Cover an underground cistern with a reinforced concrete slab 10 cm thick, with rebar mesh, resting on a concrete ring beam around the top edge, and leave a 60 cm hatch with a lockable cover. The roof must carry a person standing on it; for the 2.5 m pit the slab weighs about 1,200 kg. Keep the cistern covered but not airtight, with a screened vent, and keep the hatch locked so animals and children cannot get in.
Part of Cisterns and Rainwater Storage
A cistern roof is the cover over an underground tank: it keeps out light, animals, debris and people, carries a person’s weight, and holds the hatch, vent and pipe openings. Its design is as important to water quality as the lining.
The Roof Keeps The Water Dark, Clean And Safe To Walk Over
Light grows algae, open water breeds mosquitoes, and an uncovered hole is a hazard to anyone walking past. The roof also carries the ground-level weight of a person and sets where the hatch, inlet and vent sit.
The cisterns page gives the standard design: a reinforced concrete slab 10 cm thick with rebar mesh, bearing on a concrete ring beam cast around the top edge of the pit, with a 60 cm access hatch with a lockable cover. For a cheaper cover, Cistern Building describes timber beams at least 15 cm in diameter spaced 40–50 cm apart, covered with poles, brush and 30–40 cm of earth, with a hatch at least 60 × 60 cm.
What You Need
- Concrete, rebar mesh, and a form (temporary support) for the slab
- A ring beam — a concrete collar around the pit rim that the slab bears on
- A hatch frame and lockable cover, 60 cm wide
- A screened vent pipe with fine mesh, and sleeves for the inlet and overflow pipes
- Props and boards to hold the form
Method: Ring Beam, Form, Reinforce, Pour, Cure
Step 1: Cast the ring beam around the rim, solid and level, so the slab bears on firm ground rather than on the pit wall’s edge.
Step 2: Build the form across the pit to carry wet concrete. Design it so it can come out in pieces through the 60 cm hatch, because it must be removed from inside.
Step 3: Lay the rebar mesh in the slab, and set the hatch frame, vent pipe and pipe sleeves in place before pouring.
Step 4: Pour the slab to 10 cm and keep it wet while it cures (Curing).
Step 5: Remove the form only when the concrete has cured. Taking the props out is work inside a closed pit; see the safety block.
Step 6: Fit the cover and lock it.
Worked Example
For the 2.5 m pit the slab area is π × 1.25² = 4.91 m² and its volume is 4.91 × 0.10 = 0.491 m³. Reinforced concrete is about 2,400 kg/m³, so the slab weighs about 0.491 × 2,400 = 1,180 kg, or about 1,200 kg, a load of 1,180 × 9.81 = 11,600 N, about 12 kN. A person is about 1 kN. The ring beam must carry the slab on firm ground; do not build the edge on loose soil.
Do not extend this design to a wider pit. The 10 cm slab is the cisterns page’s figure for this opening; no tested figure is given here for a larger span. For a wider pit use two smaller cells, or add a central support designed for the load.
Common Mistakes
- No vent. A completely sealed cistern can go anaerobic, and the water smells and tastes bad.
- No lock on the hatch. Children and animals can fall in.
- Pipe sleeves added after the pour. They leak at the join.
- Parking a cart, animal or vehicle on the roof. The design carries a person only.
- Pulling the form early. An under-cured slab cracks or collapses.
Safety
The space under the slab is a confined space and the form can fall
A roofed cistern may be short of oxygen, and may hold hydrogen sulphide or carbon dioxide, which pools at the bottom. Stripping the form means working inside it. Do it only with the cistern ventilated at two openings, the entrant in a harness tied to a line held by an attendant, and a second person outside who pulls the line and never climbs in after a collapsed colleague, the commonest fatality pattern. Do not test the air with a flame. Keep out from under a form while props are being knocked out.
What’s Next
- Learn next: Inlet Filtering — what happens at the inlet pipe
- Combine with: Water Extraction — how the hatch should and should not be used