A roof laid at an angle of less than 10° to the horizontal is known as flat roof. Flat roof may be of reinforced cement concrete, reinforced brick work, precast concrete with channel units, waffle units etc, flag stones supported on rolled steel joists or jack arch type.
A roof that is nearly flat or slightly pitched is called a flat roof or low slope roof – No roof should be dead flat because it must have at least a slight slope to drain. Low-slope & steep-slope terms describe roof slope, the slant of a roof. A low-slope roof is one that has a slope of less than 3-in-12
A flat roof is generally any roof with a pitch of 1-10 degrees. That means flat roofs aren't actually flat. It may look horizontal but often has a minimum slope of 1/4 inch per foot. This allows water to run off the roof to prevent damage.