Modified bitumen is the evolution of built-up roofing - a multi-ply bitumen system reinforced with polyester or fiberglass mats that performs in the Triangle's humidity and temperature extremes. We install, recover, and repair modified bitumen systems on commercial buildings across the Raleigh-Durham market.
Modified bitumen roofing bridges the performance gap between traditional built-up roofing (BUR) and the single-ply membrane category. It is a factory-manufactured sheet product - either SBS (styrene-butadiene-styrene) rubber-modified or APP (atactic polypropylene) plastic-modified - that delivers the multi-ply redundancy of BUR with more consistent quality control than field-applied hot asphalt. For commercial buildings in the Triangle that are recovering from BUR systems or that need a roof with specific dimensional stability and redundancy requirements, modified bitumen is often the right specification.
Raleigh commercial buildings with original BUR systems - particularly the mid-rise office and institutional buildings constructed along Fayetteville Street and in the Downtown core between the 1960s and 1990s - are common recover candidates for modified bitumen. The existing gravel-surfaced BUR provides an excellent substrate for a modified bitumen recover when moisture core sampling confirms that the insulation is sound.
Modified bitumen's performance in the Triangle is well-established. SBS systems handle the Triangle's temperature flexibility requirements better than APP - SBS remains flexible at January ice storm temperatures where APP can become brittle. For Raleigh buildings that face the full range of the Piedmont climate, from summer surface temps above 150F to winter ice events, SBS modified bitumen is our standard specification in this system category.
SBS vs. APP Modified Bitumen
SBS-modified bitumen is rubberized asphalt. The SBS polymer gives the membrane elasticity - it can stretch and recover, which is what allows it to accommodate the thermal cycling between a July Raleigh rooftop and a January ice event without cracking. SBS systems are typically installed with cold-applied adhesive or with hot asphalt mopping, both of which avoid the open-flame application risk of torch-applied systems. We specify SBS for most Raleigh commercial buildings and for all buildings where open-flame torch application is prohibited by the building's fire suppression system or occupancy requirements.
APP-modified bitumen is plasticized asphalt - stiffer than SBS, better UV resistance, and typically torch-applied. APP systems perform better under sustained UV exposure and are appropriate for buildings in south-facing exposures where solar gain is a primary aging factor. The downside in the Triangle climate is APP's reduced flexibility at low temperatures: in January 2022's ice event, APP systems that were already in their second decade showed more thermal cracking at lap edges than comparably aged SBS systems. For new installations, we specify SBS as the standard.
The cap sheet - the top surface layer - on modified bitumen systems is either granule-surfaced for UV protection or smooth-surfaced for coating applications. Most Raleigh commercial buildings receive granule-surfaced cap sheets. Buildings that are candidates for future coating restoration within 5-10 years receive smooth cap sheets, which provide better adhesion for silicone or acrylic coating systems.
Recovering BUR Systems in Downtown Raleigh
Downtown Raleigh's commercial office inventory includes a significant stock of buildings with original gravel-surfaced BUR systems - some installed in the 1970s and long past their original service life, maintained through periodic flood coating and repair. These buildings are the primary recover market for modified bitumen in the Triangle.
BUR-to-modified-bitumen recover works well when moisture core sampling confirms that the original insulation is sound - less than 15% saturation - and the BUR surface is stable and sound. The gravel ballast on a BUR system must either be removed (preferred) or left in place as a substrate if the structure can carry the additional weight. We evaluate the structural loading question as part of every BUR recover scope.
The crane and material logistics for Downtown Raleigh modified bitumen projects follow the same constraints we described for replacement work: City of Raleigh permitting for lane closures, early-morning delivery scheduling on Fayetteville Street and adjacent streets, and coordination with the building's facility manager on access and staging. Modified bitumen recover projects on occupied Downtown office towers are sequenced to keep the building dry-in secure at end of every production day.
Modified Bitumen and Hurricane Remnant Events
Multi-ply modified bitumen systems perform well under sustained water loading - the redundancy built into the system means that a single-ply failure does not immediately result in interior penetration, as it might on a single-ply TPO or EPDM system. This redundancy made modified bitumen a popular specification for Raleigh healthcare buildings and institutional facilities where interior water intrusion is a high-consequence event.
Hurricane Florence's remnant rainfall in 2018 exposed the vulnerability of modified bitumen systems that were past their designed service life and had suffered lap seam fatigue. When the seams failed under three days of sustained water loading, water wicked under the cap sheet and into the base sheet layer, saturating insulation from the top before showing as an interior ceiling stain. We inspected numerous modified bitumen roofs post-Florence and found insulation saturation levels that the exterior condition had not suggested.
The lesson from Florence and Helene for modified bitumen building owners: a visual inspection is not sufficient for aging modified bitumen systems. Moisture core sampling that goes all the way to the deck is required to determine actual insulation condition. We perform moisture surveys as the first step in any modified bitumen scope development, regardless of what the surface inspection suggests.
Frequently asked questions
How long does modified bitumen roofing last in Raleigh?
A properly installed two-ply SBS modified bitumen system carries a 15-20 year manufacturer warranty and typically performs 15-25 years with semi-annual maintenance. The limiting factor in the Triangle climate is generally cap sheet granule loss - once granules have worn past the point where they protect the asphalt from UV, the cap sheet degrades rapidly. Most maintenance programs include an annual granule count assessment to track this degradation and forecast the remaining service life.
Can modified bitumen be installed without open flame (torch)?
Yes. SBS modified bitumen is available in cold-applied and hot-mop versions that do not require open flame. Many occupied commercial buildings in Raleigh - healthcare facilities, buildings with combustible interior materials, and buildings where fire watch requirements make torch application impractical - specify cold-applied SBS as a standard requirement. We install cold-applied SBS systems using manufacturer-approved cold adhesive to the same seaming and detailing standards as torch-applied systems.
Is modified bitumen a good choice for a Raleigh building being recovered from BUR?
Often yes, particularly if the BUR system's insulation is confirmed sound by moisture core sampling. Modified bitumen recover over sound BUR substrate is a well-established scope that produces a warranted multi-ply system at roughly 50-60% of full replacement cost. The key qualification is that the BUR surface must be stable - we walk and probe the surface, document the condition, and confirm structural loading calculations for the recover weight before recommending this path. If the BUR is beyond recover candidacy, we will say so and present the full replacement scope.
