Flat roofing is not actually flat; it has a very low slope—between 1/4 to 1/2 inch per foot—so that it drains water. But such a low slope holds snow and water much longer than a steeply pitched roof and therefore needs a very different material to stay watertight. While standard sloped roofs typically have shingles that are overlapped like fish scales so that water slides over them, a flat roof is designed as a continuous, or monolithic, surface that can hold some standing water for a limited time.
There are three main types of flat roof systems:
- Membrane or single-ply
- Built-up roofing (BUR)
- Modified bitumen (MBR)
Membrane (Single-Ply) Roofing
There are several types of membrane roofing materials, including rubber and plastic formulations. The most common type used for residential flat roofing is EPDM (ethylene propylene diene monomer), a synthetic rubber sheet material that is also commonly used as a pond liner.
Membrane roofing consists of a layer of insulation board topped by sheets (the membrane) of EPDM rubber or other material. The EPDM membrane can be loose-laid and held in place with ballast, such as river rock or masonry pavers. The membrane can also be fastened or glued to the insulation layer.
- Repairs are relatively simple and inexpensive; homeowners may be able to make some repairs themselves.
- The roof deck doesn’t need reinforcement because EPDM roofing is lightweight.
- Leaks are very rare with EPDM roofing, provided no surface damage occurs.
- EPDM roofing can retain heat to lower heating bills; other types of membrane roofing can reflect heat to keep the home cooler.
- Roof penetrations, such as pipes, HVAC systems, and chimneys, make installation more difficult and costly; penetrations can be a source of leaks if not flashed properly.
- Membranes can be punctured by falling branches, foot traffic during installation or maintenance, or storm damage, leading to leaks.
- Seams between membrane sheets, while sealed, are common areas for leaks.
Built-Up Roofing (BUR)
Built-up roofing (BUR) was the most common type of flat roof before modified bitumen and membrane roofing were developed. BUR consists of many layers: a bottom layer or two of insulation board, multiple intermediate layers of tar or asphalt alternated with layers of roofing felt, and a top layer of gravel. The result is a thick, tough, seamless roof assembly that is highly resistant to damage.
While built-up roofing is still used in some commercial applications, it is not a common roof type for houses, primarily due to is weight and thickness as well as the strong odors and mess it creates during installation.
- BUR offers excellent protection against water, UV rays, and inclement weather.
- BUR low-maintenance and costs very little to maintain throughout its lifetime.
- It’s easy to remove layers when repairing or resurfacing the roof.
- The gravel in built-up roofing makes it highly resistant to normal foot traffic.
- Installing BUR is slow and labor-intensive, due to the many layers and materials involved.
- Potentially hazardous fumes and vapors are emitted during installation.
- The roofing assembly is very heavy and often requires that roof joists are strengthened before it’s installed.
- Finding the source of a leak can be difficult and sometimes requires dismantling the whole roof.
- Built-up roofing is not flexible in cold temperatures, making it susceptible to damage.
Modified Bitumen Roofing
Modified bitumen roofing (MBR) was developed in the early 1960s as a lighter-weight alternative to BUR. It also comes without much of the mess, heat, and smell associated with installing BUR. Modified bitumen roofing is a flexible, asphalt-based material with a mineral top coating, similar to traditional asphalt shingles. It comes in rolled sheets that are 3 feet wide and up to 36 feet long. The sheets are rolled onto the roof atop a base sheet membrane.
The conventional method of installation, called "torch-down," involves heating the backside of the roofing as it is unrolled, essentially melting the material to the base layer. There are also self-adhesive versions of modified bitumen that install in a peel-and-stick fashion.
- Factory-applied mineral surfacing ensures consistent installation.
- MBR is much simpler to install than BUR, saving labor and reducing installation error.
- Offers better elasticity and flexibility at low temperatures, compared to BUR.
- MBR is low-maintenance and durable.
- Self-adhesive roll roofing can be installed by homeowners.
- Most MBR material can be recycled, just like asphalt shingles.
- Provides better durability than a BUR with similar ease of installation like EPDM.
- Some application techniques require an open flame/torch, which requires special skills and safety considerations.
- Overlapping joints must be correctly adhered to prevent leaks.
- Modified bitumen is generally less attractive than BUR or membrane roofs with gravel or river rock ballast.