Flat roofs usually have at least a slight slope to assist in the shedding of water thus they are also referred to as "low slope roofs ’. Flat roofs are typically a more economical roof to build given that it requires less material. While being cheaper to intially build a flat roof will require re-roofing more often with many materials lasting 10-20 years versus 25-50 years for many pitched roof materials.
Flat roofs are susceptible to failure if pooled water is left for long periods of time. Most recent flat roofs are covered by a continuous membrane to help prevent such water pooling. Still, flat roofs are not an ideal choice for areas that get a lot of rain and/or snow.
