Built-up roofing (BUR), which consists of multiple plies of membranes soaked with asphalt or coal tar pitch, has existed in its present form for close to a century. It continues to be the standard against which all other roofing systems are compared, according to the Asphalt Roofing Manufacturers Association (ARMA).
BUR is a continuous, semi-flexible roof membrane assembly that combines plies of saturated felts, coated felts, fabrics or mats between which alternate layers of bitumen are applied. Generally surfaced with mineral aggregate, bituminous materials or a granule-surfaced roofing sheet, BUR systems that incorporate asphalt have the longest performance record of any roofing system.
Another feature that distinguishes BUR from the newer single-ply roofing systems is its multiple plies, which provide redundancy in the event that a problem arises in one of them. Roofing crews have a familiarity with BUR that is unmatched by any other roofing system, not only during application but also in making repairs and modifications. After decades of use, there are seldom any "surprises" involved with BUR applications. BUR also is competitive in cost with other systems, according to ARMA.
The advantages of BUR have not come at the expense of technical improvements, however. These include the development of synthetic materials such as those used in modified bitumens, the advent of cold application processes that reduce the fire hazard associated with torched applications and improvements in the insulation used in the systems.
Additionally, "hybrid" BUR systems have been developed that use two or three plies along with a modified bitumen cap sheet. These systems offer the advantages of multiple plies as well as a modified bitumen product for improved weathering characteristics. Combined with the modified bitumen flashing already in use on most BUR roofs, the result is an extremely durable membrane system.
The use of more complex roof systems, such as cap sheets using modified bitumen as saturants, is a significant BUR trend. Previously, modified bitumens in BUR were used primarily as a mopping asphalt. While the addition of a modified bitumen cap sheet may add from 5 percent to 10 percent to the cost of a roofing project, it can extend the roof's service life by at least 40 percent.
Roofing contractors have worked with BUR systems for decades, giving them a familiarity that becomes a major advantage during the application process.
Modified bitumens (known as "mod bits") are asphaltic roofing materials modified with an elastomeric compound that is compatible with roofing asphalt. This blending improves the asphalt's low temperature flexibility and extensibility characteristics. Although officially classified as single-ply membranes, the increasing incorporation of mod bits into BUR systems makes them an integral part of the BUR market. When elastomers are added to the bitumen, the bitumen can be used either as a mopping asphalt or to saturate glass or fiber scrims. These applications represent the most common uses of mod bits.
Modified bitumen systems originated in Europe during the 1950s, with the first atactic polypropylene modified (APP) system applied in Italy. In the 1960s, the first styrene-butadiene-styrene (SBS) modified system was installed in France. The first modified bitumen systems were seen in the United States in the mid-1970s.
Modified bitumen membranes are composed of asphalt flux and polymer modifiers. They are typically reinforced with either polyester or fiberglass. The rubber- or plastic-based modifiers extend the membrane's aging abilities and improve both high and low temperature performance. Fillers, fire retardants and surfacing are added to improve the longevity and safety of the membranes, and offer protection from UV exposure.
Modified bitumens are generally installed in two plies — a base sheet and a cap sheet. The cap sheet is also used for base flashing around the roof perimeter and penetrations. Modified bitumens may be torched or mopped on, heat welded or adhered with a self-adhesive backing. Hybrid systems become an intrinsic part of BUR systems when used as a cap sheet, offering greatly enhanced heat resistance, low temperature flexibility and elongation properties. Cap sheets generally incorporate either granule surfacing for added UV protection, or are unsurfaced, which allows a wide range of surfacing materials to be applied.
From 1995 through 2000, modified bitumen's commercial roof market has grown from 886 million square feet to 1.92 billion square feet. Fire hazards associated with torching have been a major factor in the decline of torching modified asphalt from 47 percent to 14 percent of applications since 1991, and a corresponding growth in cold applied applications from 10 percent to 38 percent, according to Business Communications Co. Inc., which reports on trends in roofing materials.
APPs have traditionally been torch-applied, because when they were mopped the asphalt often would not remain hot long enough to melt the back of the APP sheet into the asphalt to form an acceptable bond. However, a good chemical bond can be obtained with cold adhesives, which is beneficial when it is difficult to get hot asphalt onto the roof.
Cold process applications growing
The industry has seen significant asphalt-based roofing system improvements in cold process applications using cold adhesives to install modified bitumens (a component of flexible membrane systems) and in some cases, the underlying base plies. In addition to the fire hazards associated with torch applications, an increase in governmental regulations is favoring the growth of "cold process" BUR. The cold process not only greatly reduces the amount of noxious volatile organic compounds (VOCs) released into the environment, but eliminates the danger of injury to roofers working with hot kettles. New material developments and an increase in situations where "hot" systems prove impractical have resulted in a situation in which the cold-applied growth trend is expected to continue.
Fire hazards associated with torching have been a major factor in the decline of “hot” BUR applications and a corresponding growth in cold-applied systems.
Cold-applied systems use solvent- or water-based cold adhesives instead of hot asphalt. Available in the United States for more than 50 years, they were developed as substitutes for hot-mopped bitumen as the interply and surfacing bitumen.
Regulations covering asphalt fuming in BUR applications have not prevented the use of the material. But many owners find the odor associated with asphalt roofing applications to be undesirable, and restrict or eliminate the use of hot asphalt strictly because of the fumes generated.
The asphalt industry has been successful in greatly reducing fuming during roofing applications. In many cases, fume recovery systems can filter as much as 99 percent of the fume at the kettle, although the negligible odor at the mop bucket cannot be eliminated.
One nonfilter method draws the fume through a hose to a thermal converter, where it is mixed with fresh air. The mixture is then superheated, eliminating up to 99 percent of the odor and visible fume. Another method features a recovery system that utilizes five separate filtration modules, with each one working to eliminate a designated airborne pollutant. As with any filtration system, there is a cost associated with regularly replacing the filters.
A low-fuming asphalt also has been developed. Divided into two small pieces, which are more easily handled than one very large package, the packaging of this product creates a skim layer on the surface of the hot asphalt when it melts, trapping the fume and odor inside the kettle.
Special considerations must be observed when insulation is used in a "hot" BUR application, whether torching or mopping down hot asphalt is involved. The most commonly used insulations are rigid foam board products. Foamed plastic insulation is manufactured from either thermoplastic or thermoset materials. Expanded and extruded polystyrenes are thermoplastic foams that soften at approximately 165 F and melt at approximately 200 to 210 F, according to the Polyisocyanurate Insulation Manufacturers Association (PIMA). Because this temperature range is reached with all hot asphalt systems, it is almost impossible to incorporate polystyrene insulations in hot asphaltic systems without damaging the insulation.
Some solvents used in cold-asphaltic mastics can also damage thermoplastic foams. For this reason, thermoplastics, which are excellent products for single-ply membrane applications, are seldom used in BUR applications. But if they are, a cover board is required, and taping of the seams is recommended.
Polyisocyanurate insulation products, on the other hand, are thermosets, which are manufactured as a rigid foam board. They will not soften or melt at temperatures used with the hot asphalt systems. Also, thermoset foams hold up well to exposure to adhesives.
Rigid insulation boards made of perlite are also commonly used. Perlite is a naturally occurring siliceous rock that expands when heated. This expansion results in millions of small bubbles that trap air, resulting in good insulating properties. The perlite is typically combined with cellulite fibers and binders, with a specially treated top surface to adhere to facilitate the adhesion of bitumens. Dimensionally stable, fire-resistant and compatible with other roofing materials, perlite is not usually used with ballasted, loose-laid membranes as it is prone to moisture absorption. While it doesn't have an especially high R-value (less than 3.0 per inch), perlite boards are often used to protect foam insulations from hot asphalts, or as a protective layer to minimize roof traffic damage.
While single-ply technologies continue to be developed, BUR's traditional benefits continue to make it a popular roofing option. With ongoing improvement, it is expected to remain a system of choice for decades to come.