Cistern Building

Part of Lime & Cement

Designing and constructing waterproof water storage cisterns using lime-based cement and plaster.

Why This Matters

Clean water storage is one of the most critical infrastructure needs in any settlement. Rain falls intermittently, streams may be distant or seasonal, and wells can go dry. A properly built cistern captures rainwater or stores water from other sources, providing a reliable supply through dry periods. The difference between a settlement that thrives and one that fails is often the ability to store water.

Building a cistern that actually holds water — without leaking, cracking, or contaminating the contents — requires waterproof construction. Lime-based cement and plaster, when properly formulated and applied, create structures that hold water for decades or centuries. The Romans built massive cisterns throughout their empire, some of which still hold water two thousand years later. The Basilica Cistern in Istanbul, built in 532 AD, held 80,000 cubic meters of water.

You do not need Roman engineering resources to build a functional cistern. A household cistern holding 2,000-10,000 liters can be built by a small team using locally available stone, lime mortar, and waterproof lime plaster. This article covers the complete process from site selection through waterproofing.

Site Selection and Design

Location Requirements

1. Higher than the water table. If your cistern sits in groundwater, external water pressure can crack the walls or contaminate stored water. Dig a test pit first — if it fills with water, the site is too low.

2. Below collection surfaces. If capturing rainwater from a roof, the cistern must sit lower than the roof gutters so water flows in by gravity.

3. Stable ground. Avoid sites on fill, loose sand, or clay that swells when wet. Settling ground cracks cistern walls. Bedrock or firm, undisturbed soil is ideal.

4. Away from contamination. Minimum 15 meters from latrines, animal pens, and waste disposal areas. Minimum 3 meters from large trees (roots crack masonry).

5. Accessible for maintenance. You will need to enter the cistern periodically for cleaning and repair. Provide a hatch large enough for a person.

Sizing

Calculate based on water needs and rainfall:

Household Size	Daily Need	30-Day Storage	Cistern Volume
2-4 people	40-80 liters	1,200-2,400 liters	2,000-3,000 liters
5-8 people	100-160 liters	3,000-4,800 liters	4,000-6,000 liters
10-20 people	200-400 liters	6,000-12,000 liters	8,000-15,000 liters

Shape Options

• Cylindrical: Strongest shape — the circular walls distribute water pressure evenly. Most efficient use of material. Harder to construct without formwork.
• Rectangular: Easier to build with straight walls. Corners are stress points — reinforce them. Less efficient shape but simpler construction.
• Bottle-shaped: Narrow top, wide bottom. Traditional in Mediterranean and Middle Eastern regions. Reduces evaporation and contamination. The overhanging top acts as a compression arch.

Construction Process

Step 1: Excavation

For a partially or fully buried cistern (recommended — underground is cooler, reducing algae growth and evaporation):

1. Excavate the pit 30-50 cm larger than the finished interior dimensions on all sides. This provides working space for wall construction.
2. Excavate to planned depth plus 20-30 cm for the floor slab.
3. Level and compact the bottom firmly.
4. If groundwater seeps in during excavation, the site is too low — choose another location or build above ground.

Step 2: Foundation and Floor

1. Lay a gravel drainage layer (5-10 cm) at the bottom of the excavation. This prevents moisture from wicking up through the floor.

2. Pour a concrete floor slab. Mix lime concrete: 1 part hydraulic lime (or lime with pozzolan) : 2 parts sand : 3 parts gravel. Make the slab 15-20 cm thick.

3. While the concrete is still damp, roughen the surface where the walls will sit. This provides a key for the wall mortar.

4. Cure the slab for at least 7 days, keeping it damp by covering with wet cloth or straw. Lime concrete gains strength slowly and must not dry out too fast.

Step 3: Wall Construction

Build walls using local stone or brick laid in hydraulic lime mortar:

1. Select dense, non-porous stone. Granite, basalt, or dense limestone are ideal. Avoid sandstone or soft limestone — they absorb water.

2. Lay stones in courses with full mortar beds. Every joint must be completely filled — no voids. Voids become leak paths.

3. Wall thickness: Minimum 20 cm for cisterns up to 2 meters deep. For deeper cisterns, increase wall thickness by 5 cm per additional meter of depth.

4. Bond stones across the wall thickness. Periodically lay stones that span the full wall width (through-stones or bonding stones). These tie the inner and outer faces together and prevent delamination under water pressure.

5. At corners (rectangular cisterns), interlock stones from each wall. Corners are the weakest points and must be reinforced.

Step 4: Mortar Specifications

Standard lime mortar is not waterproof. For cistern construction, you need hydraulic mortar that sets underwater and resists water penetration.

Hydraulic mortar formula:

Component	Parts by Volume
Hydraulic lime OR lime + pozzolan	1
Clean sharp sand	2
Crushed brick dust (pozzolan)	0.5

If natural hydraulic lime is unavailable, add pozzolanic material to ordinary lime:

• Crushed fired brick, ground fine
• Volcanic ash (if available)
• Calcined clay (any clay heated to 700 degrees Celsius and ground)

Mix to a stiff consistency. The mortar should hold its shape when squeezed and placed, not slump.

Step 5: Waterproof Plaster (Cocciopesto)

The interior of the cistern must be plastered with a waterproof render. The Roman technique — cocciopesto — uses crushed brick mixed with lime to create a hydraulic plaster that is essentially impervious to water.

Cocciopesto formula:

Component	Parts by Volume
Lime putty (aged 3+ months)	1
Crushed brick dust (fine, < 2mm)	2
Fine sand	1

Application:

1. Dampen the wall surface thoroughly. Plaster will not bond to dry stone.

2. Apply a scratch coat (8-10 mm thick). Press firmly into the stone joints and surface. Score the surface with horizontal lines using a trowel edge or stick while still soft. Allow to set for 24 hours.

3. Apply the finish coat (5-8 mm thick) over the dampened scratch coat. Work it smooth with a wooden float or flat stone. The smoother the finish, the more waterproof and the easier to clean.

4. Apply to the floor as well. The floor-wall junction is a critical leak point. Build a cove (a rounded fillet of plaster) at the junction to eliminate the sharp corner where cracks tend to form.

5. Cure slowly. Keep the plaster damp for at least 14 days by covering with wet cloth or filling the cistern with a few centimeters of water. Rapid drying causes shrinkage cracks that destroy waterproofing.

Inlet and Outlet Design

Inlet (Water Entry)

1. Build a stone or clay pipe through the upper wall for water to enter.
2. Place a screen (woven cloth, basket, or stone grid) over the inlet to filter debris, leaves, and insects.
3. Slope the inlet pipe slightly downward into the cistern so water cannot back up.
4. For rainwater collection, connect to roof gutters via a channel or pipe.

First-Flush Diverter

The first rain after a dry spell washes dirt, bird droppings, and debris off the roof. Divert this contaminated “first flush” away from the cistern:

1. Install a small side chamber or pipe that fills first before water enters the main cistern.
2. Size for roughly 1 liter per square meter of roof area.
3. Include a slow-drain hole so the diverter empties between rain events.

Outlet (Water Extraction)

1. Draw pipe: Install a pipe through the lower wall, 10-15 cm above the floor (not at floor level — sediment collects on the bottom). Seal around the pipe with extra plaster.
2. Overflow pipe: Install near the top of the cistern to prevent overfilling, which can crack walls. Route overflow away from the foundation.
3. Access hatch: A covered opening in the roof/top large enough for a person to enter for cleaning. Seal with a tight-fitting stone or wooden lid to exclude light (prevents algae), insects, and animals.

Drain

Include a drain at the lowest point of the floor for periodic cleaning. This can be a plugged pipe through the wall or a simple removable stone in the floor. Route the drain away from the cistern foundation.

Maintenance

Regular Tasks

Task	Frequency
Check inlet screens for blockage	After each rain
Inspect for cracks or plaster damage	Monthly
Clean sediment from floor	Annually
Recoat interior plaster (touch-up)	Every 3-5 years
Full re-plastering	Every 15-25 years

Water Quality

• Keep the cistern covered and dark to prevent algae growth.
• Add a small amount of lime water (1 tablespoon of lime putty per 1,000 liters) to keep pH slightly alkaline, which inhibits bacterial growth.
• If water tastes flat or stale, aerate it by pouring back and forth between clean containers before drinking.

A Properly Built Cistern Lasts Generations

Roman cisterns in North Africa, the Middle East, and Mediterranean islands are still functional after 2,000 years. The technology is proven. Take the time to source good materials, apply the waterproof plaster correctly, and cure it properly. A cistern built well today may serve your community for centuries.