Drainage on a flat roof (or nearly flat) is very important. A minimal slope of ΒΌ is needed for water to drain off the roof properly to prevent buildup of water and leaks and other roof problems. Usually you are not seeing a flat roof, but a parapet or wall that contains the flat roof. A parapet is a low wall projecting from the edge of a roof, platform or terrace.
Drain spouts that protrude through the roof parapets are called canales. Canales are commonly made of wood reinforced with a metal lining. Southwestern adobe style flat roofed homes traditionally use canales instead of gutters to drain water from a flat roof. Parapets are very common in London; in 1707 projecting wooden eaves were banned due to fire risk.
Typically Canales Will Drain 400 sf of a Roof
Sometimes heavy rain fall has trouble passing through the canales openings fast enough to prevent water from puddling on the roof. Regular yearly inspections of the canals can insure nothing is blocking them and they are in good condition. Also check where the roof meets the parapet to look for possible places for leaks that may need roof repair. Other ways to allow more water to drain is by making the opening in the parapet larger than the canals. Canales should be avoided on the north sides of roofs since the parapet can give too much shade causing freezing and ice blockage and damaging the roof.
You can prevent erosion in your yard from the canales drainage by using rain barrels under the spouts. The water saved can water your yards and growing beds and used for other non-potable water needs.
