Rainwater
harvesting

Sizing your array

The collection formula is simple: roof area in square feet × rainfall in inches × 0.623 = gallons, then multiply by about 0.85 for real-world losses from splash, wind and first-flush diversion.

Look at the annual column and then at what you can actually store. A 1,500 square foot roof theoretically yields 27,000 gallons a year in Austin, and a two-tote array captures 550 gallons at a time. The constraint is never the rain. It is always the storage on the day it falls, which is why arrays beat single totes so decisively here.

Building it in the right order

1. Leaf screen at the gutter or the inlet. Stainless mesh, cleanable, sloped so debris sheds rather than mats.
2. First-flush diverter. A standpipe that fills with the first two gallons or so and self-drains between events. This is where the bird droppings, the pollen and the asphalt grit end up instead of in your tank.
3. Inlet with an insect screen. Mosquitoes will find any opening, and a screened inlet is the whole of mosquito control on a sealed tote.
4. Storage, shaded. See below — this is the step people skip.
5. Overflow, sized to match the inlet and routed away from the building. An overflowing tote against a slab foundation is a genuinely expensive mistake in expansive Central Texas clay.
6. Draw point. Bottom-linked if you have multiple totes, so you draw from one tap and the array equalises itself.
7. Elevation if you want gravity pressure. Four feet of stand gives about 1.7 psi — enough for drip irrigation, not enough for a sprinkler.

Linking multiple totes

Bottom-linking is the standard approach: connect the outlets of adjacent totes with 2-inch pipe or hose so they behave as one vessel, and draw from a single tap at one end. Everything fills together, everything empties together, and you have one connection to maintain.

• Put an isolation valve between each pair. When one tote needs work, the rest of the array keeps running.
• Link at the bottom, vent at the top. Without a top vent, the far tote fills slowly and drains slower.
• Keep the array level. Bottom-linked totes on uneven ground will not equalise and the low one carries the load.
• Give the base real support. A filled 275 gallon tote is 2,420 lb on a 48 × 40 footprint — that is roughly 180 lb per square foot, which will sink into unprepared soil and crack thin concrete.

Freeze, and the Texas mistake

Every few years Central Texas gets a hard multi-day freeze, and rainwater systems designed for a mild climate fail all at once. A full tote will not freeze solid in a two-day event — the thermal mass is large — but the plumbing will, and the valve will, and the valve is the expensive part.

• Drain and open the valve before a hard freeze if the tote is not in use.
• Insulate or heat-trace exposed pipework, especially the run between totes.
• Leave headspace. A brim-full tote has nowhere to expand.
• Never leave a pump primed and exposed. That is the second-most expensive part.

Texas law, in brief and without pretending to be legal advice

Texas is unusually supportive of rainwater harvesting. State law protects a property owner's right to harvest rainwater, restricts homeowners' associations from prohibiting systems outright (though they may regulate appearance), and exempts qualifying rainwater harvesting equipment from state sales tax when purchased for that purpose. Some municipalities and utilities offer rebates on top.

What that does not do is make roof-collected water potable, or exempt a system connected to a building's plumbing from plumbing code and backflow prevention requirements. Irrigation and outdoor use is straightforward. Anything crossing into the house is a licensed plumber's job. Ask us for the sales-tax exemption form with your order — most customers do not know it exists.