Plaster a ferrocement tank from inside and outside at the same time with two people, using 1:3 cement:sand mortar pressed through the mesh and built up to 20–25 mm total, with a smooth interior. Because ring and vertical bars cross at 12–14 mm of steel, a 20 mm wall leaves as little as 3 mm of mortar over the steel on each side, so check thickness as you go and cover every wire completely.
Part of Cisterns and Rainwater Storage
Plastering technique is the way mortar is forced into and built up over the mesh cage of a ferrocement tank so that the steel is fully encased and the wall is dense and uniform. Most leaks in a ferrocement tank trace back to voids, thin spots and cold joints made at this stage.
Two Workers, One Wall: Press From One Side, Back From The Other
The method on the cisterns page is to apply the mortar from both sides at once. The outside worker presses mortar into the mesh while the inside worker supports it with a flat hand or board, so the mortar passes through the wires and fills behind them instead of bridging across the mesh and leaving a hollow. Then both build up the thickness in layers to 20–25 mm. The interior is smoothed.
The Steel Takes Up Most Of A Thin Wall
Take the bars from Frame Construction. At a point where a ring crosses a vertical bar the steel stacks up:
- 8 mm ring + 6 mm vertical = 14 mm
- 6 mm ring + 6 mm vertical = 12 mm
If the steel is centred in the wall, the mortar cover on each side is (wall − steel) ÷ 2:
| Wall | Steel at crossing | Cover each side |
|---|---|---|
| 20 mm | 14 mm | 3 mm |
| 20 mm | 12 mm | 4 mm |
| 25 mm | 14 mm | 5.5 mm |
| 25 mm | 12 mm | 6.5 mm |
The mesh layers add a little more. The tolerance is tight: every bar must be encased, and the wall should be built to the 25 mm end of the range if 8 mm rings are used. Any wire showing at the surface will rust and weep.
What You Need
- 1:3 cement:sand mortar, mixed by volume
- About 250–310 L of placed mortar for the 5,000 L tank (12.6 m² of wall at 20–25 mm), which is 25–31 buckets if your bucket holds 10 L
- Two people, boards, trowels, floats
- A thickness gauge — a wire or nail marked at the wall thickness, pushed in while the mortar is wet
- Shade and water, to keep the work damp
Method: Press, Back, Build, Smooth
Step 1: Mix a stiff mortar. It should be workable but hold its shape when squeezed, and wet enough to be pressed through the mesh. Use each batch before it starts to stiffen, and never re-wet a stiffened one.
Step 2: Press the first layer from the outside while the inside worker backs it up. Cover the mesh completely and make sure mortar comes through to the inside face.
Step 3: Build up in layers to the full thickness, checking with the gauge. Fill every low spot and hollow.
Step 4: Smooth the interior with a steel trowel. The inside surface is the waterproofing face.
Step 5: Finish in one continuous session if you can. Do not leave a horizontal break part way up the wall; if you must stop, end at a rough, wetted edge and dampen it before starting again.
Step 6: Cover and cure immediately (Curing).
Common Mistakes
- Bridging the mesh. Mortar sits on the outside only and leaves hollows; tap the wall and listen for a dull or hollow sound.
- A cold joint. Fresh mortar against dried mortar does not bond.
- Overworking the surface. Troweling too much brings water to the top and weakens it.
- Skipping the gauge. Thin spots show up as seeps later.
- Plastering in the afternoon sun. The surface dries before it can be cured.
Safety
Working inside a tank can kill if it has a lid or roof, and the rescuer dies second
An open-topped tank is safer than a roofed one, but a tank with a lid or roof can be short of oxygen, and carbon dioxide or hydrogen sulphide can pool at the bottom. The commonest fatality pattern is a second person climbing in after the first collapses. Keep the tank open to the air at the top while anyone is inside, and use a harness tied to a line held by an attendant outside, who pulls the line and never climbs in. Never run a petrol or diesel mixer beside the tank: carbon monoxide pools in low places. Do not test the air with a flame.
Wet mortar burns skin and eyes
Wet cement is strongly alkaline. Wear gloves and eye protection and wash splashes off at once.
What’s Next
- Learn next: Curing — what must happen in the 14 days after plastering
- Combine with: Cement Plaster — the richer 1:2 mix used to render an underground cistern