Built Up Roof Construction for Flat Roof Systems

Built up roof construction (BUR) is a long-standing solution for flat and low-slope roofs, combining multiple asphalt or coal tar layers with reinforcing felts and a protective cap layer. This method offers robust waterproofing, durability, and excellent resistance to weathering when properly installed. The following guide explains BUR components, variants, installation steps, performance considerations, and maintenance best practices to help building owners, designers, and contractors select and manage this traditional roofing system effectively.

What Is Built Up Roof Construction

Built up roofs are composed of alternating layers of bituminous materials and reinforcing fabrics, terminated with a protective top layer. The classic BUR stack includes a ply or ply sheet, asphalt or coal tar bitumen, and a surfacing layer such as gravel, reflective coating, or cap sheets. The “built up” phrase stems from stacking these materials into a single roof assembly. The result is a watertight, self-flashing system that tolerates foot traffic, temperature swings, and ponding conditions common on flat roofs.

Key Components Of BUR Roofs

Understanding each layer clarifies how BUR achieves durability and waterproofing. The main components are:

• Payload Deck: The structural substrate, usually wood or lightweight concrete, that supports the BUR system.
• Ply Sheets: Fabric felts or fiberglass mats that provide tensile strength and reinforcement for the bitumen layers.
• Bituminous Apply: The hot or cold asphalt, asphaltic mastic, or coal tar pitch applied between plies to bond the assembly.
• Surfacing Layer: The cap layer that protects the roof from UV exposure and physical damage; options include gravel, mineral granules, or a reflective coating.
• Waterproofing Integrity: Each ply and the bond between plies contribute to the overall waterproof performance and redundancy.

Common BUR Variants

Different BUR formulations cater to climate, budget, and performance goals. Notable variants include:

• Asphalt BUR: The most common variant, using asphalt as the bitumen and reinforcing plies.
• Coal Tar BUR: An older option with excellent chemical resistance and longevity but limited use in some jurisdictions due to health concerns and odor.
• Modified Bitumen BUR (MBR): An evolution that blends traditional BUR with polymer-modified bitumen for enhanced elasticity and hail resistance.
• Gravel-Surfaced BUR: Gravel ballast adds UV protection, ballast weight, and additional thermal mass.

Design Considerations For BUR Roofs

Effective BUR design balances waterproofing, energy performance, and long-term maintenance. Key considerations include:

• Insulation Integration: Roof insulation should be installed beneath or above the deck, with attention to thermal bridging and vapor diffusion.
• Ventilation: Proper attic or roof deck ventilation helps control moisture and reduces condensation risk in unvented assemblies.
• Flashing Details: Mechanical penetrations, curbs, and parapets require meticulous flashing and detailing to prevent leaks.
• Drainage: BUR relies on slope and drainage design to prevent ponding, which can degrade the cap sheet and asphalt.
• UV Exposure: Cap sheets and surfacing materials must resist sun exposure to extend service life.

Installation Process And Best Practices

Proper BUR installation involves a sequence of preparation, laying, detailing, and curing. General steps include:

1. Deck Preparation: Repair any surface irregularities, clean debris, and install a suitable vapor barrier if required by the climate and code.
2. Base Ply Application: Apply the first layer of bitumen and embed the first ply sheet, ensuring full adhesion and alignment.
3. Practical Ply Stacking: Interleave additional plies with adhesive and maintain consistent coverage to prevent air pockets.
4. Bitumen Bonding: Apply successive coats of bitumen between plies, creating a continuous seal and redundancy.
5. Surfacing Layer: Install gravel ballast or a cap sheet according to the selected BUR variant, ensuring proper protection against UV and impact.
6. Detailing: Flash around penetrations, curbs, and edges with compatible materials to maintain continuity of waterproofing.
7. Quality Assurance: Inspect for gaps, blistering, or misalignment and perform repairs promptly.

Maintenance And Longevity

BUR roofs are known for durability, but proper maintenance extends service life. Frequency and tasks typically include:

• Regular Inspections: Visual checks for blisters, cracks, ponding, and delamination after weather events.
• Flashing and Penetrations: Periodic inspection of sealants around vents, skylights, and HVAC equipment; re-seal as needed.
• Surfacing Layer Care: If gravel ballast is used, ensure adequate covering and replace lost ballast to protect cap sheets.
• Cleaning: Remove debris to prevent moisture retention and algae or mold growth on the surface.
• Repair Strategies: Address blisters or cracks promptly with patching and resealing to prevent leaks.

Advantages And Drawbacks

The BUR roofing system offers several benefits and some limitations:

• Advantages: Excellent waterproofing redundancy, proven long-term performance, good impact resistance, and compatibility with extensive insulation strategies.
• Limitations: Heavier roof assembly, relative labor intensity during installation, potential odor from coal tar variants, and periodic maintenance requirements.

Code Compliance, Sustainability, And Safety

Code compliance ensures BUR installations meet fire, energy, and structural standards. Considerations include:

• Fire Ratings: BUR systems typically achieve high fire ratings when installed with proper flashing and ignition barriers.
• Environmental Impact: Modern asphalt and MBT formulations aim to reduce volatile organic compounds and improve recycling potential of asphalt-based components.
• Worker Safety: Installation involves hot asphalt work; proper PPE, ventilation, and fire safety protocols are essential.
• Recycling And Disposal: Reuse of asphalt materials and proper disposal of coal tar components should follow local regulations.

Choosing BUR For Your Project

Selecting BUR depends on climate, roof size, load capacity, and maintenance plans. Consider:

• Climate Compatibility: Regions with heavy rainfall, hail, or UV exposure can benefit from reinforced and capped BUR configurations.
• Insulation Strategy: Pair BUR with appropriate insulation to achieve desired energy efficiency and thermal comfort.
• Budget And Lifecycle: Initial costs are offset by long service life when the system is well maintained.
• Local Code And Contractor Expertise: Engage qualified roofing contractors familiar with BUR installation to ensure proper detailing and workmanship.

Common Issues And Troubleshooting

Awareness of common problems helps prevent costly repairs. Typical issues include:

• Blistering: Water vapor or poor bonding can cause blisters; fix by venting moisture and resealing.
• Delamination: Separation between plies may indicate water ingress or improper adhesion; remedy with patching or ply replacement.
• Cap Layer Wear: Gravel loss or UV damage reduces protective strength; reapply ballast or coats as needed.
• Ponding: Insufficient slope leads to water accumulation; consider regrading or adding drainage improvements.

The BUR roofing system combines time-tested waterproofing with versatile design options. When installed by skilled professionals, BUR delivers reliable protection, energy efficiency, and a durable roof assembly that can be repaired and extended as needed.