The design rainfall is a variable used for calculating the rainwater quantities to be expected. Canals and other wastewater or infiltration systems must not indicate an overload for the design rainfall. A common method for design in accordance with DIN 1986-100:2007-04 is that the design rainfall for roof drainage is a rain event lasting 5 minutes with a statistical frequency of 5 years. The notation for this is r(D;T). The unit of measure for design rainfall is litres per second and hectare.