System Overview
Modified bitumen roofing combines the proven waterproofing properties of asphalt with modern polymer modifiers (SBS - Styrene-Butadiene-Styrene) to create a durable, flexible roofing membrane. These multi-ply systems are popular for small commercial buildings, additions, and retrofit applications.
Unlike single-ply membranes that come in large sheets, modified bitumen is installed in overlapping layers - typically a base sheet followed by one or more cap sheets. The redundancy of multiple layers provides excellent waterproofing and longevity.
Safety Note: Torch-applied systems involve open flame and require proper safety training, hot work permits, and fire watch procedures. Always check local code requirements and insurance restrictions.
Application Methods
Torch-Applied (Heat Welding)
The most common application method. Propane torch melts the bottom surface of the membrane, bonding it to the substrate or previous layer as it is rolled out.
- • Strongest bond to substrate
- • Immediate seal - no waiting for adhesive
- • Works in cold weather
- • Visual confirmation of proper heating
- • Fire hazard - hot work permits required
- • Cannot use on wood decks (most codes)
- • Weather dependent (wind, rain)
- • Requires skilled, certified crews
Self-Adhered (Peel & Stick)
Membrane with factory-applied adhesive backing protected by release paper. No heat required - simply peel and press into place.
- • No fire risk - safer for occupied buildings
- • Can use on wood decks
- • Cleaner installation
- • Less specialized labor required
- • Higher material cost
- • Temperature sensitive (50°F+ required)
- • Substrate must be perfectly clean/dry
- • Difficult to reposition
Cold-Applied Adhesive
Trowel or roll liquid adhesive onto substrate, then roll membrane into wet adhesive. Eliminates torch but requires proper cure time.
Occupied buildings where hot work is prohibited, wood decks, or buildings with extensive combustible materials nearby.
Key Materials & Components
1. Base Sheet
First layer applied to roof deck. Provides foundation for cap sheet and redundant waterproofing.
Smooth or granulated surface, fiberglass mat
36 inch wide × 65 ft (1.5 squares per roll)
$30-50 per square
1:1 with 3-6 inch side laps
2. Cap Sheet (Finish Layer)
Final layer with weather-resistant granulated surface. Available in various colors for aesthetics and solar reflectance.
Granulated (most common), mineral surface, reflective coating
36 inch wide × 33.5 ft (1 square per roll)
$50-90 per square
White (reflective), tan, grey, black
3. Primer/Adhesive
Surface preparation for smooth substrates or cold-applied systems.
4. Flashing Materials
Modified bitumen or metal flashing for walls, penetrations, and terminations.
5. Insulation (Optional but Common)
Rigid insulation boards installed before base sheet for energy efficiency.
Estimation Considerations
System Configuration
Two-ply systems (base + cap) are standard. Three-ply systems add a ply sheet between base and cap for enhanced performance.
Application Method
Torch-applied is most common but local codes may restrict. Always verify fire department and insurance requirements.
Existing Roof Condition
Modified bitumen can often be installed over existing smooth-surfaced roofs with proper preparation.
Project Size
Modified bitumen is economical for smaller roofs (under 10,000 SF). Large roofs often favor single-ply for speed.
Material Takeoff Details
Step-by-Step Takeoff Process
Measure total roof area in squares (100 SF each). Low-slope roofs use actual area.
Base sheet has 3-6 inch side laps and 6 inch end laps. Add 8-10% for laps.
Cap sheet also requires laps. Typically 6 inch side laps, 6 inch end laps.
Calculate linear feet of walls/curbs and multiply by height (typically 8-12 inches).
If using primer or cold adhesive, calculate gallons based on coverage rate.
Budget propane fuel for torching. Varies by crew and conditions.
Labor Estimation Guidance
Baseline Productivity Rates
| Task | Hours/Square | Crew Size | Notes |
|---|---|---|---|
| Tearoff existing roof | 1.5-2.5 | 3-4 | BUR slower than single-ply |
| Insulation install | 0.8-1.2 | 2-3 | If specified |
| Base sheet - torch | 1.5-2.0 | 3 | 2 torchers + 1 helper |
| Cap sheet - torch | 1.5-2.5 | 3 | Requires more precision |
| Self-adhered install | 2.0-3.0 | 3 | Per layer |
| Flashing details | Variable | 2 | $10-18 per LF installed |
Example Labor Calculation
Key Takeaways
- ✓Two-ply systems (base + cap) are the standard configuration
- ✓Torch-applied is fastest but requires permits and certified crews
- ✓Self-adhered eliminates fire risk but costs more in materials
- ✓Most economical for small to medium roofs (under 15,000 SF)
- ✓Add 10% waste factor for laps and overlaps
Get the Complete Modified Bitumen Estimating Template
Excel template with 2-ply and 3-ply system calculators, torch vs. self-adhered cost comparison, lap calculations, primer/adhesive estimators, and complete labor breakdowns.
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