Pros and Cons of Flat Roofing Systems

Orange brick building facades above retailers

Barry Winiker / Getty Images

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.

  • 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.

Pros

  • 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.

Cons

  • 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. 
EPDM covered flat roof
KVDP/Wikimedia Commons/Public Domain

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.

Group of worker installing tar foil on the rooftop
PhanuwatNandee / Getty Images

Pros

  • 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.

Cons

  • 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.

Roofer preparing part of bitumen roofing felt roll for melting by gas heater torch flame
Nenov / Getty Images

Pros

  • 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.
  • BUR is low-maintenance and durable.
  • Self-adhesive roll roofing can be installed by homeowners.
  • Most BUR material can be recycled, just like asphalt shingles.
  • Provides better durability than a BUR with similar ease of installation like EPDM.

Cons

  • 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.