Rain
catchment
bank.

Step 1 — size it from the drought, not the storm

Yield is roof plan area × rainfall inches × 0.623 × 0.9. A 1,200 sq ft roof in a 1-inch storm gives about 673 gallons, which fills two and a half totes in one ordinary rain. So the storm is never the constraint — the dry spell is.

A 1,000 sq ft vegetable garden needs roughly 600 gallons a week in a Midwest July. To ride out a two-week gap you need 1,200 gallons, which is four and a half totes. Round up to five. To ride out one week, three. The sizing article has the full yield table.

Step 2 — prepare the base properly

This is the step people skip and regret. The bank must be level within about half an inch across its whole length, because a filled tote on an uneven base loads two cage corners instead of four and bends its lower horizontals.

• Compacted gravel at least 4 inches deep, or a poured pad.
• A paving slab under each corner of each tote spreads the point load.
• Check level across the bank and along each tote, in both directions.
• Leave 24 inches of working space behind the bank. You will need it, and you will not have it if you do not plan for it.

Step 3 — manifold at the bottom

Fit a 2-inch bulkhead into each tote low on the side wall — not through the existing valve — and connect all three to a common header with unions. Then take a single outlet off the header.

Why bulkheads rather than the factory valves: the valves become your isolation points, so you can shut any one tank and work on it while the other two keep running. Why unions: you can add a fourth tote next year without cutting anything you have already built. A bulkhead fitting is $45 fitted; the three-tote unionised manifold is $420 from our fabrication shop.

Step 4 — first flush, and why it is not optional

The first 10 to 20 gallons off a roof carries most of its accumulated bird droppings, dust, pollen and asphalt granules. A first-flush diverter is a vertical standpipe that fills before the flow continues to the tanks, then drains slowly through a small orifice between storms.

$95 plumbed. Skip it only if the water is purely ornamental. If anything touches an edible crop, you want it and you want a food-grade tank as well.

Step 5 — screen everything, including the overflow

1 mm mesh or finer, sealed rather than draped, on every opening: the inlet, the vent and the overflow. A single unscreened 2-inch overflow will breed thousands of mosquitoes in a week, and it is the single most common fault we see on home-built systems. Screening is physical exclusion; do not substitute chemicals for it.

Step 6 — overflow, directed away

A 2-inch standpipe near the top of the last tote, piped to daylight at least ten feet from any foundation. Your bank will overflow — regularly, in the first good storm after it fills — and where that water goes is a decision you should make rather than discover. $78 fabricated.

Step 7 — UV protection

Sunlight does two things to an uncovered tote: it chalks and embrittles the HDPE over two Midwest summers, and it grows algae inside. A fitted sleeve is $38 and roughly triples outdoor service life. Paint works too; shade works best. Siting the bank on the north side of a building is free.

Step 8 — get the outlet to a usable height

A tote on the ground produces almost no head pressure. On a 24-inch stand it gives roughly 2.5 psi at the outlet, which runs a soaker hose or a gravity drip line out to about fifty feet. A welded stand in reclaimed cage tube is $180. For sprinklers or anything uphill you need a pump, around $150 for a small on-demand unit.

Step 9 — winterise

Drain to at least 10% headspace before the first hard freeze, or drain fully and disconnect the downspout. Water expands about 9% on freezing and a brim-full tote will bulge permanently against the cage. Most people run three seasons and drain in November.