Modified Bitumen Roofing - SBS and APP Systems

Modified bitumen remains the right system for Atlanta commercial buildings where slope, substrate, or existing system conditions make it the logical choice - and for repairs on the large existing mod-bit inventory across the metro's industrial corridors and older commercial stock.

Modified bitumen has been in Atlanta commercial roofing service since the early 1980s. It sits between traditional built-up roofing and single-ply membranes in system architecture: factory-manufactured asphalt sheets reinforced with polyester or fiberglass, modified with either SBS (styrene-butadiene-styrene) rubber for flexibility or APP (atactic polypropylene) plastic for high-temperature performance. In Atlanta, modified bitumen has a large installed base - most of the industrial and warehouse building inventory from the 1980s and 1990s across Marietta, Gwinnett, and the South Fulton industrial corridors carried modified bitumen at original construction.

Atlanta's climate imposes specific performance requirements on modified bitumen systems. SBS-modified bitumen performs better in Atlanta's thermal cycling than APP - SBS rubber modification maintains flexibility at the low end of Atlanta's winter temperature range, while APP formulations can become brittle in extended cold snaps below 20 degrees Fahrenheit, conditions that occur in Atlanta roughly one to two weeks per year. For new installation in Atlanta, we default to SBS unless there is a specific reason to specify APP.

The Atlanta modified bitumen market is currently more repair-and-extend than new installation. Most of the active reroof cycle on the metro's aging commercial building stock is moving to single-ply TPO or EPDM - not because modified bitumen is an inferior system, but because single-ply has lower installed cost per square and longer manufacturer warranty terms. Modified bitumen SBS with a granulated cap sheet carries a 10 to 15-year manufacturer warranty; TPO carries 20 years. For owners optimizing capital allocation, the single-ply path usually wins on a lifecycle cost basis. Where we continue to specify modified bitumen for new installation is on buildings where slope, substrate conditions, or existing system compatibility makes it the most practical choice.

SBS modified bitumen: The rubber-modified formulation that remains flexible across Atlanta's temperature range - from summer peaks above 95 degrees Fahrenheit to winter cold snaps that occasionally drop below 20. SBS systems are installed by cold-adhesive, heat welding, or hot-mopped application. The cold-adhesive method is the most common current installation approach because it eliminates open-flame hazards. SBS cap sheet granules provide UV protection and improve impact resistance against Atlanta's frequent hail events, which are common in the spring severe weather season across North Georgia.

APP modified bitumen: The plastic-modified formulation with higher heat resistance but less cold-temperature flexibility. APP is installed primarily by torch application - the torch-applied installation method that is the source of the 'torch down' roof term. Atlanta's summer surface temperatures make APP's high-heat resistance appealing in theory, but in practice the cold-weather brittleness risk and the open-flame installation requirement make SBS the preferred specification for most Atlanta applications. We install APP where the building's owner or architect specifically requires it and the building's permit authority accepts torch application.

Granulated cap sheet vs. smooth cap sheet: Granulated cap sheet is the standard for Atlanta commercial work - the mineral granule surface provides UV protection, impact resistance, and improved fire classification. Smooth cap sheet is specified under a coating system - silicone or aluminum - where the smooth surface is required for coating adhesion. Most of Atlanta's existing smooth cap sheet inventory has had one or more coating applications and is approaching the end of the substrate's ability to support additional coating layers.