Flats roofs are really not flat… They do have a angled slop to them to drive water down and away from the property. System works by providing a waterproof membrane over a building. It consists of one or more layers of hydrophobic materials that is placed over a structural deck with a vapor barrier that is typically placed between the deck and the roof membrane.
Flashing, or thin strips of material such as copper, intersect with the membrane and the other building components to prevent water infiltration. The water is then directed to drains, downspouts, and gutters by the roof’s slight pitch.
There are four most common types of flat roof systems. Listed in order of increasing durability and cost, they are: roll asphalt, single-ply membrane, multiple-ply or built-up, and flat-seamed metal. They can range anywhere from as low as $2 per square foot for roll asphalt or single-ply roofing that is applied over and existing roof, to $20 per square foot or more for new metal roofs.
Used since the 1890s, asphalt roll roofing generally consists of one layer of asphalt-saturated organic or fiberglass base felts that are applied over roof felt with nails and cold asphalt cement and usually covered with a granular mineral surface. The seams are typically covered over with a roofing compound. It can last about 10 years.
Single-ply membrane roofing is the newest type of roofing material. It is often used to replace multiple-ply roofs. 10 to 12 year warranties are typical, but proper installation is crucial and maintenance is still required.
Multiple-ply or built-up roofing, also known as BUR, is made of overlapping rolls of saturated or coated felts or mats that are interspersed with layers of bitumen and surfaced with a granular roofing sheet, ballast, or tile pavers that are used to protect the underlying materials from the weather. BURs are designed to last 10 to 30 years, which depends on the materials used.
Ballast, or aggregate, of crushed stone or water-worn gravel is embedded in a coating of asphalt or coal tar. Since the ballast or tile pavers cover the membrane, it makes inspecting and maintaining the seams of the roof difficult.
Lastly, flat-seamed roofs have been used since the 19 th century. Made from small pieces of sheet metal soldered flush at the joints, it can last many decades depending on the quality of the material, maintenance, and exposure to the elements.
Galvanized metal does require regular painting in order to avoid corrosion and split seams need to be resoldered. Other metal surfaces, such as copper, can become pitted and pinholed from acid raid and usually requires replacing. Today copper, lead-coated copper, and terne-coated stainless steel are favored as long-lasting flat roofs