Installing an underground rainwater tank is a fantastic way to save water, reduce utility bills, and contribute to sustainable living. Whether you're looking to water your garden, flush toilets, or even use rainwater for laundry, a buried tank is a smart and efficient solution. However, the installation process requires careful planning and execution to ensure durability and functionality. In this guide, we'll walk you through the entire process—from choosing materials to final assembly—so you can successfully install your own underground rainwater tank.
Materials Needed
- Underground rainwater tank (polyethylene or concrete, 1,000–5,000 liters recommended for residential use)
- Excavation tools (shovel, spade, pickaxe, or mini excavator)
- Level tool (bubble level or laser level)
- Gravel or crushed stone (for base and backfill, ¾" to 1" diameter)
- Geotextile fabric (to prevent soil contamination)
- PVC or HDPE pipes (for inlet, outlet, and overflow)
- Pipe fittings (elbows, tees, reducers, and connectors)
- Waterproof sealant (for joints and tank seams, if needed)
- Non-perforated pipe (for inlet from roof gutters)
- Perforated pipe (optional, for drainage around the tank)
- Filter system (leaf eater, first-flush diverter, or mesh filter)
- Tank riser and manhole cover (for access and maintenance)
- Backhoe or digger (for larger installations)
- Safety gear (gloves, boots, safety glasses, and reflective vest)
Preparation Steps
Step 1: Choose the Right Location
Select a site that meets local regulations and practical requirements. Key considerations include:
- Distance from house: At least 1 meter from the foundation to prevent water damage.
- Accessibility: Ensure the tank can be accessed for cleaning and maintenance via a manhole.
- Roof catchment: Place the tank near a downspout or gutter system to maximize rainwater collection.
- Soil conditions: Avoid areas with high water tables or unstable soil. Conduct a soil test if unsure.
- Local codes: Check zoning laws and building codes regarding rainwater harvesting systems.
Mark the area with stakes and string to outline the excavation zone. The tank should be placed on flat, level ground to prevent shifting.
Step 2: Calculate Tank Size and Excavation Depth
The size of your tank depends on your water needs and roof area. A general rule is:
- 1,000 liters for small gardens or flushing toilets.
- 2,000–5,000 liters for larger gardens, laundry, or backup household use.
Calculate excavation depth as follows:
- Tank height + 30 cm (12 in) base of gravel.
- Add 60 cm (24 in) of clearance above the tank for soil cover and future landscaping.
For example, a 1.5-meter (5 ft) tall tank will require an excavation depth of approximately 2.1 meters (7 ft).
Assembly
Step 3: Excavate the Site
Begin digging the hole using appropriate tools based on soil type and tank size:
- Use a shovel and pickaxe for small tanks and soft soil.
- Hire a mini excavator for larger tanks or hard soil.
Excavate the hole slightly larger than the tank dimensions to allow for backfilling and adjustments. Ensure the sides are vertical and the base is flat and level. Check the level frequently using a tool placed across the base.
Once the hole is dug, lay a 30 cm (12 in) layer of compacted gravel at the bottom. This provides a stable, drainage-friendly base that prevents shifting and promotes groundwater drainage.
Step 4: Install the Tank
Carefully lower the tank into the excavation using straps or a mechanical lift. Position it centrally and check for level in both directions. Adjust the gravel base as needed until the tank is perfectly level.
Once level, begin backfilling around the tank in 30 cm (12 in) layers. Use clean, well-compacted gravel or crushed stone. After each layer, compact it with a hand tamper or plate compactor to prevent future settling.
As you backfill, install the following components:
- Inlet pipe: Connect a non-perforated PVC pipe from the roof gutter to the tank inlet. Install a first-flush diverter or mesh filter to remove debris and prevent clogging.
- Overflow pipe: Place an overflow pipe at the tank’s top level, leading away from the foundation to a drainage area or storm drain. Ensure it slopes downward to allow water to flow freely.
- Outlet pipe: Install a pipe at the bottom for water extraction. Use a ball valve or pump for controlled access.
- Access hatch: Install a riser and manhole cover flush with the ground surface for easy maintenance. Secure it with a locking mechanism to prevent unauthorized access.
After installation, conduct a pressure test by filling the tank with water and checking for leaks. If leaks are detected, apply waterproof sealant to joints and seams.
Mistakes to Avoid
- Ignoring local regulations: Always check permits and codes before installing a rainwater tank. Some areas restrict tank size, location, or usage.
- Poor site selection: Avoid placing the tank near large trees (roots can damage the tank) or on unstable soil (can cause shifting or cracking).
- Skipping the gravel base: A solid, compacted gravel base prevents the tank from settling unevenly and improves drainage.
- Forgetting the overflow pipe: Without an overflow, excess water can build pressure and damage the tank or surrounding structure.
- Using perforated pipes for inlet: Perforated pipes are for drainage, not rainwater collection. Use solid pipes to prevent contamination.
- Neglecting maintenance access: A buried tank needs a manhole for cleaning and inspection. Without it, sludge buildup can reduce water quality.
Pro Tips
- Use a first-flush diverter: This device diverts the first few liters of runoff (which contains the most debris) away from the tank, improving water quality.
- Install a leaf eater: A gutter-mounted filter that screens out leaves and twigs before water enters the tank.
- Add a tank screen: Place a fine mesh screen over the inlet to block insects and small debris.
- Use a submersible pump: For easy water extraction, install a pump inside the tank connected to an outlet pipe. Choose a model with a float switch for automatic operation.
- Label all pipes: Clearly mark inlet, outlet, and overflow pipes to simplify future maintenance and avoid confusion.
- Consider insulation: In cold climates, insulate the tank and pipes to prevent freezing. Use foam insulation or bury pipes below the frost line.
- Monitor water quality: Test rainwater periodically for pH, bacteria, and contaminants, especially if used for drinking or irrigation.
Regular maintenance is key to a long-lasting system. Clean gutters and filters monthly, inspect the tank for leaks or cracks annually, and drain and scrub the tank every 2–3 years to prevent algae and sediment buildup.
By following these steps and tips, you’ll have a reliable, eco-friendly rainwater harvesting system that serves your home for years to come.
Ready to start your project? Gather your materials, check local regulations, and begin excavating. With careful planning and execution, your underground rainwater tank will be a valuable addition to your sustainable home.
