Cistern Construction: Underground Large-Volume Water Storage

Why a Cistern Instead of Barrels

Fifty-five gallon drums are a common prepper water storage solution. They work, but the math shows their limit quickly.

A family of four at 1 gallon per person per day needs 120 gallons per month — just for drinking. Add sanitation, cooking, and minimal hygiene and you are at 300-500 gallons per month. Twenty 55-gallon drums hold 1,100 gallons and weigh 9,200 pounds when full. They take up significant space, are visible, and require manual filling.

A 2,500-gallon buried cistern holds the equivalent of 45 drums, stores underground out of sight, connects directly to a pump system, and fills passively from roof runoff. The comparison is not close for any serious long-term water security plan.

Site Selection

Position relative to the house:

• Close enough for easy connection to roof gutters (intake) and to a pump or gravity feed point (distribution)
• Far enough from the foundation that excavation does not compromise structural footings — typically 5-10 feet minimum
• Downhill from the house for gravity-fed distribution, or uphill if you want gravity to supply water inside the house (requires being at least 2-3 feet above the highest tap for meaningful pressure)

Soil conditions:

• Stable soil that will not shift significantly with moisture (avoid expansive clay soils for concrete cisterns without proper engineering)
• Adequate drainage around the buried tank (a tank surrounded by saturated soil is subject to buoyancy forces — an empty buried tank can actually float upward in waterlogged soil)
• No underground utilities in the excavation zone — call 811 before digging

Frost line: The cistern inlet and all connecting pipes must be buried below the local frost depth. In the northern US, this can be 4-6 feet. Check your county's required frost depth. Water in pipes above the frost line will freeze and burst.

Setbacks from contamination sources:

• Minimum 50 feet from any septic tank or leach field
• Minimum 100 feet from any privy, composting toilet vault, or animal waste area
• Downhill from all of these (groundwater contamination moves downhill)

Polyethylene Tank Installation

The fastest and most accessible approach for most preppers.

Tank selection: Poly tanks (also called poly cisterns or leg tanks) are available from agricultural supply stores, tank dealers, and online. Common sizes: 500, 1,000, 1,500, 2,500, 5,000, 10,000 gallons.

Look for:

• Food-grade polyethylene (FDA-compliant, UV-stabilized for any above-ground portions)
• Black or dark blue color (blocks light to prevent algae growth inside)
• Inlet opening at the top (minimum 12-16 inch manway for cleaning access)
• 2-inch outlet fitting at the base (for pump connection or gravity drain)

Tanks are available with or without pre-installed fittings. Fittings can be added with a hole saw and bulkhead fittings.

Excavation: Dig a hole 12 inches wider and 12 inches deeper than the tank dimensions on all sides. This gap allows a gravel bed and backfill that distributes load evenly around the tank.

A 2,500-gallon vertical poly tank is typically 8-10 feet tall and 5-7 feet in diameter. Excavation depth: 9-11 feet. This requires a small excavator — hand digging at this depth is practical only for smaller tanks.

Installation:

1. Line the base with 6-12 inches of clean gravel (not soil — gravel distributes load and prevents upward pressure from groundwater)
2. Lower the tank into the hole using a rented equipment or working with the tank supplier
3. Fill the tank with water before backfilling — a filled tank resists buoyancy from groundwater, while an empty tank can float
4. Backfill around the tank in 6-inch lifts, compacting each layer — use clean fill (not clay), or gravel for the first 12 inches around the tank walls
5. Install inlet and outlet piping before the tank is fully covered
6. Leave the manway accessible — the lid should be at or slightly above grade

Inlet connection: Route your roof gutters to a first-flush diverter, then to the cistern inlet pipe. The first-flush diverter diverts the first 10-20 gallons of each rainfall — this water carries accumulated bird droppings, dust, and debris from the roof — before allowing the cleaner remainder to flow to the cistern.

A first-flush diverter is simply a vertical pipe that fills with contaminated first-flush water (tipping the flow to the cistern after it fills) and then drains slowly between rain events. DIY from 4-inch PVC with an end cap that has a small hole for slow drainage.

Concrete Cistern Construction

For those building permanent homestead infrastructure.

Dimensions for a 5,000-gallon cistern: Interior dimensions: 10 feet long × 8 feet wide × 8 feet deep. The volume is 640 cubic feet — approximately 4,800 gallons (minus the rounding of walls and floor).

Construction method:

Step 1: Excavation Excavate to 12 inches deeper than the cistern depth (for the gravel base) and 12 inches wider on all sides (for forms).

Step 2: Gravel base Install 6-12 inches of compacted crushed gravel. Slope the base very slightly (1%) toward one corner — this allows full drainage for cleaning.

Step 3: Footings Pour a 12-inch-wide, 12-inch-deep concrete footing around the perimeter of the cistern base. Allow to cure 72 hours.

Step 4: Floor slab Pour a 4-inch reinforced concrete floor slab (1/2-inch rebar on 18-inch grid) over the gravel base, tying it to the footing. Slope as planned. Allow 72 hours to cure.

Step 5: Walls Use concrete block (8×8×16 CMU) for accessible DIY construction, or poured concrete with forms for the strongest result. If using block:

• Fill all voids in blocks with concrete and rebar vertical columns every 4 feet
• Two rows of horizontal rebar in mortar joints every 24 inches
• 8-inch block is the minimum for a water-holding structure

Poured concrete walls: 6-inch minimum thickness, 1/2-inch rebar at 18-inch grid (horizontal and vertical).

Step 6: Waterproofing This is the most critical step. Concrete is porous and must be sealed for water storage.

• Apply two coats of crystalline waterproofing compound (Xypex, Kryton, or equivalent) to all interior surfaces — this penetrates the concrete and forms crystals that block water passage
• Coat the exterior walls with waterproofing before backfilling to prevent groundwater infiltration
• Install a drain collar around the exterior at grade to direct surface water away from the cistern

Step 7: Cover and access A reinforced concrete lid, or concrete blocks in a series of lintels supporting an accessible panel. The cover must support the weight of any vehicle or equipment that might drive over it. Standard residential cistern covers are rated for foot traffic only — if you need vehicle-crossing capability, engineer accordingly.

Access opening: minimum 18×18 inches for cleaning access. Cover with a lockable or tight-fitting lid.

Step 8: Inlet, overflow, and outlet Install 4-inch PVC through the wall (with a sleeve and waterproofing compound around the penetration) for:

• Inlet: positioned high on one wall
• Overflow: 6 inches below the top of the wall (prevents overfilling)
• Outlet: at the very base of one wall, for pump connection or gravity feed

Pumping Systems

Hand pump: For small cisterns (under 1,000 gallons) or as a backup, a deep well hand pump (pitcher pump) handles cisterns down to 15-20 feet. Below that, a force pump is needed.

Submersible electric pump: The standard for residential cisterns. A 1/2-HP submersible pump lowered into the cistern through the manway, connected to a pressure tank and pressure switch inside the house, supplies water on demand like a well pump. Requires 110V power — plan for generator or battery backup.

Gravity feed: If the cistern is higher than the points of use, gravity provides pressure. Every 2.3 feet of elevation provides 1 psi. You need 20-30 psi for adequate flow — meaning the cistern bottom needs to be 46-70 feet above the lowest tap, or 23-35 feet above for half-adequate pressure. This works in terrain-constrained applications (hillside cisterns above the house) but is rarely available in flat suburban settings.

Maintenance and Water Quality

Annual inspection:

• Remove all water, clean interior surfaces with a 1:10 bleach solution applied by brush
• Inspect for cracks (concrete) or wall deformation (poly)
• Check all inlet, outlet, and overflow connections
• Inspect and clean first-flush diverter
• Refill and treat

Ongoing treatment: Maintain a chlorine residual in the cistern to prevent bacterial growth. Add 1/4 teaspoon of unscented household bleach per 50 gallons monthly, or as needed to maintain 0.5-1 ppm free chlorine. Test with pool test strips.

Before drinking: Always treat cistern water through a complete purification chain before drinking: pre-filter (screen), sediment filter, slow sand filter or ceramic filter, and UV or chemical disinfection. The cistern is storage — the treatment chain makes it drinking water.