Terminology

• Ply: one layer of felt; three- and four-ply systems common
• Base sheet: first heavier ply, nailed or adhered to the substrate
• Interply mopping: hot asphalt between plies
• Flood coat: heavy top coat of asphalt or coal tar that holds aggregate
• Aggregate surfacing: gravel or slag embedded in the flood coat for UV and fire protection
• Smooth-surfaced BUR: coated with emulsion or reflective coating instead of gravel
• Cap sheet BUR / hybrid: BUR plies with a mineral-surfaced or modified bitumen cap
• Coal tar pitch: dead-level-capable bitumen with self-healing properties; strong fumes
• Asphalt types: ASTM D312 Types I to IV by softening point; higher types for steeper slopes and hot climates
• Kettle / tanker: equipment that heats asphalt on site
• Equiviscous temperature (EVT): application temperature for proper bonding

Standards

Ratings

• Ply count, felt type, and surfacing set durability and fire rating
• Gravel-surfaced four-ply systems are known for damage resistance
• Wind uplift and fire ratings by tested assembly

Test standards

• ASTM D312: asphalt for roofing
• ASTM D1863: mineral aggregate for surfacing
• ASTM D4990: coal tar glass felt
• ASTM D6083: acrylic coatings
• FM 4474, UL 580, UL 1897: wind uplift
• ASTM E108 / UL 790: fire

Materials

• Glass-fiber felts: standard ply sheets
• Asphalt or coal tar pitch: bitumen; coal tar for very low slopes
• Gravel or slag: aggregate surfacing
• Reflective coatings and white cap sheets: cool roof options
• Insulation: perlite, fiberglass, or polyiso with cover board compatible with hot asphalt

Detailing

• Check structure for gravel and multi-ply dead load
• Flash walls and curbs with base flashing and counterflashing; avoid pitch pockets where possible
• Keep aggregate out of gutters and drains with gravel stops and strainers
• Use cover boards that tolerate hot asphalt
• Limit kettle fumes near air intakes on occupied buildings

Uses

Specific uses

• Large commercial, institutional, and government low-slope roofs
• Roofs with heavy foot traffic or equipment
• Hybrid systems with modified bitumen caps
• Very low-slope roofs using coal tar
• Long-life roofs where redundancy is valued

Pros and cons

Pros

• Multi-ply redundancy and long service record
• Gravel surface protects from UV, fire, hail, and traffic
• Easy to inspect and repair layer by layer
• Wide contractor familiarity in some regions

Cons

• Heavy, especially gravel-surfaced
• Hot asphalt fumes, odor, and burn hazards
• Labor-intensive and weather-dependent
• Gravel makes leak finding and reroofing harder

Manufacturing and cost

Construction

• Asphalt heated in kettles; felts rolled into mopped asphalt ply by ply
• Flood coat and aggregate applied; flashings installed with felts and cap sheets

Main manufacturers and suppliers

• Johns Manville
• GAF
• Garland
• Tremco Roofing
• Siplast

Trade associations: National Roofing Contractors Association and Asphalt Roofing Manufacturers Association.

Typical costs

• Built-up roof, installed: $5 to $22 per sq ft depending on plies, surfacing, and insulation

Sources

• Roofing Contractor: modified bitumen and BUR damage resistance, https://roofingcontractor.com/articles/84061-modified-bitumen-1
• UpCodes: 2024 IBC Chapter 15, Table 1507.10.2 BUR material standards, Chapter 15 Roof Assemblies and Rooftop Structures: Department of Defense Building Code 2024 | UpCodes
• Johns Manville: ER10167-03 evaluation report (07 51 13), https://www.jm.com/content/dam/jm/global/en/commercial-roofing/Codes-Approvals/ER10167-03 SBS.pdf
• Terrapin Construction Group: commercial roofing cost per square foot, 2026, https://terrapincg.com/news/commercial-roofing-cost-per-square-foot-2026
• Roof Observations: roof cost guide, https://roofobservations.com/roof-cost-guide-general-roof-prices/
• Wikimedia Commons: Flat roof bitumen felt.jpg, by Htm (CC BY-SA 3.0), https://commons.wikimedia.org/wiki/File:Flat_roof_bitumen_felt.jpg