What is a flat roof?

Flat roofs are staples of metropolitan areas, but this roof type certainly isn't only for the major cities of America. Flat roofs are a common alternative to the pitched roofs we find on so many traditional homes. Making use of membranes and sealants, flat roofs are a cost-effective solution for roofing modern buildings.

Flat roofs aren't actually flat. They tilt at a slight slope, up to around a 2:12 pitch, according to the resource New England Metal Roof , and drain rain or snowmelt in specific places. For this reason, they're more accurately referred to as low-slope roofs. Whether you're a homeowner considering a flat roof for a new building, or you bought a home, apartment or business with a flat roof, here's what you need to know about the different types of flat roofs and the upkeep required to keep them in operation.

Why choose a flat roof?

Flat roofs have numerous benefits, depending on the materials used. Metal can provide extraordinary longevity, while more common materials, like modified bitumen, are cheap to install. As a general rule, flat roofs are a less-expensive alternative to pitched roofs.

Flat roofs have a smaller surface area than pitched roofing, so they require less material. Even with an expensive roof material like metal , a flat roof will use less of it than a pitched alternative. 

Flat roofs are also easier to maintain than a pitched roof. Because they can be walked on with less risk, homeowners or building superintendents can check for fallen branches, snow or water build-up themselves.

Some flat roofs can also be used as extra outdoor space. The right flat roof can double as a deck in the sky, supporting outdoor furniture and houseguests. 

How do flat roofs drain?

While the slight grade of a flat roof will encourage water to flow off of it, they need to be supported with a system of drains. These drains can be installed in the center of the roof or along the edges as scuppers. 

Central drains will pipe water out of the roof with an internal pipe system. Gutters support the roof drains and use downspouts to control the flow of water and move it away from the building, where it could damage the foundation.

Types of flat roofs

A survey of flat roofing materials doubles as a survey of architectural materials. "Flat roofs have been a part of residential architecture in places like Brooklyn since the early 19th century," says Michael Bianchi, owner of Phenotype Architects in Bryn Mawr, Pa.

While early systems, such as built-up roofs (BUR), are still used today, they have largely been replaced by membrane systems like thermoplastic polyolefin (TPO). "A critical advancement of these newer membranes has been their impact on energy performance. Traditional black asphalt roofs absorb a tremendous amount of solar radiation, intensifying urban heat island effects and increasing cooling demands in summer months." Bianchi says. These reflective membranes are now a code requirement in many cities.

Built-up roofs (BUR)

According to roofing supplier GAF , built-up roofs have been in use for over a hundred years and are generally recognizable for their gravel top. These roofs are built-up, as the name suggests, with layers of asphalt-saturated felt or fiberglass and sealed with bitumen, which is a viscous tar-like petroleum derivative. The crushed stone or gravel top layer is added for protection, though sometimes a cap sheet can be used instead of stone. 

The underlying layers are generally sealed with petroleum-derived bitumen, though sometimes coal-tar bitumen or coal-tar pitch is used instead. This is less common today than it was historically, due to environmental contaminants in coal tar. 

Roofers use blowtorches to melt the bitumen in the felt panels and seal them together. This creates a unified, sealed surface.

Built-up roofs are simple to repair and generally last about 20 to 30 years. That said, these roofs don't have seams when properly installed, which makes finding leaks difficult. 

Modified Bitumen

Much like the layers of BUR, modified bitumen roofing builds up. However, unlike the bitumen in BUR roofs, most modified bitumen roofing can usually adhere without requiring blow torches, which makes it safer, simpler and faster to install. CertainTeed highlights modified bitumen's easy application as its signature benefit.

Modified bitumen roofs usually rely on a bitumen or polyester fabric combined with an asphalt emulsion and a top and bottom coat.

A sturdy substance, modified bitumen can stand up to foot traffic, and is often finished with a reflective coat that insulates well and reflects sunlight.

Ethylene Propylene Diene Monomer ( EPDM)

With some EPDM formulas and products in use for over fifty years, EPDM is a thick, rubbery roof coat that's long-lasting and weather and impact-resistant, according to Heidler Roofing of Pennsylvania and the Mid-Atlantic . It's often referred to as rubber roofing, though it's not actually rubber. 

EPDM installs without heat. The membrane is rolled out onto the roof base overtop of a spread adhesive, then it is pressed into the adhesive with a broom-like tool. The sheets glue one over the next so that there are no crevices between them. Roofers then seal the edges of the roof with flashing.

If you're looking for a highly durable membrane, EPDM is a good pick. In an EPDM Roofing Association study , five EPDM roofs were all performing satisfactorily when inspected at between 28 and 35 years of service.

Polyvinyl Chloride (PVC)

Installed in rolls, PVC is a membrane roofing material that's simple to apply. PVC is a plastic that provides strong weather resistance, and as it's generally white or light-colored, it can reflect and resist sunlight. 

While it's less relevant for residential buildings, another benefit of PVC is that it resists grease and chemical incursion. Because of this, it's a popular pick for commercial buildings like restaurants, where grease and chemicals could emit with their exhaust. According to Colony Roofers , an Atlanta-based roofing company, PVC has a lifespan between 20 and 30 years.

PVC can be covered with stones and "ballasted," or fully adhered and effectively glued to the roof decking . The downside of a PVC roof is that it is hard to repair, so repairs will generally require a full teardown and reapplication. 

Thermoplastic Polyolefin (TPO)

TPO is a single-ply synthetic rubber that's rolled out and adhered, much like other membranes such as PVC. Often reflective, it's a strong choice for homes that are looking for an energy-efficient option, as it reflects heat away from the roof. TPO is highly cost-effective, making it so popular that GAF says it's used on "over 50% of single-ply roofs installed today."

Weather, puncture and UV resistant, TPO is a long-lasting material. According to Peterson Roofing , a California-based roofer , TPO can last from 15 to 30 years. Heat-welded to the roof deck, it forms a tight seal. Because TPO requires heat-welding, it isn't as simple to install as some cold-adhering membranes; however, its various benefits make it a strong pick for a flat roof.

Metal

When we think of metal roofs, most of us imagine pitched roofs, but flat-seam metal is also a strong pick for flat roofs. According to the Frank Lloyd Wright Building Conservancy , flat metal roofs are usually constructed from galvanized steel or copper. Metal panels are fastened to roof decking, then welded together to make seams.

While metal is a strong roofing choice and is generally highly weather-resistant, it can rust. If the metal seams are improperly welded, they can leak or rust through. 

Other roofing materials

While we've covered the standards above, there are always outliers. Spray polyurethane foam roofing (SPF) is one of them. SPF is sprayed onto roofs in foam form, then hardens to form a protective layer. It has been used since the 1980s, according to manufacturer American Weatherstar . 

Other fluid-applied membranes include neoprene, butyl rubber and asphalt emulsion. In some buildings, roofs are constructed entirely of concrete, which provides exceptional longevity and weather resistance.

The drawbacks of a flat roof

While flat roofs offer many benefits, they can have their drawbacks, which vary depending on the type of material picked. Still, there's one drawback that's universal to flat roofs. Because low-pitch roofs have only a slight grade, they're more susceptible to water ponding and risky snowpack.

Since traditional pitched roofs have steep sloped surfaces, gravity does the work to pull snow and water away from the roof surface. While the slight grade of flat roofs moves water to drains, uneven installation, leaf build-up and other factors can cause water to pool on the roof. When this happens, water can slowly infiltrate the material. As with cleaning gutters on homes with pitched roofs , it's important to clean off your low-slope roof.

Historically, snowpack could be a real problem with flat roofs. Snow and ice can layer atop flat roofs, slowly leaking water and weighing down the roof. In some cases, the weight alone could be too much for poorly designed flat roofs, causing damage to the foundation. That's all changed with modern steel and concrete foundations. "Today, modern flat roofs can usually hold about 20 to 30 pounds of snow per square foot without any trouble." Says Robert O'Sullivan, president of Ranch Roofing , a roofer in Massachusetts. 

Beyond water accumulation and weight, flat roofs have another drawback: no attic space. With pitched roofs, the triangular structure of the roof generally provides attic space, while flat roofs do not. 

More to consider for flat roof owners

When it's time for a reroof, you usually won't get to choose between pitched and flat. Most of the time, this choice was made by an architect when your home was designed. That said, when maintaining a flat roof, homeowners do have some choice, though city codes might limit what's available. 

If you get a roofing inspection and your roofer tells you it's time to replace your flat roof, do the research to decide on the roof investment that works for your building. While we broke down the general types of flat roofs available above, many of these materials are available from numerous suppliers, often with substantial variation in their composition and design. Ask questions about what's available. For example, if your roofer suggests modified bitumen, ask whether it will be installed with heat or with adhesives. 

If you have specific goals for your flat roof — such as installing a solar bank or creating a patio area — ask your roofer whether the material you're considering and your roof decking can support it.

See more Yahoo Local-approved U.S. roofers

Best California roofers

Fresno / Los Angeles / San Jose / San Diego

Best Florida roofers

Cape Coral / Fort Myers / Jacksonville / Lakeland / Miami / Orlando / Sarasota / St. Petersburg / Tampa

Best New York roofers

Brooklyn / Rochester / Staten Island

Best Texas roofers

Austin / Dallas / Fort Worth / Houston / San Antonio

• Best national roofers in the U.S.

• See all Yahoo Local roofing