Rainwater harvesting is legal in most states now, and a basic system pays for itself in the first year. Here's how to build one that actually works.
Why Rainwater Harvesting?
In most of the US, rainwater harvesting is now legal and increasingly encouraged. A single inch of rain on a 1,000 square foot roof produces about 600 gallons of water. That's free water — if you capture it.
A 500-gallon system costs about $300 to build and can supply all your garden irrigation needs through a dry summer, saving $50-150 on water bills annually.
Legal Considerations
Check your state and local regulations before building. Most states now allow rainwater harvesting, but some have restrictions on tank size or use. Texas, Colorado, and Arizona have specific programs that even offer rebates for rainwater systems.
The System Components
First-flush diverter: The first water off a roof after a dry period carries the most contamination — bird droppings, dust, debris. A first-flush diverter automatically discards the first 10-15 gallons before routing water to your tank.
Storage tanks: Food-grade polyethylene tanks (IBC totes) are the best value. A 275-gallon IBC tote runs $50-100 used. Two of them give you 550 gallons of storage.
Overflow pipe: Your system must have an overflow that directs excess water away from your foundation.
Outlet and filter: A spigot at the bottom of the tank, with a mesh filter to keep debris out.
The Build
Step 1: Site selection. Place tanks as close to your downspout as possible, on a level surface. Elevate them 18-24 inches on a sturdy platform to allow gravity flow to a garden hose.
Step 2: Install the first-flush diverter. These are available pre-made for $30-50, or you can build one from 4" PVC pipe. Install it in the downspout before the water reaches your tank.
Step 3: Connect the tanks. Link multiple tanks with 1.5" PVC pipe at the bottom — this equalizes water levels between tanks.
Step 4: Install the overflow. Run a 2" pipe from the top of the last tank to a safe discharge point, away from your foundation.
Step 5: Install the outlet. A 3/4" ball valve at the bottom of the tank connects to a standard garden hose.
Keeping Water Clean
For irrigation use, basic filtration is sufficient. For any other use, additional treatment is required.
Keep tanks covered to prevent algae growth and mosquito breeding. Clean tanks annually by draining completely and scrubbing with a mild bleach solution (1 tablespoon per gallon of water), then rinsing thoroughly.
Expanding the System
- Underground cisterns (1,000-10,000 gallons)
- Pump systems for pressurized delivery
- Additional filtration for non-potable uses (toilet flushing, laundry)
- Gravity-fed drip irrigation systems
A well-designed whole-property rainwater system can supply 50-80% of a household's non-drinking water needs in most of the US.
