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Built up roof construction (BUR) is a time-tested roofing method that uses multiple asphalt-saturated felts or fiberglass mats layered with hot or cold adhesives to form a durable, weather-resistant membrane. This article explains the materials, design considerations, installation steps, maintenance, and common issues associated with BUR systems, and highlights best practices for performance and environmental considerations in the United States.

Overview Of Built Up Roof Systems

Built up roofs consist of alternating plies of reinforcing felt and bitumen, topped by a protective surface such as aggregate, gravel, or a coating. The number of plies typically ranges from three to five, depending on climate, building code requirements, and roof slope. The finished system provides waterproofing, UV resistance, and substantial long-term durability. Modern BUR installations often integrate a reflective or white cap sheet to improve energy efficiency in hot climates.

Materials And Layers

A BUR assembly follows a predictable stack from deck to surface. The key components include:

• Roof deck: A solid, structural substrate such as plywood, oriented strand board (OSB), or concrete.
• Waterproofing plies: Typically asphalt-saturated felt or fiberglass-mat felts, bonded with hot asphalt or with cold adhesives for cold-process installations.
• Bitumen: Either hot-applied asphalt, coal tar, or, in some modern systems, polymer-modified bitumen for enhanced durability and flexibility.
• Surfacing: Mineral granules, gravel ballast, or reflective coatings to protect the roof and influence energy performance.
• Adhesives and fasteners: Used to bond plies and secure ballast or surface coatings, depending on the design.

Different BUR configurations include:

• Hot-built BUR: Felt plies are mopped with hot asphalt, forming a cohesive, solvent-resistant membrane.
• Cold-applied BUR: Solvent-free adhesives bond plies, reducing fumes and improving worker safety during application.
• Granite or gravel-ballasted BUR: A ballast layer protects the surface and adds weight for wind uplift resistance.

Design And Performance Considerations

Designing a BUR system involves climate, slope, and building use. Key considerations include:

• Slope: BUR performs well on slopes typical for commercial roofs; low-slope roofs (<2:12) are most common.
• Wind uplift: Ballast or mechanical fasteners provide resistance to wind uplift, especially at edges and penetrations.
• Thermal performance: Reflective cap sheets or cool coatings can reduce cooling loads in hot climates.
• UV exposure: Surfacing materials protect the bitumen from sun damage; high-quality granules extend membrane life.
• Fire resistance: Some BUR systems incorporate mineral granules or ceramic cap sheets to improve fire performance.

Installation Process

The installation of a BUR system follows a sequence that ensures proper adhesion and performance. Typical steps include:

1. Deck preparation: Clean, repair, and ensure a smooth, dry surface free of voids and moisture.
2. Base plies: Apply the first ply with hot asphalt or cold adhesive, ensuring full embedment and no air pockets.
3. Additional plies: Add each ply in sequence, typically alternating felt direction for balanced bonding, and maintain appropriate temperatures during hot applications.
4. Interply bonding: Ensure uniform laydown and overlap to prevent blistering or delamination.
5. Surfacing: Apply ballast, granules, or reflective coating as specified to protect the membrane and meet energy targets.
6. Flashing and detailing: Install edge details, penetrations, curbs, and vents with compatible BUR materials to maintain waterproof integrity.

Maintenance And Longevity

BUR systems offer long service life when properly maintained. Maintenance activities include inspections after severe weather, cleaning drains, repairing blisters or splits promptly, and re-coating or re-surfacing when the surface shows wear. Typical lifespans range from 15 to 30 years depending on climate, foot traffic, and maintenance. Regular inspections identify aging asphalt, weakened plies, or ballast displacement that could compromise performance.

Advantages And Disadvantages

Understanding the trade-offs helps in selecting BUR for a project.

• Advantages: Excellent waterproofing, proven track record, durable under heavy foot traffic, and effective for flat or low-slope roofs.
• Disadvantages: Heavier system with more labor-intensive installation, longer cure times for hot-applied versions, and potential environmental concerns related to asphalt VOCs during installation.

Environmental Considerations And Energy Performance

Modern BUR systems can improve energy efficiency and reduce environmental impact through:

• Reflective surfacing: White or light-colored cap sheets reduce heat absorption and cooling loads.
• Recycled materials: Some manufacturers incorporate recycled asphalt or fibers in plies to reduce waste.
• VOC management: Cold-applied or low-VOC bitumen options minimize emissions during installation.

Common Issues And Troubleshooting

Common BUR problems include:

• Blistering: Caused by entrapped air or moisture; addressed by venting and resealing affected areas.
• Delamination: Poor interply bonding due to moisture, improper adhesion, or aging; requires patching or ply replacement.
• Cracking and Shrinking: Result of temperature fluctuations and aging; may necessitate resurfacing or additional protective coatings.
• Ballast displacement: Wind uplift concerns; ensure ballast is properly secured and edges are sealed.

Code, Standards And Best Practices

BUR installations should comply with local building codes, manufacturer specifications, and industry standards. In the United States, project teams often reference:

• ASTM Standards: Durable roofing materials and testing methods for bituminous membranes.
• NRCA Guidelines: National Roofing Contractors Association recommendations on installation quality and safety.
• FM Global and Underwriters Laboratories: Standards related to fire performance, wind uplift, and low-slope roofing.

Selecting A BUR System For A Project

Choosing the right BUR configuration involves collaboration among architect, contractor, and building owner. Consider climate, roof slope, intended energy performance, ballast preferences, and maintenance plans. Evaluate different ply counts, bitumen types, and surface finishes. Request detailed specifications, warranties, and long-term service expectations from manufacturers and installers to ensure compatibility with existing structures and local codes.