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Roof design evolved around 1850 in the American Southwest. Three inches (8 cm) of adobe mud was applied on top of the latillas, then 18 inches (45 cm) of dry adobe dirt applied to the roof. The dirt was contoured into a low slope to a downspout aka a 'canal'. When moisture was applied to the roof the clay particles expanded to create a waterproof membrane. Once a year it was necessary to pull the weeds from the roof and re-slope the dirt as needed.[citation needed]