5,000-Gallon Underground Tank
The 5,000-Gallon Underground Tank provides 5,000 gallons of rainwater storage in a underground polyethylene or fiberglass container measuring 12' x 6' x 5' (buried). Priced between $4,000 and $8,000 ($0.80-$1.60 per gallon of capacity), this tank is a full-scale solution for properties seeking significant water independence.
About the 5,000-Gallon Underground Tank
A 5,000-gallon underground tank provides major storage capacity with zero above-ground footprint. This is a popular choice for suburban properties where outdoor aesthetics matter, for commercial buildings with large roof areas, and for properties in areas with strict visual standards. The tank is buried below the frost line, preventing freeze damage in northern climates. Installation is a significant construction project requiring heavy equipment excavation, engineered bedding, anti-flotation anchoring (to prevent the empty tank from rising due to groundwater pressure), and professional plumbing connections. The system includes a submersible pump, float switch, overflow drain to storm sewer or dry well, and access risers for periodic inspection and cleaning. Despite the higher upfront cost, underground systems have longer lifespans and maintain consistent water temperature year-round, improving water quality for irrigation use.
Installation Guide
A 5,000-gallon underground tank is a professional-grade installation requiring experienced contractors, heavy equipment, and potentially engineering permits. Excavation must account for the tank dimensions plus backfill space, and the hole bottom must be prepared with a compacted gravel bed and anti-flotation anchoring. The tank must be rated for below-ground burial — standard above-ground tanks will collapse under soil pressure. Backfill with clean sand or pea gravel, never clay or rocky soil that could damage the tank walls.
At this scale, the plumbing system is equally important. Plan for pre-filtration (leaf screen + first-flush diverter), a calmed inlet to minimize sediment disturbance, a floating suction filter for the outlet, an adequately sized overflow pipe, and a distribution system with a variable-speed pump and pressure tank. If the system will supply indoor non-potable use (toilets, laundry), you will also need a backup connection to municipal water with an air gap or reduced-pressure backflow preventer.
Professional Installation Considerations
- Building permit may be required — check local codes
- Engineered foundation or excavation plan
- Anti-flotation anchoring for underground installation
- Multi-stage filtration system for water quality
- Variable-speed pump with pressure tank
- Municipal water backup with backflow prevention
- Electrical supply for pump and any UV treatment
- Annual maintenance contract recommended
Best Uses for the 5,000-Gallon Underground Tank
The 5,000-Gallon Underground Tank is designed for whole-property water management, rural homesteads, small farms, commercial buildings, and any application where rainwater is a primary rather than supplemental water source. At 5,000 gallons, it provides the capacity to store several months of harvest for use during extended dry seasons. This size is common on properties that use rainwater for all outdoor needs plus indoor non-potable uses like toilet flushing and laundry.
States Where the 5,000-Gallon Underground Tank Is Recommended
Based on our calculations for a 1,500-square-foot asphalt shingle roof used for garden irrigation, the 5,000-Gallon Underground Tank is the recommended storage size in the following states:
- Alabama — 56" annual rainfall
- Arkansas — 50.6" annual rainfall
- California — 22.2" annual rainfall
- Connecticut — 50.3" annual rainfall
- Delaware — 45.7" annual rainfall
- Florida — 54.5" annual rainfall
- Georgia — 50.7" annual rainfall
- Hawaii — 63.7" annual rainfall
Your actual recommended tank size depends on your specific roof area, material, intended uses, and household size. Use our calculator for a personalized recommendation.