6 Most Common Commercial Roof Types

roof installation

Over the past several decades, modern advancements in the roofing materials industry have greatly expanded the number of different types of commercial roofing systems available. While this certainly creates more options for developers and contractors, it may also generate a bit more confusion around which types of commercial roofs are optimal for certain buildings and projects.

To alleviate some of this confusion, we’ve compiled the following guide outlining the six most common commercial roof types in use today. We’ll highlight some of these systems’ pros and cons and which types work best with different settings, conditions, and budgets.

Commercial Roof Types

Before diving into the various roofing systems available, let’s review the three most common categories of roofs used for commercial buildings: flat, low-slop, and steep-slope roofs. All the roofing systems described below will be specified for one or more of these roof types.

Flat Roofs

The term “flat roof” is a bit of a misnomer. Almost no roofing system is completely flat because there would be no rain or snow drainage otherwise. Rather, most “flat roofs” are actually sloped very slightly — typically a half-inch pitch or less per 12 inches (or half in 12) — to allow for water runoff. Many commercial buildings benefit from flat roofs because they are more affordable to install, and they are easier to access and maintain on larger structures. They also offer more usable space — many buildings install their HVAC units on flat roofs, for example — and they can also be constructed to accommodate foot traffic if desired.

Low-Slope Roofs

Any roofing system with a slope of three inches per foot (3:12) or less is considered a low-slope roof. Low-sloped systems are quite popular in commercial buildings because they combine the benefits of affordability and ease of installation/maintenance with more efficient drainage. Low-slope roofs (and flat roofs) also reduce the amount of unused space between the ceiling and the roof, making it easier to keep the building cool in summer and warm in winter.

Steep-Slope Roofs

Steep-sloped roofing systems are less common with commercial buildings than residential structures, mainly because they tend to be too expensive and impractical for larger commercial buildings. However, steep-slope roofs provide excellent water drainage for buildings that can accommodate them and require less cleanup of accumulating debris. On the other hand, the steeper slope makes these roofs more difficult and dangerous to install and repair when needed.

6 Commercial Roof Systems

Having established the various types of commercial roofs, let’s now discuss the seven most common commercial roofing systems and for which types of roofs they work best.

Built-Up Roofing

As one of the oldest and time-tested systems for commercial roofs, built-up roofing (BUR) has been in use for more than a century, and you’ll still find it commonly used today. Also known as “tar and gravel” roofs, BUR systems consist of multiple layers of asphalt/bitumen, and roofing felt topped with gravel. These roofing systems work best on flat or low-slope roofs.

Advantages of BUR systems

  • Durable, waterproof, and fire-resistant: BUR systems are seamless and highly durable, with limited, vulnerable places for water to seep through. Additionally, the gravel top layer makes this roof more fire-resistant.
  • Easy to maintain: Once installed, a BUR roof requires relatively little maintenance over time.
  • UV resistant: Built-up roofing is resistant to the sun’s damaging UV rays, which can deteriorate roofing materials over time.

Disadvantages of BUR systems

  • Labor-intensive to install: The various BUR layers require a lot of time and labor, adding to the installation costs.
  • Challenging to repair: While leaks are uncommon, when they do occur, it can be difficult to pinpoint the source in a BUR roof. It may be necessary to do a full resurfacing to solve the problem.
  • Prone to damage in cold temperatures: BUR systems hold up better in warmer climates, less so in colder climates.

Modified Bitumen

Hailed as the next evolution of BUR roofing, modified bitumen was developed in Europe in the 1960s and came into wide use here in the U.S. by the mid-1970s. Modified bitumen is essentially a membrane of asphalt combined with rubber or plastic polymers and reinforced with fiberglass for added flexibility. Used on flat and low-slope commercial roofs, modified bitumen roofing systems are typically installed in two layers (base sheet and cap sheet), but three-layer systems are common.

Advantages of modified bitumen

  • Better in extreme temperatures: Modified bitumen’s added flexibility solves BUR systems’ problem with cold temperatures.
  • Easy to install: A two-layer installation makes it much less labor-intensive to install modified bitumen than its BUR counterpart.
  • Tear-resistant: Modified bitumen can handle a reasonable amount of foot traffic without damage. If a tear does occur, it’s also easy to repair.
  • UV protection and “cool roof” effect: Modified bitumen’s lighter color makes it reflect the sun’s rays, making buildings easier to keep cool.

Disadvantages of modified bitumen

  • Lower life expectancy: The trade-off with modified bitumen is that it has one of the shortest life-cycles of any other commercial roofing material.
  • Odor complaints: Modified bitumen may emit a foul odor during installation and for some time afterward.
  • Fire danger during installation: Standard installation processes involve using a blow torch, creating a fire hazard with some structures. Recently, cold adhesive options have created a safer alternative.

Single-Ply Roofing

Over the past 50 years, the single-ply membrane roofing industry has grown to dominate the market for flat and low-slope roofs. They may be installed by ballasting, adhesives, or mechanical attachment, and they are well-known for their versatility and ease of installation. Single-ply membranes generally fall into two categories:

  • Thermoset (EPDM): Made from synthetic rubber, EPDM roofs are among the longest-lasting on the market, with a life span of up to 60 years with proper care.
  • Thermoplastic (TPO, PVC): TPO and PVC membranes are made from plastic polymers. TPO roofs are valued for their reflective “cool-roof” features, while PVC roofs offer high resistance to chemicals, high winds, and extreme weather elements.

Taken together, the overall advantages of single-ply membrane roofs are that they are lightweight, versatile, fire-resistant, and UV resistant, providing excellent coverage without overburdening the structure. The downside is that single-ply membranes are more prone to punctures and leakage.

Metal Roofing

Among the commercial roofing materials, metal is one of the few that works well on all three types of commercial roofs — flat, low-slope, and steep slope. In use since the mid-1800s, metal roofing is one of the oldest roof types still in use on commercial structures today. Many metal types can be used for metal roofing, including aluminum, copper, zinc, steel, and tin.

Advantages of metal roofing

  • Very lightweight: Provides excellent coverage without overburdening the structure.
  • Energy efficient: Reflects the sun’s rays, keeping the building cooler in hot temperatures.
  • Fire resistant: Metal roofs are fire-proof, and so provide ample protection against fire.
  • Long-lasting: With proper care, metal roofs can last 50 years or longer. (Copper and zinc roofs can last up to 100 years.)

Disadvantages of metal roofs

  • Cost: Metal roofs are among the most expensive to install and might be cost-prohibitive for larger structures.
  • Noisy: Unless they cover other materials, like asphalt, metal roofs can be very noisy in rain, wind, or hail conditions.
  • Prone to denting: Falling hail or debris can cause dents in many types of metal roofs.

Vegetative Roofs

While vegetative (or “green”) roofs have been around in one form or another since the 1960s, they have seen a resurgence in popularity in recent years as the public has become more environmentally conscious. True to its name, a vegetative roof consists of plant life (in a growing medium like soil) placed over a waterproof membrane. Vegetative roofs typically appear on flat or low-slope roofs, but they can also be installed on some steeper slopes, as well.

Advantages of vegetative roofs

  • Benefits to the environment: Green roofs don’t only help reduce carbon dioxide and release healthy oxygen but also greatly reduce air conditioning in the building itself.
  • Natural insulators: Plant life on the roof keeps the building cooler in summer and warmer in winter.
  • Better for drainage: Vegetative roofs absorb water and release it naturally back into the environment, reducing storm runoff.

Disadvantages of vegetative roofs

  • Heavyweight load: Green roofs place greater stress on the structure, which must often be reinforced to hold them.
  • Higher cost: Vegetative roofs are more expensive to install.
  • Higher maintenance: A vegetative roof is essentially a garden, and it must be cultivated as such.

Liquid-Applied Roofs

Liquid roof coatings have been around in some form for many decades, but the concept of liquid-applied membrane roofing came into its own starting in the 1980s. Liquid-applied roofing essentially comprises several chemicals sprayed onto the roof in liquid form, which then congeal to form a seamless, self-flashing waterproof membrane.

Liquid-applied membranes (LAMs) can be installed on most roof types and can even be applied over existing roofing materials like asphalt, bitumen, or concrete.

Advantages of liquid-applied roofs

  • Easy application: LAMs can be installed with minimal time and labor costs.
  • Lightweight yet durable: Liquid-applied roofing provides highly effective coverage with minimal stress on the structure.

Disadvantages of liquid-applied roofs

  • Strong smell: Many people complain of the chemical odor during application.
  • Requires skill to apply: While easy to apply in general, it requires a steady hand and practice to apply the layer evenly.
  • Limited shelf life: While liquid applied roofs have a long lifespan once applied, the liquid form can separate if not used within a reasonable amount of time.

Which is the right system for your needs?

With all these choices of commercial roofing systems available, how do you determine which is best suited for your project? Working with a manufacturer representative like RoofSource can help. With our vast knowledge of the roofing products industry, we can help you evaluate your needs and budget to narrow your options to your perfect roofing solution.