Rain barrels that do not breed mosquitoes

I have pulled more larvae out of other people's barrels than I have out of stock tanks. Rain collection is simple. Leaving standing water open to the air is also simple, and the mosquitoes vote first.

Catchment math

One inch of rain on a thousand square feet of roof is about 600 gallons. A modest ranch in a two-inch storm will overflow a pair of 55s before breakfast. Size the overflow as carefully as the inlet.

The lid

A tight lid with a fine screen (window screen is not fine enough; I use 20-mesh or a dedicated barrel screen). The downspout should dump through that screen, not around it. I add a first-flush diverter on the shop roof because the first five gallons off asphalt shingles are grit and bird work.

Overflow

A hose near the top that runs to a swale or a second barrel, never onto the foundation. I have seen more wet crawlspaces from proud rain systems than from bad gutters.

What I use it for

Garden and livestock, not drinking, unless I filter and disinfect on purpose. If you want potable rainwater you need a cleaner roof, a settling tank, a filter, and a treatment step. Do not skip the last one because the water looks clear.

Mosquito dunks (Bti) are a backup, not a substitute for a lid. If you can see the water, so can they.