As an affiliate, we may earn a commission from qualifying purchases. We get commissions for purchases made through links on this website from Amazon and other third parties.
A cistern may be built under a house to save space, protect stored water from sunlight and heat, and place the tank close to the plumbing that uses the water. Underground or under-house cisterns can work well, but they are harder to inspect, repair, and replace than an above-ground tank.
The location also raises important concerns about structural loads, leaks, drainage, access, and water quality. A cistern should not simply be placed beneath a house because there happens to be room.
Why Put a Cistern Under a House?
Several practical reasons can make an under-house cistern attractive.
It Saves Yard Space
Large cisterns take up a lot of room.
A tank that holds thousands of gallons may be difficult to fit beside a house, especially on a small property. Placing the cistern below the building keeps that space available for gardens, driveways, patios, or other uses.
This can be especially useful when the cistern is planned as part of the building from the start.
It Keeps the Water Out of Sunlight
Sunlight can encourage algae growth when light reaches stored water.
A cistern below a house is normally dark. That does not make the water clean or safe by itself, but darkness can reduce one condition that supports algae.
An enclosed tank also protects the stored water from direct heating by the sun.
Water Temperature Changes More Slowly
Water stored underground or below a building usually experiences smaller temperature swings than water in an exposed tank.
That can be helpful in hot climates because the water stays cooler. In cold climates, surrounding soil and the building may also offer some protection from freezing.
This does not guarantee freeze protection. Pipes, pumps, valves, vents, and shallow parts of the system can still freeze.
It Can Be Close to the Point of Use
A cistern below the house can shorten some plumbing runs.
For example, a household using rainwater for toilet flushing or other approved non-potable uses may have the tank close to its pump and indoor plumbing.
Non-potable means water that is not intended for drinking.
Shorter plumbing may reduce some installation challenges, but the system still needs proper pipe sizing, controls, backflow protection where required, and safe separation from drinking-water plumbing.
Some Houses Were Designed Around a Cistern
Under-house cisterns are not always a later addition.
In some homes, especially older rural houses or buildings in areas where reliable water supplies were limited, the cistern was part of the original structure.
The building may have been designed so roof runoff entered a masonry or concrete storage chamber below the floor.
Modern systems may use reinforced concrete tanks or manufactured tanks installed in a purpose-built space.
A cistern that was included in the original building design is very different from placing a large tank beneath an existing house without engineering review.
The Weight of the Water Matters
Water is heavy.
One U.S. gallon of water weighs about 8.34 pounds. A 2,000-gallon cistern can therefore hold roughly 16,700 pounds of water, not including the tank itself.
That load must be supported safely.
A cistern should not place unintended pressure on:
- Foundations
- Footings
- Floor framing
- Retaining walls
- Soil beneath the building
- Nearby utility lines
Excavating under an existing house can also weaken soil supporting the foundation.
Structural work beneath a house is not a simple DIY tank installation. A qualified engineer or other appropriate building professional should evaluate the design when the tank or excavation could affect the structure.
Leaks Are More Serious Under a House
A leaking outdoor tank may create a muddy area.
A leaking cistern beneath a house can cause much larger problems.
Water can affect:
- Foundations
- Crawl spaces
- Basements
- Insulation
- Wood framing
- Electrical equipment
- Indoor humidity
Persistent moisture can also create conditions that support mold and wood decay.
The cistern needs a reliable overflow route as well. During heavy rain, excess water should be carried safely away from the building rather than released next to the foundation.
Access Can Be Difficult
Every cistern eventually needs attention.
Depending on the design, maintenance may include checking:
- Screens
- Inlets
- Outlets
- Pumps
- Float switches
- Sediment
- Overflow piping
- Vents
- Tank condition
An under-house tank should have enough access for the parts that require normal service.
Do not assume a person should enter the tank for cleaning. Tanks and other enclosed storage spaces can present serious confined-space hazards, including unsafe air conditions. Cleaning should be designed so routine work can be done from outside whenever possible.
The Pump May Be Inside or Outside the Cistern
An under-house cistern often supplies water through a pump.
For rainwater storage options, first compare cleaning access for sediment and odor control.
The exact arrangement depends on the system.
A submersible pump sits in the water. Other pumps may be installed outside the tank and draw water through an intake line.
Pump selection depends on more than tank size. Important factors can include:
- Required flow rate
- Required pressure
- Vertical lift
- Pipe length
- Pipe diameter
- Intended water use
Flow rate is how much water moves through the system during a given time, often measured in gallons per minute.
Head height describes the resistance a pump must overcome, including vertical lift and pressure losses through pipes and fittings.
Easy pump access is especially important with an under-house cistern. A pump that can only be reached through major demolition makes future maintenance unnecessarily difficult.
Roof Water Still Needs Debris Control
Putting a cistern below the house does not remove the need for good collection practices.
Roof runoff can carry leaves, pollen, dust, bird droppings, insects, roofing particles, and other material.
A typical rainwater system may include:
- Gutters and downspouts
- Leaf screens or debris guards
- A first-flush or similar runoff-diversion method when appropriate
- A screened cistern inlet
- Storage
- A pump
- Treatment suited to the intended use
A first flush device diverts some of the earliest roof runoff so part of the dirt that accumulated between rains does not immediately enter the tank.
It does not make the remaining water potable.
A Cistern Under a House Does Not Make Rainwater Drinkable
Location does not determine whether rainwater is safe to drink.
Roof-collected rainwater can contain microorganisms, chemicals, metals, and other contaminants. A dark, cool cistern may be good for storage, but it is not a complete water-treatment system.
For drinking-water use, the whole system must be considered. That can include suitable collection surfaces, debris control, treatment designed for the contaminants of concern, current laboratory testing, regular maintenance, and applicable local requirements.
One filter, UV unit, test strip, or home water meter should not be treated as proof that stored rainwater is safe to drink.
Under-House Cisterns Need Venting
As water enters and leaves a sealed tank, air must also move.
A cistern normally needs a properly designed vent. The vent should allow airflow while helping keep insects, animals, and debris from entering.
The overflow should also be screened where appropriate.
Poorly protected openings can allow mosquitoes or other pests into stored water.
Drainage Around the Cistern Matters
Water outside the cistern can be almost as important as water inside it.
Groundwater or surface runoff may collect around an underground tank. Depending on the site and tank design, outside water pressure can damage the tank or cause an empty tank to shift or float.
Site drainage should direct unwanted water away from the foundation and cistern area.
The correct approach depends on:
- Soil
- Groundwater level
- Tank construction
- Building foundation
- Local rainfall
- Site slope
These conditions should be considered before installation rather than after water begins appearing beneath the house.
Should You Put a New Cistern Under an Existing House?
Usually, this is more complicated than installing an accessible tank beside the building or in another suitable location.
An under-house cistern may make sense when it is carefully integrated into new construction. Retrofitting one beneath an existing house can involve excavation, structural work, drainage changes, plumbing, electrical work, and difficult future access.
Before choosing the location, consider whether you can safely:
- Support the full tank weight
- Excavate without affecting the foundation
- Reach pumps and controls
- Inspect the tank
- Manage leaks
- Route overflow away from the house
- Maintain inlet screens and filters
- Replace major components later
If those issues are difficult to solve, another tank location may be more practical.
Frequently Asked Questions
Are cisterns commonly built under houses?
They can be, especially in older homes, rural properties, and buildings designed around rainwater storage. Modern under-house cisterns also exist, but the tank should be planned around structural support, drainage, plumbing, and maintenance access.
Does an underground cistern keep water cleaner?
It can keep sunlight out and reduce temperature changes, which may help storage conditions. However, it does not remove contaminants that enter with roof runoff. Debris control, cleaning, treatment, and testing still matter.
Can a cistern damage a house foundation?
A poorly designed installation can. Excavation, leaks, overflow, soil movement, and the weight of stored water can all affect a building. Structural concerns should be evaluated by a qualified professional.
Can you put a plastic water tank under a house?
Possibly, but only if the tank is suitable for that installation and has the support, clearance, access, ventilation, and protection required by its design. A tank intended for above-ground use should not be buried unless its manufacturer specifically allows it.
How do you clean a cistern that is under a house?
The safest design allows sediment removal and servicing from outside the tank. Entering a cistern can create serious confined-space hazards and should not be treated as normal DIY maintenance.
Will an under-house cistern freeze?
It may be less exposed to freezing temperatures than an outdoor tank, but pipes, valves, vents, pumps, and shallow sections can still freeze. Freeze protection should be designed for the local climate.
Can water from an under-house cistern be used for drinking?
Potentially, but the location of the tank does not make the water potable. Drinking-water use requires a suitable collection and treatment system, current laboratory testing, careful maintenance, and compliance with applicable local requirements.
