Selecting a Roof System: Modified Bitumen

With the waning popularity of gravel surface and asphalt built-up roof systems as well as the complete disappearance of coal-tar pitch roof systems, the use of modified bitumen roof systems has gained wide popularity. Modified bitumen roof systems are primarily composed of the following:

  • Rigid or tapered isocyanurate foam roof insulation that meets local code requirements. In Alabama, the state requires a minimum Thermal Resistance (R) Value of 25. This insulation is mechanically-fastened to metal or wood substrates and is adhered to concrete substrates. Use of a vented, mechanically-attached base sheet over lightweight insulating concrete is recommended.
  • A ¼” to ½” hard coverboard is adhered over the isocyanurate insulation. The coverboard safeguards and strengthens the roof system so that it can better withstand foot-traffic and weather events such as hail storms.
  • One ply of smooth-surface, polyester-reinforced modified base sheet is adhered to the coverboard through a cold adhesive or heat-applied melding process. In some cases, two plies of base sheet are used.
  • The base sheet application is followed by the application of one ply of polyester-reinforced, granular-surfaced modified bitumen cap sheet. There are multiple cap sheet options including ultra-reflective white to increase energy efficiency.
  • Modified flashing at curbs and walls is constructed through a similar process.
  • Modified bitumen systems can carry 5, 10, 15, 20, 25, and 30-year manufacturer warranties on labor and material. Typically, approved applicators of the systems must have an additional, special designation to install the 25 and 30-year systems.

In addition to the cold adhesive or heat-applied melding process, a self-adhering modified bitumen system has made strong headway into the market. Both the base sheet and cap sheet have a plastic liner on the underside of the sheet. As the roll is laid out, a crew member peels off the plastic, revealing a self-adhering mating surface that adheres the base sheet to the coverboard face and the cap sheet to the top of the modified base sheet. The edges of the overlapping rolls (laps) are fused together by a “robot-welder”, which applies high-temperature air to meld the two mating surfaces. The self-adhering system is especially convenient when access to the building is difficult. Also, the fumes which accompany these self-adhering systems are virtually non-existent, so this system is accommodating when working around fresh air intake points—especially on healthcare and educational facilities.

Of course, modified bitumen roof systems are still specified to be adhered with hot asphalt – an application which has stood the test of time. However, on facilities with sensitive occupants (healthcare patients, students, office staff), the fumes from the hot asphalt become a problem when this system is applied in the vicinity of fresh air intake points or with open windows nearby. Moreover, working at the rooftop level with hot asphalt at temperatures of 425 degrees Fahrenheit, crew and building occupant safety is also an additional consideration. And finally, with surges in oil prices, so goes the cost of asphalt to install this application of a modified bitumen roof.

As to the building types to which modified systems can be applied, the use of adhered and self-adhered modified bitumen systems is nearly limitless. And by incorporating “liquid flashing” systems into modified bitumen applications, almost any type of roof detail can be addressed effectively.

No matter what kind of commercial roofing system you are considering for a replacement project; or, even if you are not certain what type of roof system would be ideal for your application, the experienced team at Standard Commercial Roofing and Building Envelope Solutions can work with you (or your designer/consultant) to make sense of the options and offer proposals on your re-roofing program.

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