How much rain a roof collects
Every inch of rain falling on one square foot of roof is 0.623 gallons of water. Part of it never reaches the tank: it splashes off, evaporates, soaks into the roofing or is diverted by the first-flush filter that discards the dirtiest water.
- Yearly yield (gallons) = roof area (sq ft) × rainfall (in) × 0.623 × runoff coefficient × filter efficiency
- Use the footprint of the roof seen from above, not the sloped surface. A steep roof does not catch more rain than a flat one with the same footprint.
Runoff coefficients
| Roof surface | Coefficient |
|---|---|
| Metal | 0.95 |
| Asphalt shingles | 0.90 |
| Tile or slate | 0.85 |
| Flat roof with gravel | 0.60 |
| Green roof | 0.30 |
Worked example
A house has a 1,500 sq ft asphalt shingle roof and gets 38 inches of rain a year. The filter keeps 90% of the runoff.
- Yield per inch of rain: 1,500 × 0.623 × 0.90 × 0.90 = 757 gallons.
- Yearly yield: 757 × 38 = 28,782 gallons.
- Planned use of 50 gallons a day for the garden and toilets needs 18,250 gallons a year, so the roof covers 100% of it.
- At $12 per 1,000 gallons, the water saved is worth about $219 a year.
Sizing the tank
Rain does not fall evenly, so the tank has to bridge dry spells. Two simple rules give a starting point:
- Three weeks of use: 50 gallons × 21 days = 1,050 gallons.
- 5% of the yearly yield: 28,782 × 5% = 1,439 gallons.
In regions with long dry summers, such as California, a larger tank or a smaller use plan is needed. A single 55-gallon rain barrel overflows after a light shower on most roofs, so linking several barrels is common.
Good practice
- Screen gutters and install a first-flush diverter to keep leaves and debris out.
- Cover the tank to block light (which grows algae) and mosquitoes.
- Add an overflow that drains away from the foundation.
- Use it for non-potable needs: irrigation, toilet flushing and laundry. Drinking rainwater requires filtration and disinfection designed for it.
Is it allowed?
Rainwater harvesting is legal in every state, but a few, such as Colorado, cap the volume a household can store, and some cities require permits for large tanks or indoor plumbing. Several states and utilities also offer rebates on barrels and cisterns.