This type of roof is usually more of an industrial application. You start with a flat roof, then build it up with layers of asphalt and material coated with asphalt. It's cool because of the top layer, which can be made of something like white-colored gravel.
According to the National Roofing Contractors Association, this type of roof has been used in the United States for more than 100 years, and it's commonly known as a tar and gravel roof.
It's considered "built-up" because it's made up of at least four alternating layers of tar and reinforcing material such as glass fiber mats or organic mats. Because of the layering, this type of roof is very insulating, and with careful selection of materials, the roof can be a cool roof as well.