Catch the rain. Use it. Send the rest to the bioswale.
A 2,500 ft² roof in Bexar County catches 46,000 gallons a year. The question is whether you keep it.
Cistern sizing · first-flush · overflow designUp to 46,000 gal/yr
A 2,500 ft² roof in Bexar County catches about 46,000 gallons of rain a year. Most of it runs into the street.
Rainwater harvesting in the Hill Country isn't a hobby — it's economic infrastructure. SAWS bills climb every year. Boerne and Bulverde residents on wells watch their static water levels drop after every drought. Meanwhile, the rain that lands on your roof — softer, cleaner, free — washes off the property in 8 minutes.
We size cisterns the way an engineer does — by use case, by roof area, by storm event, by dry-spell length. We site them on the slope so gravity does the work. And we route overflow to a bioswale or rain garden so big storms don't blow out the system. Catch what you can use. Slow what you can't.
§ 1 · How it works
Sized for your use case.
Cistern sizing is the most common mistake. Homeowners buy a 1,500-gallon tank thinking it's huge, then run it dry in 6 weeks of summer. Or they install 5,000 gallons of storage they never fill. Right size = annual capture × seasonal demand × dry-spell buffer.
Use case demand. Garden irrigation, tree establishment, pond feed, potable. Each has different storage requirements.
Dry-spell buffer. Hill Country typical dry stretch is 30-60 days. Target 30 days of use storage minimum.
Site placement. Cistern on the high side of the property, downhill from the longest gutter run. Gravity beats pumps.
First-flush diverter. The first 10 gallons per 1,000 ft² of roof carry the pollen, bird droppings, and dust. Diverter sends them around the tank.
Once sized, we spec the tank (poly for most, metal for premium installs, food-grade for potable), the inlet (with leaf screen + first-flush), the outlet (gravity-fed where possible, pump where required), and the overflow (always to a bioswale or rain garden, never to the street).
§ 2 · What we build
Real installations.
Included
Cistern install
500 to 10,000+ gallons. Poly, metal, or steel-lined concrete. Sized to use case.
Included
First-flush diverters
Stops the dirty initial water from entering the tank. Self-cleaning. ~10 gal per 1,000 ft² roof.
Included
Gutter + downspout work
Most existing gutters underperform. We resize, add leaf screens, and route to inlets.
Included
Irrigation distribution
Drip lines, soaker hose, garden valves. Gravity-fed when grade allows, pumped where needed.
Included
Potable conversion
For off-grid or supplemental. First-flush + sediment + 5-micron + UV. County-code compliant.
Included
Overflow to bioswale
Big storms = big overflow. We design the bioswale at the same time as the cistern.
§ 3 · Pricing
Honest pricing.
Rainwater harvesting cost is driven by tank size, material, access, and whether plumbing or electrical is involved.
Starter
$1,500-$4,000
1,000-2,500 gal poly tank · first-flush · gravity irrigation · garden use
5,000-10,000+ gal · metal or concrete · full filtration + UV · whole-house
§ 4 · Why it pencils
The math.
Outdoor water is 30-50% of a residential SAWS bill in summer. A 2,500 gallon cistern fed from a 2,500 ft² roof typically covers a third to half of summer landscape irrigation. The cistern pays for itself in 6-12 years on water bill savings alone.
On a well property in Boerne or Bulverde, the value isn't dollars saved — it's pressure preserved. Every gallon you irrigate from a cistern is a gallon your well doesn't pull. Static water levels stay deeper, longer.
And on every property, captured rain is better for plants than treated water. Lower pH, no chlorine, no chloramines, soft. Your trees, your beds, your soil microbiome all do better on it.
The math is simple: roof_sqft × annual_rain_inches × 0.623 × efficiency (~85%). A 2,500 ft² roof in San Antonio (32 in/yr) catches ~42,000 gallons annually. A 1,500 ft² roof in Boerne (34 in/yr) catches ~27,000. Most properties only need to capture 25-35% of that to cover most garden irrigation.
Is rainwater safe to drink?
Yes, with the right system: first-flush diverter, sediment filter, 5-micron filter, and UV sterilization. The math is the easy part — the build is regulatory. Confirm county code first. Most clients use captured rain for landscape, garden, and (filtered) toilet flushing without going full potable.
How big a tank do I need?
Depends on use. For garden irrigation only, aim for 30 days of dry-spell storage. A typical household garden needs 1,500-3,000 gal. For tree establishment plus garden, 2,500-5,000. For potable, 5,000+. Our free Rain Capture Estimator calculator handles the math.
What's a first-flush diverter and why do I need one?
The first 10 gallons of rain per 1,000 ft² of roof carry the accumulated pollen, dust, bird droppings, and leaf bits. A first-flush diverter sends that water around the tank instead of into it. Keeps your tank clean and your water usable. Mandatory on any system above 500 gallons.
What about big storms — won't the tank overflow?
Yes, and that's by design. Every cistern overflows in big storms. The question is where the overflow goes. We always route it to a planted bioswale or rain garden — never to the street. That way the property captures the smaller storms in the tank, and the bigger storms infiltrate through the bioswale. Two systems, one outcome: less water leaves the property.
How much does a rainwater system cost in San Antonio?
A starter system (1,000-2,500 gal poly, first-flush, gravity irrigation) runs $1,500-$4,000. A standard system with pump and zoned irrigation is $4,000-$12,000. Estate-scale or potable is $12,000-$25,000+. The Site Read gives you the right size for your property and use case.
Start here
Book a Site Read.
Site Read + cistern sizing + first-flush spec + overflow plan. Written summary in 48 hrs. From $299.