As I waded through the lake of a parking lot this morning, I noticed the incredible speed at which the runoff collected and found its way to the storm drains. This is often how many fearless drivers get caught during floods. The power of water is usually underestimated.
The same can be said about roof runoff. Especially with rain barrels, the power of the runoff is often underestimated when making an overflow. The overflow needs to be at least the same diameter as the pipe/downspout leading into the tank. If not, water can spill out of the top of the barrel and run down the sides. This concentrated spilling water has the potential to erode away or ruin the integrity of the ground beneath the barrel.
Pipes from the overflow should lead to another barrel or be directed to an area where the water can drain safely. It is recommended that this be at least 10 feet away from a building's foundation.
Another important step to remember is equipping the overflow outlet with a device to prevent the entry of pests. A 1/16" mesh screen will keep out mosquitoes. If larger pests, such as mice, are an issue, larger, more sturdy screen may be needed. Do not use one with openings larger than 1/4".