Build the ferrocement frame from rebar rings of 6–8 mm bar spaced 15 cm apart up the height, vertical bars of 6 mm at 15 cm around the rings, and two layers of chicken wire tied tightly over the whole cage. For the 5,000 L tank that is about 17 rings and 34 vertical bars, roughly 170 m of rebar, and 25 m² of wire mesh before overlaps.
Part of Cisterns and Rainwater Storage
The frame is the steel cage inside a ferrocement tank wall: rings that resist the water’s outward push, vertical bars that hold the rings in place, and mesh that holds the mortar. The mortar can only be as good as the cage it sets around.
The Rings Carry The Water’s Sideways Push
Water presses outward on the wall, harder with depth. The rings carry that pull. The vertical bars hold the rings apart and in place while the mesh is tied on and the mortar goes in. The two layers of chicken wire hold the mortar and spread any cracking into fine lines.
What You Need
- Rebar, 6–8 mm, for rings and 6 mm for verticals
- Chicken wire, two layers, enough to wrap the whole cylinder
- Tying wire and pliers
- A pad, 2 m across and 10 cm thick, with the rebar stubs or a keyed edge to tie to
- Gloves for handling cut wire and rebar
Method: Rings, Verticals, Mesh
Step 1: Bend rings to the tank diameter. For the 1.6 m tank each ring is a circle of circumference π × 1.6 = 5.03 m, with an overlap at the join tied tight.
Step 2: Space the rings 15 cm apart up the 2.5 m height: 2.5 ÷ 0.15 = 16.7, so about 17 rings.
Step 3: Tie vertical bars of 6 mm at 15 cm around the rings: 5.03 ÷ 0.15 = 33.5, so about 34 bars, each 2.5 m long.
Step 4: Wrap two layers of chicken wire around the cage, pulling it tight, overlapping the edges and tying it to the rebar at close intervals. Loose mesh leaves hollows in the mortar.
Step 5: Check the cage. It should be round, plumb and rigid, with nothing sticking out of the planned wall surface.
Worked Example: Steel Quantities And Hoop Pull
Quantities for the 5,000 L tank. Rings: 17 × 5.03 = 85.5 m. Verticals: 34 × 2.5 = 85 m. Total about 170 m, before overlaps and waste. Mesh: wall area π × 1.6 × 2.5 = 12.6 m², two layers, 25.1 m², about 25 m².
Why the lowest rings matter most. Pressure at the bottom of a full 2.5 m tank is 1,000 × 9.81 × 2.5 = 24,500 Pa. The pull in the wall per metre of height is pressure × radius = 24,500 × 0.8 = 19,600 N, about 19.6 kN, at the base. It falls steadily to zero at the top.
Rings per metre of height at 15 cm spacing: 1 ÷ 0.15 = 6.67. A 6 mm bar has a cross-section of π × 3² = 28.3 mm². Steel area per metre = 6.67 × 28.3 = 188 mm². Stress = 19,600 N ÷ 188 mm² = 104 N/mm² at the base. Assuming plain mild steel of unknown grade with a yield strength of about 250 N/mm² (an assumption, since salvaged bar is unlabelled), that is 42% of yield, with the mortar’s own help ignored.
This shows the steel will not snap. It does not prove the tank will stay watertight, because mortar cracks at a far smaller pull than steel yields; the mesh and the tight spacing are what keep those cracks hairline. If you have rebar to spare, put the extra rings in the bottom third of the height, where the pull is greatest.
Common Mistakes
- Loose tying. The cage moves under the plastering.
- Rebar too thick for the wall. See Plastering Technique for the cover left on each side.
- A gap in the mesh. Mortar sags through.
- Sharp wire ends facing outward. They snag hands and stop the mortar smoothing.
Safety
Cut wire and rebar tear skin
Chicken wire and cut rebar ends are sharp; wear gloves and keep face and eyes clear when twisting wire. Bent rebar can spring back. A cage taller than a person can fall over in wind until it is plastered; brace it.
What’s Next
- Learn next: Plastering Technique — putting mortar on the cage
- Combine with: Ferrocement Tanks — the whole tank