Modified Bitumen
San Jose & the South Bay
Modified bitumen — 'mod-bit' to the trade — is an asphalt-based roll roofing modified with polymers for flexibility, durability, and weather resistance.
Modified Bitumen
Done Right Since 1952
Modified bitumen — 'mod-bit' to the trade — is an asphalt-based roll roofing modified with polymers for flexibility, durability, and weather resistance. Installed torch-on (heat-welded) or self-adhered, it's a workhorse commercial flat-roof system that's been standard for 40+ years. Keith Roofing has been installing mod-bit since it became widely available and we're certified installers for GAF, CertainTeed, Firestone, and Johns Manville mod-bit systems.
Mod-bit comes in two primary formulations. SBS (styrene-butadiene-styrene) is rubber-modified asphalt — flexible in cold temperatures, excellent recovery from deformation, typically self-adhered or hot-mopped. APP (atactic polypropylene) is plastic-modified asphalt — higher heat resistance, typically torch-applied. Both have 20-30+ year service lives with proper installation, both handle Bay Area climate well, and each has applications where it's the better choice.
Our mod-bit work includes: new mod-bit installation on new commercial construction; full replacement of aging BUR or other systems with mod-bit; mod-bit on low-slope residential applications (particularly Eichler homes and modern flat-roof residential); mod-bit repair and restoration work; and coordination with property managers and building owners on commercial portfolios.
Understanding Modified Bitumen Systems
SBS vs. APP: Choosing the Right Formulation
SBS mod-bit (styrene-butadiene-styrene) is the choice for most Bay Area commercial and residential flat-roof applications. SBS remains flexible across temperature ranges — doesn't stiffen in cold weather or soften excessively in summer heat. Typical application methods: self-adhered (peel-and-stick) or hot-mopped (traditional hot asphalt between plies). Self-adhered SBS is particularly popular for Eichler and residential low-slope applications where torch work isn't practical.
APP mod-bit (atactic polypropylene) is typically torch-applied — open-flame heating of the underside of the membrane as it's rolled into place. APP is more heat-resistant than SBS and preferred for some high-heat applications. Torch application requires specific safety protocols, trained operators, and insurance coverage for open-flame work.
Installation Methods
Torch-applied installation: traditional method for APP mod-bit. Propane torch heats the underside of the membrane as it's rolled into place, melting the modified bitumen into adhesive bond with the substrate. Requires trained operators, fire safety protocols, and site hot-work permits in many jurisdictions. Delivers strong bond but safety concerns mean many projects prefer alternatives.
Hot-mopped installation: traditional method using hot asphalt between plies (similar to BUR installation). Requires specialized equipment (kettles, asphalt transport) and trained crews. Delivers strong bond but declining in use due to equipment complexity.
Self-adhered installation: peel-and-stick membrane with factory-applied adhesive on underside. Significantly safer and simpler than torch or hot-mop methods. Typical choice for residential applications and increasingly common for commercial. SBS modifier works particularly well with self-adhered technology.
Cold-applied installation: liquid-applied cold adhesive between plies. Safer than torch or hot-mop but requires specific adhesive products and careful temperature-controlled application.
Common Applications
Mod-bit works well for: mid-size commercial buildings where a proven traditional system is preferred over newer single-ply; Eichler and mid-century modern residential flat/low-slope roofs (self-adhered SBS is particularly common); low-slope residential additions and modern flat-roof architecture; replacement of failing BUR systems where a similar traditional system is preferred; and high-heat applications favoring APP mod-bit.
Signs You Need
Modified Bitumen
Existing BUR System at End-of-Life
Aging BUR systems are commonly replaced with mod-bit for commercial applications. Similar installation approach, proven performance, and familiar maintenance.
Eichler Flat-Roof Re-Roof
Self-adhered SBS mod-bit is particularly well-suited to Eichler low-slope roof geometry. Avoids torch safety concerns on residential property.
Mid-Century Modern Residential Flat Roof
Mid-century architectural homes with low-slope or flat roof sections benefit from mod-bit durability and compatibility with the architectural style.
Modern Residential Flat-Roof Addition
New flat-roof residential architecture (modern and contemporary homes) works well with mod-bit — typically self-adhered for safety.
Commercial Project Preferring Traditional Systems
Some property owners prefer traditional systems over newer single-ply for perceived long-term reliability.
High-Wear or High-Heat Applications
Restaurant rooftops, industrial applications, or other high-wear environments can benefit from mod-bit's durability. APP formulation preferred for high-heat applications.
Modified Bitumen
Options
GAF Mod-Bit Systems
Complete mod-bit product line including SBS and APP formulations, various base and cap sheets, self-adhered options.
CertainTeed Mod-Bit
Alternative mod-bit manufacturer with strong commercial and residential product lines.
Firestone Mod-Bit
Holcim-owned mod-bit systems with good Bay Area distribution.
Johns Manville Mod-Bit
Mod-bit products with solid Bay Area service network and competitive pricing.
Base Sheets
Modified asphalt base sheet material, mechanically-fastened or adhered foundation for mod-bit system.
Cap Sheets with Granule Surfacing
Mineral-surfaced cap sheets provide UV protection and aesthetic finish. Various colors available.
How We Handle
Your Modified Bitumen
Existing Roof Assessment
For replacement projects, evaluation of existing roof condition, substrate integrity, drainage, and whether overlay or tear-off is appropriate.
System Specification
SBS vs. APP selection, application method (torch, hot-mop, self-adhered, or cold-applied), base sheet specification, and cap sheet selection.
Permit & Scheduling
Building permit, Title 24 compliance documentation. For torch-applied work, hot-work permit and safety protocols.
Installation
Tear-off or overlay preparation. Insulation installation where required. Base sheet, ply sheet installation via specified method. Cap sheet installation with granular surfacing.
Final Inspection & Warranty
Seam integrity verification. Flashing detail review. Manufacturer warranty registration and documentation package.
Why Homeowners Choose Us
for Modified Bitumen
All Mod-Bit Installation Methods
Torch-applied, hot-mopped, self-adhered, and cold-applied — we do all installation methods and recommend based on project specifics, not what's easiest for us.
Safety Culture for Torch Work
Torch applications require rigorous safety protocols. We maintain training, equipment, and insurance coverage for torch work and have excellent safety track record.
Eichler & Mid-Century Specialty
Significant portion of our residential mod-bit work is Eichler and mid-century modern homes — we know the architectural details and proper installation approach.
40+ Years of Mod-Bit Experience
Installed mod-bit since the material was introduced commercially. Pattern recognition on what works long-term vs. what fails prematurely.
Recent Modified Bitumen
Projects
Projects Featuring
Our Modified Bitumen Service
Real Keith Roofing projects where we delivered this service. Click any case study for the full story.
San Jose Commercial Maintenance
Ongoing commercial flat-roof maintenance with solar panel coordination, single-ply install, and active leak repair.
View Case Study →
San Jose Apartment Re-Roof
Multi-unit apartment re-roof — Landmark shingle on pitched wings, single-ply on flat carports, full gutter replacement.
View Case Study →Crown Blvd Single-Ply Commercial
Commercial single-ply roof install on a low-slope building along Crown Boulevard.
View Case Study →