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Part VIII: Wind, Water, and BiofuelsChapter 12. Small Wind and Micro-Hydro

Rainwater Catchment and Storage

Roof area, a first-flush diverter, and tank volume for garden water.

Difficulty
Beginner
Cost
$50–400
published

A roof is a wide bucket. One inch of rain on one square foot is 144 cubic inches, and a gallon is 231, so the catch is about 0.623 gallons per square foot per inch. A thousand square feet and one inch is 623 gallons. That water is heavy, dirty at the start of the storm, and not a drinking supply. It is a reasonable way to water a garden if you divert the first wash, screen the mosquitoes, and put the tank where the weight can sit.

Rules differ. As of 2026 some places encourage barrels and some places want a permit once the tank is large. Ask the local building or health office before you plumb anything into a house or call the water potable. This page covers garden irrigation only.

If the garden needs the water uphill, the solar pump page is the next machine. The catchment projects in the links below are other people's. Read them there.

Difficulty

Level
Beginner
Time
A weekend for gutters, a diverter, and a ground-level tank
Skills
Respecting weight and not drinking the barrel

Cost estimate

$50–400

A used food-grade barrel, a diverter, and screen can land near the low end if the gutters already exist. New gutters, a larger tank, and a proper stand use the rest and pass it. A cistern is a different budget. Once you count the tank and the overflow piping, the water is not free.

Materials

ItemQtyNotes
A tank that never held chemicals1Food-grade or a purpose-made water tank. Not a fuel drum. Not a pesticide barrel.
A first-flush diverter1The first water off a dirty roof goes aside. Then the cleaner flow may enter the tank.
Screen on every openingAll of themMosquitoes breed in an open tank. The overflow needs screen too.
A level pad or a stand built for the weight1About 8.3 pounds per gallon, plus the tank. A deck is not a pad unless it was designed as one.

Catch, divert, overflow

Measure the flat area of roof that drains to the downspout you will use, not the slanted area of every shingle, if you want the horizontal footprint. The sketch uses the plan area. Gutters clog. A leaf guard that you will actually clean is part of the yield.

The first minutes of a storm rinse dust, pollen, and droppings. A small chamber that fills and then lets the rest of the flow continue is the idea of a first flush. Empty it between storms so the next storm gets a fresh divert. If you never empty it, you have built a stagnant pocket.

The tank will fill. Overflow has to leave by gravity to a place that does not soak the foundation, the neighbor's basement, or a slope that slides. A fitting high on the tank, larger than the inlet so it cannot be the bottleneck, is the usual arrangement. Do not seal a tank that has no overflow. You will discover the lid is the overflow.

Roof, first flush, tankA roof draining to a small diverter and then a tank, with overflow leaving the foundation.First flushTankOverflowAbout 8.3 pounds per gallon, plus the tank. Not drinking water.
Roof, a first-flush chamber, a tank, and an overflow that leaves the foundation.

Gallons from roof and rain

Gallons are roof area in square feet, times inches of rain, times 0.623. One inch on one square foot is 144 cubic inches, and a gallon is 231 cubic inches. The weight uses 8.34 pounds per gallon of water and does not include the tank.

623 gallons

About 5,196 lb of water. That is what the stand has to hold, plus the tank. A first-flush diverter throws some of this volume away on purpose.

Painted metal is a common surface when the water is for a garden. Asphalt shingles add grit. Many gardens tolerate that. A food crop you will eat raw is a stricter question, and drinking is stricter again. Wood shakes and any roof with a biocide are a poor source for water on food plants. Ask the health department before you aim higher than ornamental or established garden beds.

Composite: the deck barrel

Invented. A full barrel goes on a wooden deck because the deck was near the downspout. One inch on a modest roof fills it. The water weighs more than the deck joists were asked to hold for a person and a chair. The composite ends with a cracked joist and a flood against the siding, which is the failure the overflow and the pad were supposed to prevent. The lesson is ground or a designed stand, and an overflow that is bored before the first storm. Not a real porch.

  1. Check the local rule and the roof

    Garden water is the scope. Drinking water is a health-department project. Skip treated roofs and unknown coatings.

  2. Compute gallons and pounds

    Area times inches times 0.623. Then times about 8.3 pounds. Put that weight on the ground or on a stand that was built for it.

  3. Divert the first flush and screen every hole

    Empty the diverter between storms. Mosquito screen on the inlet, the vent, and the overflow.

  4. Aim the overflow away from the house

    Larger than the inlet, downhill, and not into the neighbor. Leave a way to drain and clean the tank.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.