Advances in Modified Bitumen Roofing Systems
There have been a number of improvements to modified bitumen roofing systems, both in the way they are manufactured and applied, since they were introduced originally to this country. While modified bitumen products always have provided excellent handling and weathering characteristics by combining modifiers with high-quality asphalt and reinforcements of fiberglass, polyester or a combination of both, they have evolved to better meet the needs of today’s roofing contractor and building owner.
Initially, atactic polypropylene (APP) was the first modified bitumen product brought from Europe, finding great success in reroofing the cut-up, smaller buildings of the “torch-on” market of the Northeast coast. This was followed closely by styrene butadiene styrene (SBS) hot-mopped asphalt systems, which enjoyed success by ultimately capturing 50 percent of the overall market nationally. In addition to working as a roofing membrane system, both APP and SBS modified products also have been used successfully for many years as flashing for built-up roofing systems. As a membrane system, both APP and SBS products have had their shortcomings, which led to some of the more recent changes in the way they are applied.
One of the first advances was the use of hot-air welders on field seams, which addressed the concern about the potential fire hazard of the “torch-on” systems. A second advance was the development of a cold adhesive application to address the complaints from building occupants about the odor from hot asphalt used with the SBS systems. As the use of cold adhesives became more prevalent, concerns about the time — sometimes days — for the laps to set-up, led to hot-air welding and more recently, to the newer self-adhered laps. These advances are generating a great deal of interest and corresponding questions by roofing contractors.
What are the shortcomings of a torch-on system?Torch-on products have long been recognized as premier flashing materials, due to their great flexibility, adaptability and adherence to wall details in very difficult areas. The torch-on flashing can be flame heated and bent to conform to very irregular surfaces and angles, better than most other types of flashing. It is unparalleled in adhering to difficult inside and outside corners. Its adhesion is so strong that often higher wind resistant ratings can be obtained. Perhaps the only flaw lies in an unskilled applicator inadvertently overheating and damaging the product, or torching into blind areas where the potential to start a fire exits. Misused torch-on systems have been known to start fires, which have burned entire buildings.
Because of this fire hazard, and also because of the great success that heat-welded thermoplastic membrane systems enjoy, the use of hot-air welding of modified bitumen products has been increasing significantly of late. Hot air welders with higher heat and wider welding shoes or nozzles have been developed specifically to apply modified bitumen products without the need of dangerous open-flame torches. As with any product, the skill and training of the applicator are key to the success of the system, by ensuring that the products are properly attached. Overheating during application would destroy or significantly shorten the product’s life span, while under-heating would only create a cold weld or tack bond and ultimately, a bond failure and resulting leak.
Are cold adhesive systems being used to replace hot asphalt BUR?Building owners sometimes request a system other than hot-applied asphalt to avoid the complications arising from the odor and fumes associated with it when used in sensitive areas such as schools, hospitals and occasionally, government buildings. In cases where hot asphalt is undesirable or prohibited, “cold process” cements are used. These cements use a special mastic adhesive containing low levels of solvent, typically one third of that used in traditional cold application cements. The adhesive can be hand applied, at the rate of 1 1/2 gallons maximum per square, using a 1/4-inch (maximum) saw-toothed trowel or rubber squeegee. For larger areas or when desired, the adhesive also can be spray applied.
Once the solvent flashes off (from a few days to as much as a month depending upon climatic conditions and time of year), the adhesive sets-up into a durable, elastomeric and waterproof bond. The rate of application is critical, as more is not better. Only enough adhesive should be used to “wet-out” both the base sheet and the back of the modified cap sheet. This will cause the modified asphalt in the rolls to soften and adhere to each other. Once the solvent flashes off, the bond becomes as strong as if the sheets were joined together using hot asphalt. Too much adhesive could cause a sheet to breakdown from solvent attack or possibly never bond properly. In addition, excessive solvent could lead to an odor problem almost as significant as that perceived to come from hot asphalt fumes.
How long do cold adhesive seams take to set up?Cold adhesive seams take from a few days to as long as a month to bond completely, depending upon the climatic conditions of the area. For the bond to set up completely, all solvent in the adhesive must flash-off entirely. If the adhesive was manufactured with too much solvent or applied too heavily, it will take longer for the bond to form. Cooler weather will slow down the process as well.
Despite concern that the lap may be pulled apart, the lap seam still is waterproof. The old, incorrect perception still exists in the minds of the applicators that if a lap can be peeled apart it is not waterproof. Experience using the system is the only way to overcome this misperceptio
Are self-adhering seams the future of modified bitumen systems?While cold adhesive seams are watertight as they are applied, many contractors feel that any sheet or lap area that doesn’t set up tightly upon installation is suspect to leakage or even blow-off. The self adhering lap products allows the sheet to be set in cold adhesive without the use of torches or hot-air welders but still have a tightly bonded lap area upon application.
The self-adhering lap products come with a 3- to 4-inch factory-applied self-stick lap adhesive along the selvage edge, which has been covered with a release film during manufacture. This film remains in place until the rolls are applied. The film is then stripped off, allowing an instant bond with the overlapping sheet. The factory-applied adhesive bonds instantly so there is no cure-time of days or weeks to have a watertight seal. Combining this with the cold adhesive application gives the modified bitumen roll products an application method that doesn’t require torching, hot-air welding or hot asphalt.
How are the fully self-adhered products affecting the commercial roofing market?
This new generation of easy-to-install modified bitumen products are aimed at the smaller roofing contractor. These products provide contractors with an alternative to BUR systems and APP torch-applied systems without making a major initial investment.
Self-adhered products offer a number of advantages. Installation is up to a third faster than conventional application methods. They are ideal for areas where hot asphalt or torching is prohibited or not desirable. As there isn’t any solvent odor, they offer a solution where cold-applied adhesive odors may be a problem. The strong adhesive properties provide an immediate watertight seal. The sheet is reinforced with an SBS rubber and asphalt blend, which offers elasticity for full recovery. The base sheet has a fiberglass reinforcing mat, adding durability. The cap sheet with a polyester-reinforcing mat adds high tensile strength for durability and can accommodate typical rooftop expansion and compression forces.
Despite these benefits, it is important to keep in mind that these products are difficult to install in cold weather and should not be installed in temperatures below 40 degrees F. It also is very important that the deck is free from debris and contaminates, especially water. They are not able to adhere to every substrate. While the products are easy-to-install and form a strong bond, it is important for contractors to invest the time in training so proper installation is ensured.