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Modified Bitumen roofing systems have been available in the roofing marketplace for more than 20 years, but their popularity has increased in recent years. As contractors, specifiers, and maintenance and engineering managers become more familiar with their positive attributes, modified bitumen systems in many cases are replacing conventional built-up roofing (BUR) and single-ply systems.
Like many other new technologies, some early modified bitumen systems performed poorly due to poor selection, poor quality materials or untrained installers. Some modified bitumen membranes weathered poorly, lost granules or lacked the physical properties to be installed in hot or cold climates.
Fortunately, manufacturers have addressed most early problems with these systems by adopting material standards, better worker training, and a better understanding of product limitations. Today, modified bitumen systems offer some significant advantages for roofing specifiers and maintenance and engineering managers.
One attribute driving modified bitumen’s growing popularity is the variety of application methods it affords managers. These methods are among the most common:
Hot-asphalt method. Modified bitumen systems can be installed in conventional hot asphalt, allowing the use of common BUR equipment and application techniques. A roofing crew familiar with the BUR application will require limited additional training to become proficient in this application.
Torch method. This application method is probably the second most common. When this application is to be used, a manufacturer typically increases the thickness of modified bitumen on the backside of the roll and adds a thin plastic burn-off film. The additional bitumen and burn-off film melt under the application of a propane-fired torch, and they adhere the membrane to the roof insulation or substrate.
Torch application does not require the use of hot asphalt, eliminating the need for an asphalt kettle. In areas where asphalt fumes become an issue or where asphalt cannot be transported to a roof, the torch method might be a viable alternative.
Cold-adhesive method. This third application option is gaining in popularity. In locations where hot asphalt cannot be used due to fumes or accessibility restrictions and where torch application might not be allowed or prudent, cold-adhesive application might be more appropriate. In such applications, the modified bitumen membrane is installed in the manufacturer’s cold adhesive. Because cold adhesives take time to flash off and reach proper bonding strength, some manufacturers require that the seams be hot-air welded or torched to assure they are watertight.
Some manufacturers require a waiting time after the first modified ply is installed but before the second or successive plies are installed so each layer of adhesive can cure. Since the adhesives contain a fair amount of petroleum solvents, the modified bitumen ply often is quite soft underfoot while curing and more susceptible to damage. Managers should keep this issue in mind on new-construction projects, where a roof often serves as a work surface for other trades.
Self-adhesive method. “Peel and stick” modified bitumen products have been available for more than 20 years. Most self-adhered membranes have been used on steep-slope roofing and as an underlayment for eaves and valleys. Modified bitumen manufacturers have developed a membrane that has a very sticky surface on the back of the roll. A release paper is mated to this sticky side during manufacturing. When installing a self-adhering membrane, the contractor unrolls the membrane, positions it, peels off the backing, and then presses it into place, commonly with a weighted lawn roller.
The appeal of self-adhering roof membranes is related to concerns about asphalt fumes, torch safety and solvents in cold adhesives. Self-adhering membranes eliminate the need to lift asphalt, propane or adhesives to the roof, making it perhaps more suitable for hard-to-reach locations.
Another feature of modified bitumen systems is the option to use several application methods on one project. It is not uncommon to see specifications for applying the base-ply modified bitumen in hot asphalt and torch-applying or cold-applying the top ply.
Some roofing professionals believe that using the torch or cold-adhesive method between layers of modified bitumen results in a composite membrane that achieves a better fusion than using regular mopping-grade asphalt. Without the foreign asphalt between plies, the entire composite membrane consists of a polymer-modified bitumen and is thought to perform better over time. Without entering into a discussion of the compatibility of mopping-grade asphalt and modified bitumen polymers, this argument seems to have merit.
In the interest of accuracy, it should be pointed out that APP modified bitumen membranes generally are not suitable for use with the hot-asphalt application method, but they generally are recognized as being suitable for torch and cold-adhesive applications.
Manufacturers provide specific membranes suitable for each method. For example, modified bitumen membranes made for torch applications generally are not suitable for hot asphalt or cold adhesive applications. So there is a potential for inappropriate or inadvertent use.
In addition to the wide range of membranes and application methods available, modified bitumens generally are recognized as durable under foot traffic, relatively easy to inspect and repair, easy to clean and maintain, cost competitive, widely available and good for BUR repair and reflashing. Managers should use caution when specifying modified bitumens for areas of ponded water, as the asphalt-based membranes will deteriorate under prolonged ponding conditions. Modified bitumens might be used on roof slopes greater than 1/4-inch per foot slope, but managers should provide for proper membrane attachment, as the sheets are relatively heavy and might slide if not properly designed.
As with other asphalt-based products, modifieds are susceptible to oil-based contaminants, so they should be used with caution on roofs where such contaminants might be discharged.
Modified bitumen membrane systems are usually two plies thick versus three or four plies for built-up roofing, so labor costs typically are lower for modified bitumen systems. These labor savings generally are offset by the higher material costs, and for simple, easily accessible buildings, the installed cost for an asphalt BUR usually is slightly lower compared to the modified bitumen system.
Comparing the costs of modified bitumen systems to single ply systems is like comparing the costs of shoes; a work boot is designed for a different use than a running shoe. The most appropriate shoe would depend on whether the user was going for a run or working construction.
In general, single-ply systems are lower in installed cost compared to modified bitumen systems, with some regional exceptions. Modified bitumen systems offer flexibility and proven durability. This system belongs in the arsenal of roofing solutions. Properly selected, designed and installed, these systems can provide years of watertight service.