Coming to the forefront

Polymer-modified asphalt bitumen roof systems have been installed in the U.S. for more than three decades and matured into reliable, popular low-slope roof systems. These systems have evolved from two types of polymer-modified asphalts: APP polymers that give the material a "plasticized" characteristic and SBS polymers that give the material a "rubberized" quality. Manufacturers have developed hot-applied, cold-applied, torch-applied and even hot-air welded polymer-modified bitumen roof systems.

Following is information about an emerging fully adhered modified bitumen system technology—self-adhered modified bitumen.

Advancements

Self-adhering polymer-modified bitumen roof systems do not require liquid or viscous materials to activate adhesive properties and attach to a surface; the adhesive properties improve as the ambient temperature rises. However, several manufacturers' published guidelines also state maximum allowable temperature limits regarding installation.

Self-adhering polymer-modified bitumen roof systems for low-slope applications have evolved from the self-adhering polymer-modified bitumen "peel-and-stick" steep-slope underlayment and waterproofing sheets that have been used for 20 years. Advancements in polymer technology, including selective blending and better understanding of the variability of different asphalt sources and composition, have aided the development and use of self-adhering polymer-modified bitumen products.

Several roofing material manufacturers now offer multi-ply self-adhering polymer-modified bitumen roof systems. Other manufacturers offer hybrid polymer-modified bitumen roof systems, incorporating the use of conventional SBS- or APP-modified base sheets, interply sheets and/or granulated cap sheets used with self-adhering polymer-modified bitumen sheets.

A majority of manufacturers use SBS-modified bitumen as the primary bitumen modifier though one manufacturer, Polyglass USA Inc., Fernley, Nev., also is using APP-modified bitumen and TPO. Select resins and fillers, which are added to improve adhesive characteristics, complete the product formulations.

Self-adhering polymer-modified bitumen systems were refined and reintroduced into the low-slope U.S. roofing market by several roofing material manufacturers several years ago. ASTM D1970, "Standard Specification for Self-Adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection," currently is the only ASTM International standard that addresses these products. New ASTM standards are being developed to address self-adhering APP- and SBS-modified bitumen membranes used for low-slope applications.

Self-adhering sheets

As shown in the comparison chart, a majority of self-adhering polymer-modified bitumen interply sheets are reinforced with fiberglass mats though a few manufacturers use polyester reinforcement. The self-adhering polymer blend is factory-applied to one side or both sides of the reinforced and coated sheet. The use of single-sided self-adhering sheets and double-sided self-adhering sheets is dictated by a manufacturer's roof specification with regard to nailable and non-nailable substrates.

Release films also are incorporated into the manufacturing process of self-adhering sheets to prevent sticking and aid the installation process. Some single-sided self-adhering sheets may be surfaced on the opposite side with a plastic film, talc, sand or granules depending on the manufacturer and its intended use.

Cap and flashing sheets

Self-adhering polymer-modified bitumen cap sheets incorporate granule surfacing similar to granulated SBS- or APP-polymer-modified bitumen cap sheet products. The surfacing is necessary to protect the sheet from ultraviolet (UV) degradation and rooftop traffic. Several manufacturers offer granulated cap sheets with dedicated release films not only on the back sides of the sheets but also on the end-lap areas to facilitate and promote adhesion when joining end laps of two sheets.

Most manufacturers use a self-adhering polymer-modified bitumen cap sheet as a flashing sheet. These products generally are reinforced with polyester mats though a few manufacturers promote the use of fiberglass-reinforced cap sheets.

Guidelines

You should consider the following general installation guidelines when installing self-adhering polymer-modified roof systems. These guidelines are based on information contained within manufacturers' literature.

• Follow specific membrane manufacturers' instructions for product storage at the job site when ambient temperatures are expected to drop below the manufacturer's minimum application temperature.
• Self-adhering polymer-modified bitumen roofing products are susceptible to UV sunlight degradation, which adversely may affect the product's adhesive properties. Do not remove packaging prematurely, and follow specific manufacturer's instructions for exposure precautions.
• Follow specific membrane manufacturers' instructions regarding roof slope and drainage.
• Self-adhering polymer-modified bitumen roofing products require a substrate to be free of dust, dirt or foreign matter that could hinder proper adhesion. Follow specific manufacturers' instructions and guidelines regarding the use of primers, application rates and drying times.
• When installed over breathable structural substrates and attic spaces, self-adhering polymer-modified bitumen roofing products act as vapor retarders and adversely may affect ventilation and air movement. Carefully analyze roof ventilation, insulation and project-specific detailing for particular climatic conditions to prevent unwanted condensation and/or accumulated moisture.
• Because self-adhering polymer-modified bitumen roofing products are sensitive to ambient temperature and elevated heat, consider minimum and maximum allowable recommended application temperatures.
• Because self-adhering polymer-modified bitumen roofing products do not use liquids to activate adhesion between sheet and substrate surfaces, follow specific manufacturers' instructions and guidelines regarding the use of specially formulated adhesives and cements when joining side laps, end laps, T-joints or voids behind a sheet's surface.
• Follow specific membrane manufacturers' instructions regarding the use of heat guns as aids to adhere self-adhering polymer-modified bitumen roofing products to themselves or other acceptable surfaces and substrates when ambient temperatures fall below the manufacturer's minimum recommended application temperature range.

NRCA recommends checking with specific self-adhering polymer-modified bitumen roofing manufacturers for current fire- and wind-resistance testing and approvals of these products and systems where required or mandated by building code.

Pros and cons

There are several advantages of self-adhering polymer-modified bitumen roofing products. These products do not require the use of open flames to attach to themselves or a substrate. This allows the products to be used over combustible substrates, such as wood decks, or combustible wall substrates. These products also do not generate any appreciable hazardous or noxious fumes, allowing them to be used in poorly ventilated spaces or where rooftop air intake vents might draw odors into a building.

Despite these benefits, self-adhering polymer-modified bitumen roof systems have definitive application restrictions with regard to roof surface air temperatures. Every manufacturer listed in the figure cites specific minimum ambient air temperatures at the time of application. Several manufacturers also list specific maximum air temperatures. Low temperature application limits are necessary to achieve effective adhesion between the self-adhering sheet and substrate. Colder air temperatures can cause a sheet to become less pliable, which can affect the tenacity of the adhesive bond.

Adhesion of self-adhering modified bitumen sheets also is sensitive to the amount of pressure applied to a sheet to enhance a bond. A sheet's weight typically is not sufficient to achieve an effective bond between sheets or the sheet and its substrate. All manufacturers listed in the figure recommend self-adhering polymer-modified sheets be set with weighted rollers.

Extreme care should be taken when installing these products if roof surface temperatures exceed 100 F. Some products can become overly pliable and lose their rigidity. This can cause wrinkles and difficulty in sheet placement.

Another critical application issue is the condition of the substrate surface to receive the self-adhering polymer-modified bitumen sheet. The surface must be free of dust or dirt particles to achieve proper adhesion. Primer is recommended by manufacturers on properly prepared substrates, including poured and precast concrete decks.

One modified bitumen polymer supplier recommends a "tape test" be performed on the substrate before installing a self-adhering polymer-modified bitumen sheet. This test is performed by taking a length of masking or duct tape and applying it to the surface intended to receive the self-adhering polymer-modified bitumen sheet. Once the tape is applied and pressure adequately is applied to the surface, one simply pulls up the tape and notes the resistance force and voids (if any) on the adhesive side of the tape caused by dust or dirt particles.

Provide options

Self-adhering polymer-modified bitumen products could fill a void in specialized application scenarios, such as combustible roof areas and areas where open flames are not allowed or fumes or odors cannot be tolerated. The dependence of these products' application on clean site conditions and ambient air temperatures well above freezing could limit their use. However, they provide contractors with another option.

Click here to view a self-adhering modified bitumen product comparison chart.

Chuck Scislo is an NRCA senior director of technical services.