Rain tanks that survive a second winter

First, the arithmetic, because most systems are sized wrong in the same direction.

You collect roughly 0.6 gallons per square foot of roof per inch of rainfall, after allowing for losses. A thousand square foot roof in a one-inch storm therefore yields about 600 gallons, which fills two 275 gallon totes and overflows the second one.

Size for a single significant storm and for how fast you draw the tank down between them, not for an annual rainfall figure. Six containers on a small roof spend most of the year empty.

Failure one: light

A natural translucent bottle in sunlight grows algae within weeks. It is not dangerous for irrigation but it clogs screens, fouls drip emitters and makes the water unpleasant to handle.

Over years, UV also embrittles the surface layer of the polymer, which lowers the threshold for cracking under the hoop stress a full tank produces. Both problems have the same fix: a black UV-stabilised bottle, an opaque wrap, or permanent shade.

Failure two: freeze

Water expands as it freezes. A full container left through a hard freeze takes a permanent set and bows the cage outward at the mid-height bars. The tank often still holds water afterwards; it is simply no longer square, no longer stackable, and sitting on a stand that was designed for a different shape.

Drain to below the outlet before the first hard freeze. If the system must stay live, insulate the tank, heat trace the manifold and keep water moving.

Failure three: the stand

A full 275 gallon tote is about 2,435 pounds. That is a real structural load, and it is applied through four feet onto whatever is underneath.

Build the stand first, on compacted ground or a slab, and build it level. A stand that is level on the day it is built and sitting on soft ground will not be level after two winters, and a tank that leans puts uneven load into the cage.

While you are designing it, remember that height buys you very little pressure. Gravity gives about 0.43 psi per foot of elevation, so a four-foot stand delivers roughly 1.7 psi when full. That runs drip irrigation and a soaker hose. It will not run a sprinkler, and no realistic stand height will — use a pump instead.

Failure four: unscreened openings

Fine mesh on the inlet and on the overflow. An unscreened overflow is a mosquito door and a leaf-litter inlet, and once organic material is inside a warm dark tank you have a compost bin with a tap on it.

A two-tote parts list

• Two grade B or C 275 gallon totes, black bottles if you can get them
• Two 2-inch buttress-to-camlock or hose adapters for the bottom manifold
• Linking hose or pipe with clamps
• A 3/4-inch garden hose adapter on the draw-off
• A first-flush diverter sized to the roof area
• Fine mesh screen for the inlet and the overflow
• Overflow pipe routed away from the stand and the foundation
• A level, load-rated stand or slab

One rule about what the water touches

Harvested roof water is not potable and should not be treated as such. For irrigating ornamental planting, washing equipment and topping up ponds it is excellent.

If it will irrigate edible crops, the container history matters as much as the water, because irrigation water contacts the produce. Use documented food-grade stock for that, and check local rules — a few jurisdictions regulate rainwater harvesting.

Manifolding, and why bottom-linking wins

Two containers can be joined at the top or at the bottom, and the difference matters more than it looks.

Bottom-linked at the outlets, the tanks behave as a single vessel: they fill together, they draw together, and the level equalises automatically. Top-linked, the first tank fills completely before the second starts, and drawing from one does not draw from the other.

Bottom-linking needs two buttress adapters, a short length of hose or pipe and two clamps. It is the single easiest thing to get right and the one most often done the other way.

Overflow, done properly

1. 01Take the overflow from the top of the last tank in the chain, not the first.
2. 02Size it at least as large as the inlet, or the system will back up in a heavy storm.
3. 03Screen it. An unscreened overflow is a mosquito door in both directions.
4. 04Route it away from the stand and away from any foundation. A tank that overflows onto its own footings will not be level in two seasons.
5. 05Give it an air gap rather than plumbing it directly into a drain.

Maintenance, three times a year

• Spring: check screens, clear the first-flush diverter, inspect the stand for settlement after the freeze.
• Midsummer: check for algae if any light is reaching the water, and flush the manifold.
• Autumn: drain to below the outlet, clear leaf litter from screens, cap anything that is not screened.

That is under an hour a year and it is the difference between a system that lasts a decade and one that is replaced in three.

Common questions

Can you bury a tote? Not sensibly. The cage is not designed for lateral earth pressure and the container will deform. Purpose-made buried tanks exist for a reason.

Can you pressurise the system? Only with a pump downstream of the tank. The container itself is an atmospheric vessel and should never be pressurised.

Can you drink it? No, not without a proper treatment system. Roof water carries whatever is on the roof, and no amount of filtration in a garden system makes it potable.

Will it freeze solid? In Cleveland, yes, if you leave it full. Drain it.

Questions we get about rain systems

How many tanks do I need? Size against one significant storm and against how quickly you draw down between them. On most domestic roofs, two containers is the answer and six is somebody who has not done the arithmetic.

Can I link tanks that are different sizes? Yes, bottom-linked at the outlets, provided they sit at the same level. If they sit at different heights the lower one fills first and the higher one may never fill at all.

Do I need a pump? For drip irrigation and filling containers, no. For a sprinkler, a pressure washer or a hose-end sprayer, yes. Gravity gives about 0.43 psi per foot and no realistic stand height changes that.

What about mosquitoes? Screen every opening including the overflow, and keep the tank sealed otherwise. An unscreened overflow is the usual route in and it is the one people forget.

Will the water go bad? Standing water develops biological activity, particularly if light reaches it. Blocking light solves most of it. For irrigation this is largely cosmetic; for anything else it is not.