Roof Catchment

A roof catchment is an ordinary roof put to a second use. Instead of dripping off the eave, the rain is caught by gutters and carried down a pipe to a tank or a planted basin. The roof is only the first of the four parts of a rainwater system: the catching surface, the pipes that move the water, the container that holds it, and an outlet for the excess. A catchment stores nothing and yields water only while it is raining, so it is always paired with something that can hold a wet spell over into a dry one.

How much arrives depends on the area of the roof, measured as the building's outline on the ground plus the overhang, not the sloped surface. About 80 percent of what lands reaches the tank, roughly 8 litres per square metre per centimetre of rain (a little over half a gallon per square foot per inch). Material decides quality rather than quantity. Metal sheds dirt readily and is the usual first choice; asphalt shingle is accepted where the water goes to plants rather than to a tap; wood shingles, lead flashing, and lead-, chromium- or zinc-based paints leach contaminants into the runoff and are unsafe surfaces for a catchment feeding drinking water. Catching water is not treating it, and roof runoff is not drinking water until something downstream makes it so.

The part that fails is rarely the roof. It is debris. Leaves settle on the roof and in the gutters, wash down at the next rain and block the screens that protect the tank, so a whole roof quietly delivers nothing. Packed gutters hold standing water that breeds mosquitoes, and heavy rain blows gutters out of position, which makes their hanger spacing the weak element. Inspecting twice a year, clearing screens, washing the tank out annually and cutting back overhanging limbs is most of the upkeep, about six hours a year. The overflow is often forgotten. It is a designed output rather than a spill, and belongs in a mulched basin below the eave rather than cutting the ground by the foundation.