Modified Bitumen Roofing Systems

Modified bitumen bridges Atlanta's built-up roofing history and modern single-ply technology. SBS and APP systems suit low-slope roofs with irregular geometry, high-penetration density, and existing BUR substrates where a single-ply recover is not practical.

Modified bitumen roofing - SBS (styrene-butadiene-styrene) and APP (atactic polypropylene) - emerged in the 1980s as a performance upgrade over traditional built-up roofing. In Atlanta, it filled the gap between aging BUR systems and the single-ply revolution: easier to detail around complex penetration patterns than single-ply, more durable than straight asphalt BUR, and available in torch-applied, heat-weld, and cold-process installation methods. A significant portion of Atlanta's 1990s and 2000s commercial building stock carries modified bitumen systems - mid-rise office buildings, strip retail, medical office buildings, schools, and institutional facilities.

SBS-modified bitumen remains relevant in Atlanta's current roofing market in specific applications: low-slope roofs with high penetration density that would require extensive custom detailing in single-ply, recover projects over existing BUR where the modified bitumen over-BUR stack is well-established by the existing system type, and buildings where the slope and geometry make the controlled lap installation of modified bitumen easier to execute to a high quality standard than single-ply seaming.

APP-modified bitumen, typically torch-applied, is also used in Atlanta but requires hot-work permit compliance - a requirement that adds pre-construction administration in the City of Atlanta and most Cobb and DeKalb County jurisdictions. Cold-process SBS eliminates the hot-work permit requirement and is our default modified bitumen specification where the application does not specifically require a torch-applied APP system.

SBS-modified bitumen is more flexible at low temperatures and tolerates Atlanta's thermal cycling range well. It is typically applied with cold-process adhesive, heat welding, or self-adhering systems, all of which eliminate the hot-work permit requirement that torch-applied APP requires. For Atlanta projects in the City of Atlanta, Cobb County, or DeKalb County - where hot-work permits require advance application and on-site fire watch - cold-process SBS is typically the lower-administrative-burden specification for modified bitumen projects.

APP-modified bitumen, torch-applied, produces a very robust lap seam when installed correctly and is the preferred specification for exposed (granule-surfaced) walkway and traffic applications. The torch application produces a direct-bond lap joint that some installers find more reliable to execute than cold-process adhesive seams in Atlanta's high-humidity summer installation window. The tradeoff is the hot-work permit, fire watch requirement, and the additional pre-construction administration those requirements impose.

Both SBS and APP systems carry 10 to 15-year manufacturer warranty paths - shorter than the 20-year NDL paths available for TPO and EPDM. For buildings where modified bitumen is the right system choice, we are explicit about this warranty term difference in the scope recommendation.