A 2,400 square foot roof in Boone County, Missouri sheds roughly 1,500 gallons in a typical one-inch rain event, after accounting for first-flush and a generous 90 percent collection efficiency. The annual average rainfall in central Missouri is around 42 inches, which means that roof produces something close to 63,000 gallons of harvestable water per year. Most homeowners sizing a rainwater system look at their summer garden need, undersize by half, and then watch the overflow run down the driveway every spring. Here is a worked example for sizing it right.
Pick the demand number first, not the supply
Decide what the water is for before sizing the storage. A 1,200 square foot vegetable garden in July wants about 750 gallons per week at peak. A small flock of chickens and a horse paddock add maybe 60 gallons per day. Garden irrigation alone for fourteen weeks of dry-season demand totals roughly 10,500 gallons. That is the number you size storage against, not the annual rainfall total.
The rolling-buffer rule
Storage for a roof-fed system should cover the longest realistic dry spell, not the annual demand. In central Missouri the longest typical dry stretch in growing season is about three to four weeks. Three weeks of garden demand at peak is roughly 2,250 gallons. Add the livestock load and you are at maybe 3,500 gallons of working storage — call it twelve to fourteen 275-gallon totes connected at the bulkhead.
- Calculate weekly peak demand in gallons
- Multiply by three to four weeks of dry buffer
- Round up to the next full tote (275 gal each)
- Add a 15 percent overflow margin for big events
- Plan an overflow route that does not erode the pad
Manifolding totes the right way
Connect totes through their 2-inch bulkhead valves with a common header at the bottom and a separate vent line at the top of each. Equalizing through the bottom lets the array fill and draw evenly without the front tote becoming a single point of failure. Use full-port valves on every isolation point so you can drop one tote out of the line for inspection or repair without losing the rest of the array. We use brass full-port on rainwater systems because the slight cost premium pays back the first time you have to winterize.
Overflow is not optional
A full twelve-tote array hit by a three-inch rain event will see roughly 4,500 gallons of inbound water in a few hours. The overflow needs a real plumbed path — a 3-inch PVC overflow off the highest tote, routed at least 15 feet from the pad to a swale or a graveled diffusion bed. We have seen more rainwater pads fail from overflow erosion under the totes than from any other single cause. Size the overflow for the worst storm in your county history, not the average.
A note on covers
Uncovered totes in Missouri sun grow algae inside six weeks of warm weather. A cheap fitted cover or a tarped frame extends usable life of the water and the tote itself dramatically. We will say more about UV in another post, but the short version is: cover the array, even if it looks ugly the first season.