a heavy rain rate and the coefficient:
- Roof 40 m², rain 50 mm/h (an illustrative heavy rate), coefficient 0.9 for metal
- 40 × 50 × 0.9 = 1,800 L/h = 0.5 L/s
For a 50 mm pipe flowing full on a 1% fall (0.01 m/m) with Manning’s roughness 0.011 for smooth plastic pipe: area = π × 0.025² = 0.00196 m², hydraulic radius = 0.05 ÷ 4 = 0.0125 m, so Q = (1/0.011) × 0.00196 × 0.0125^(2/3) × 0.01^½ = 90.9 × 0.00196 × 0.0539 × 0.1 = 0.00096 m³/s = 0.96 L/s. That is about twice the inflow, so a 50 mm pipe passes it, if its own entrance is not choked by a screen. The inlet pipe itself is sized by the roof, so matching the overflow to the inlet size is the simple rule.
What You Need
- Pipe at least as large as the inlet, in sound, UV-resistant plastic or clay
- A pipe sleeve cast into the wall or cut through carefully and sealed
- A fine mesh or flap valve for the outlet
- A continuous fall to a soakaway or ditch
Method: Sleeve, Pipe, Fall, Screen
Step 1: Mark the level on the inside wall: at or just below the inlet pipe.
Step 2: Fit the sleeve and seal it watertight, as with every penetration through a cistern wall.
Step 3: Lay the pipe with a steady fall away from the tank, with no sags that hold water.
Step 4: Fit the screen or flap on the open end.
Step 5: Put a splash stone under the outlet so the falling water does not scour a hole.
Step 6: Test by running a hose into the tank until it overflows, and follow the water.
Common Mistakes
- Overflow higher than the inlet. The tank backs up into the gutter.
- A sagging pipe. It holds a pool and breeds mosquitoes.
- No screen. Insects and rodents enter.
- Piping into a closed drain. See Overflow Management.
What’s Next
- Learn next: Soakaway — where the overflow goes
- Combine with: Leaf Screens — keep the inlet clear so the overflow is not the first place water finds a way out