Traditionally, SBS membranes have been installed in hot asphalt, and APP membranes by torch application. Both of these application methods have been highly successful, but they are also susceptible to workmanship problems. In response to these problems, roofing manufacturers are promoting the use of adhesives as an alternative means to attach the membrane.

Although the use of adhesives for modified bitumen attachment is not new, its use over the past few years has increased. This is primarily due to the desire of building owners to specify the modified membranes in areas and circumstances where the use of asphalt or open flames is prohibited.

The use of torch applied membranes has also become more popular as roofing professionals realize the advantages of this application method.

In addition, the industry has taken steps to properly train personnel in the use of open flames and the conditions under which they can be used.

Hot asphalt, adhesive and heat welding each have their advantages and disadvantages. Depending upon the parameters of your project, you may want to consider the following:

The use of hot asphalt to attach modified bitumen is limited to SBS material.  APP membranes require greater temperatures to soften than can be achieved by hot asphalt.

Typically, ASTM D-312 Type III or IV asphalt is used in much the same traditional method as a built-up roof, and is the least expensive of the application methods. Asphalt is heated in a tanker or kettle and transported to the roof by pumping. It is then carted across the roof where it is mopped into place and the modified material is rolled into it. In order for the sheet to achieve the proper cohesive bond, asphalt must be approximately 400 degrees F. at the point of application.

If the temperature is too cool, the modified bitumen sheet will not fuse to the asphalt and poor bonding of the sheet to the substrate occurs. This allows the membrane to separate from the substrate, resulting in blistering. As the maximum temperature that asphalt can be heated is 500-550 degrees F., the temperature window from point of heating to point of application is very narrow.

An asphalt applied modified bitumen is also subject to blistering from skips or voids in the application of the asphalt. This condition occurs when the applicator fails to apply asphalt evenly over the entire substrate and the modified sheet is not 100 percent embedded. Over time, as rooftop temperatures rise, asphalt softens and the pockets of trapped vapor expand, resulting in blisters.

The use of an open flame to apply the membrane was originally limited to APP membranes. Realizing the positive attachment advantages of the torch method, SBS membrane manufacturers have developed sheets that can be applied by the torch method.

The success of attachment is not dependent upon outside forces such as ambient temperature, but rather the skill of the mechanic and their experience at knowing the proper signature when the sheet is sufficiently softened. When properly performed, torching is the most positive attachment method for modified membranes.

Both APP and SBS membranes can be torched. However, when torching an SBS membrane the skill of the mechanic becomes even more important due to its formulation. The window between over and under torching is narrower with the SBS membrane than with APP material.

Safety concerns with the use of a torch require that a fire watch be left in place to monitor the work for 2-3 hours after the torching operations are complete. In many instances, the monitor is equipped with an infrared camera or thermometer. Careful consideration of the fire hazards must be given to this application method when combustible insulations, wood construction, or recovering an existing system is required.

The adhesive used in the application of modified bitumen is generally a solvent based material.

When properly applied, this material softens the back of the modified bitumen membrane, forming a cohesive bond between the membrane and substrate. This application method has been used for many years, but has recently gained in popularity as the industry recognized blistering problems with hot asphalt applied systems and the safety concerns of torch applied systems. Both APP and SBS systems are quite adaptable to adhesive application.

During the application process, the adhesive is applied in a solid layer and the membrane is rolled into it. In some applications both the membrane and substrate require application of the adhesive. Adhesives should not be utilized when the ambient air temperature or the membrane roll is 40 degrees F. or less. 

As the adhesive is not totally cured for several days, the laps of the systems may not be totally watertight. Many manufacturers recommend that the seams in an adhesive application be torch or hot air welded to assure watertight construction on the day of application.

This type of application is the most expensive of the application methods and may add 10 to 15 percent to the cost of materials and application labor.  However, it is more efficient as the adhesive can be stored on the roof and spray, brush or squeegee applied.

There are many successful applications of modified bitumen systems using each of these methods.  Each produces a strong, durable, long-term roof system.

The industry has recognized that the majority of problems are related to the application of the system and not the modified material. Regardless of which application method is selected, the skill of the applicators on the roof is paramount to the success of the project.