How much water a roof collects
The Texas Water Development Board uses 0.62 gallons per square foot of roof per inch of rain, times a collection efficiency of about 75–85%. That works out to roughly 600 gallons from a 1,000-square-foot roof per inch of rain.
Rainfall varies widely across Texas. TWDB’s example: a 2,000-square-foot roof yields about 34,000 gallons a year in Austin (32 inches of rain) but only about 9,000 in El Paso (8.5 inches).
- Gallons = roof sq ft × rainfall inches × 0.62 × 0.85
Sizing the tank
Size storage to carry you through your longest typical dry stretch, not just one storm. Plan for a first-flush diverter — TWDB’s rule of thumb is at least 10 gallons diverted per 1,000 square feet of roof — and an opaque tank to keep algae out.
Texas tax breaks
Texas Tax Code §151.355 exempts rainwater harvesting equipment and supplies from state sales tax. Give the seller a completed Form 01-339 exemption certificate at purchase. Some local taxing units also offer property-tax exemptions for water-conservation improvements.