Flat Roofing Systems The Best Choice For Your Commercial Building

What’s the Right Roofing System for Your Low Slope Commercial Building?

It is important to consider the many options available to building owners for flat roofing systems. Commercial buildings are usually built with flat or low slope roof structures. They are built flat instead of steeply sloped like a residential roof for several reasons:

Initial Construction Cost

On a large commercial building a sloped roof would be prohibitively expensive to build because of the amount of material it would take to construct the roof support assembly and the height it would reach.

If there are peaks, valleys, or other irregular features required, those add to the complexity and cost.

Rooftop HVAC Equipment

Flat Roof With Hvac Systems

Commercial buildings often have HVAC or other equipment on the roof because it’s a secure location and the HVAC system needs to be near the area it will heat and cool. Also, HVAC systems can be difficult or impossible to install on a steeply sloped roof.


Flat roofs are ideal for the installation of rooftop solar systems, because panels can be positioned to take advantage of the sun’s track across the sky. A large flat roof is unused real estate and solar panels can turn the area into a means of energy production.

It’s important to keep in mind that a “flat roof” isn’t perfectly flat. Proper roof design dictates that roofs have some slope to provide for drainage. But a roof can eventually have changes in slope in places because of settling or flattening caused by heavy rooftop equipment or recurring ponding water.

Roofing System Options

When it comes to roofing low slope commercial facilities, building owners and managers have numerous options. Following is a brief discussion of some of the primary commercial roofing systems on the market today.

Built-up Roofing (BUR) Systems

Built Up Bitumen Roof

Built-up roofing systems are so-named because they are basically constructed or “built” on the rooftop by the installing contractor. Layers of felt or another synthetic fabric are mopped together with hot asphalt or coal tar. They are frequently covered with pea gravel other stone for protection from the sun and weathering. Properly designed gravel surfaces may also provide high fire ratings.

This system is not a new technology (now in use for well over a century), but BUR systems are still installed today. The major concern with a built up roof application is the quality of the mopping material and felts, and competent labor to properly install the system.

Modified Bitumen Membranes

Modified bitumen membranes evolved from built-up roofing. These are factory-manufactured fiberglass-based asphalt sheets, basically “built up roof in a roll” that is installed as a single layer or multiple layers.

Two types of modified bitumen exist with different installation methods. The methods include melting the material to an approved underlayment with an open torch or mopping it down with hot asphalt or cold process cement. The open torch method can be risky due to the chance of starting a fire on the rooftop.

Mod-bit products are in a class of commercial roofing systems called “single-ply.” Single-plies are flexible sheets of synthetic materials manufactured from a variety of materials and compounds, including bitumens, polymers, fillers, plasticizers, stabilizers and other elements.

Other single-ply products include thermoset and thermoplastic membranes:

Thermoset Membranes

Epdm Roofing System Replacement

Thermoset membranes (known in the marketplace as EPDM – ethylene propylene diene terpolymer) are vulcanized (rubber) materials that provide elasticity. Once EPDM sheets are manufactured and chemically “set” they are difficult to bond. They can’t be softened by heat, so EPDM membranes require adhesives for installation to seam the sheets of material together. Although available in white, EPDM roofing systems are typically black.

They can be mechanically attached with screws and plates to the roof deck, fully adhered to an approved substrate, or held down with a layer of washed river rock ballast. For re-roofing a structural engineer should be retained to approve the installation of approximately 12-15 pounds of stone per square foot.

Thermoplastic Membranes

Thermoplastic membranes (primarily PVC and TPO formulations) were introduced in Europe in the 1960s. Unlike EPDM systems, they do not set, but soften and “flow” when heated, allowing membrane sections to be “hot air welded” to seam the material together during. They are lightweight and, because they are typically white, highly reflective.

Depending on building location and climate, a white reflective membrane roof may result in significant energy savings by reflecting the sun’s heat, helping to keep the building cooler in the summertime.

PVC Membranes

Heat Sealed Pvc Roofing System

PVC (polyvinyl chloride) roofing membranes is an excellent thermoplastic choice for flat roofs and has a 40-plus-year track record in the US. It provides superior fire, oil, and chemical resistance as well as excellent rooftop seaming ability and weathering in virtually all climate conditions. White PVC membranes are highly reflective and, because PVC doesn’t “cure” on the rooftop, it remains weldable for years after the initial installation.

PVC has another advantage in that it may be installed year round, even during cold temperatures that prevent installation of other systems.

TPO Membranes

TPO (thermoplastic polyolefin) membranes were developed to combine the best qualities of EPDM (flexibility, weather durability, low-cost) and PVC (heat-weldability, chemical resistance, reflectivity) into a single roofing membrane. TPO has had a somewhat spotty performance history but is the fastest growing roof system in the U.S.

Commercial Roof Coatings

Commercial Roof Coatings For Flat Roofs

Commercial roof coatings can be a good temporary option for extending the life of a low slope roof. Coatings can be installed on a wide variety of existing roof surfaces, deliver energy savings by reflecting sunlight, and are less disruptive to apply than a complete tear off and re-roof.

Roof coatings are typically available in two formulations: silicone and acrylic. Each has its advantages, depending on budget, ambient temperature during installation, whether there is ponding water involved, how long the coating needs to last before another solution must be installed, and other factors.


Regardless of your commercial roof needs, it’s important to bring in a commercial contractor that understands the pros and cons of all the options that are available to you. Sentry Roofing has been a commercial roofing expert in the Midwest for over three decades, specializing in single ply membrane installations, primarily with the Duro-Last single-ply roofing system.

Our team regularly receives awards from Duro-Last for the quality of our workmanship and installation volume. Please contact us when convenient for you for a detailed assessment of your commercial roof requirements. We would consider it a privilege to be of service to you!