Flat roofing is typically used on a very low pitch of 10 degrees down to a no pitch roof. The advantages of a flat roof is that it is resistant to moss and in low moisture climates it can last a extended period of time. There are many methods used on flat roofs today but the most common for residential is Torchdown and TPO. These types of roofs usually last about 10 to 20 years but if properly maintained you can get up to 25 years.
Some downfalls for flat roofs is that they are not very durable. In the winter if water is built up and then freezes it can crack the membrane leading to leaks and damaging the sheathing (plywood). When a leak occurs it may take some time before it is noticed because it has to travel very far. Once it is noticed usually there is some substantial damage that has occured. This is why it is very important to properly maintain this type of roof.