A gravity tap is a tap set through the wall of an above-ground or elevated tank about 10 cm above the bottom, so water runs out under its own weight and the sediment on the floor stays behind. In the 5,000 L tank that leaves 201 L unused, and a full tank gives 0.24 bar at the spout, falling as the level drops, so raise the tank on a base high enough to fit a bucket under the spout.
Part of Cisterns and Rainwater Storage
A gravity tap is an outlet fitting in the side of a tank that releases water by the pressure of the water above it, with no pump. It is the simplest and best way to draw from a tank that stands above ground or on a hillside.
The Tap Sits Above The Floor Because Sediment Lies On It
Dirt that gets past the screens settles to the floor. A tap at floor level would draw it out into every bucket. Setting the tap 10 cm above the bottom (the cisterns page’s figure) leaves a layer of water and sediment behind. The cost is volume you cannot draw: in the 5,000 L tank of 1.6 m diameter, π × 0.8² × 0.10 = 0.201 m³ = 201 L, which is 201 ÷ 5,030 = 4% of the 5,030 L capacity.
What You Need
- A through-wall tap fitting — a threaded bulkhead connector with a washer inside and outside, fitted into the wall before plastering or set in a cast sleeve
- A tap — a ball valve, a gate valve, or a self-closing spring tap
- A base or plinth that lifts the tank so a container fits under the spout
- A drip pad and drain to a soakaway, for spilled water (Soakaway)
Method: Fit, Seal, Raise, Drain
Step 1: Mark the position 10 cm above the inside floor.
Step 2: Fit the bulkhead through the wall during construction so the mortar is packed around it, or into a sleeve cast into the wall. Drilling a finished ferrocement wall risks cracking it.
Step 3: Raise the tank. The tap is 10 cm above the tank floor, and the tank stands on a 10 cm pad, so the spout is only about 20 cm above the ground. A bucket is taller than that, so set the tank on a plinth or have a pit at the tap. Measure your own container before building.
Step 4: Fit the tap and test it for leaks at the fitting, with the tank full.
Step 5: Lay the drip pad so spilled water runs away to the soakaway and not under the tank.
Worked Example
Pressure. A full 2.5 m tank has 2.4 m of water above the tap. Pressure = 1,000 × 9.81 × 2.4 = 23,500 Pa = 23.5 kPa, about 0.24 bar.
Flow falls as the level falls. Flow through a short outlet rises roughly with the square root of the head. With the level down to 0.6 m above the tap, the head is a quarter of 2.4 m, so the flow is about √(0.6 ÷ 2.4) = √0.25 = 0.5, half the full-tank flow. This is why the last of the tank fills a bucket slowly.
A tap left open. The water above the tap is 5,030 − 201 = 4,829 L. At an assumed 0.2 L/s, an open tap drains it in 4,829 ÷ 0.2 = 24,145 s, which is 6.7 hours. A self-closing tap prevents the loss.
Common Mistakes
- Tap at floor level. It draws sediment.
- Drilling the finished wall. It cracks the shell.
- No plinth. Nothing fits under the spout, and people dip a bucket in from the top instead.
- Letting the spout touch the container. The tap fouls and passes dirt back.
What’s Next
- Learn next: No Bucket Dipping — the habit the tap is there to end
- Combine with: Tap Types — choosing between ball, gate and self-closing taps