Built up roofing (BUR), often referred to as a “tar and gravel” roof, stands as one of the oldest and most reliable commercial roofing systems available. With a history spanning over a century, this multi-layered membrane has proven its worth on countless low-slope and flat roofs across the globe. The fundamental principle behind a built up roof is the creation of a continuous, seamless membrane through the alternation of bitumen-saturated roofing felts (or fabrics) and layers of hot-applied asphalt or coal tar pitch. The final assembly is then typically topped with a layer of aggregate, such as gravel or slag, or a smooth, mineral-surfaced cap sheet. This time-tested assembly offers exceptional durability, fire resistance, and long-term performance, making it a formidable choice for building owners seeking a robust and time-proven solution.
The construction of a built up roof is a meticulous process that builds strength and redundancy from the ground up. It begins with the structural deck, which can be made of concrete, wood, or metal. Upon this deck, a vapor retarder is often installed to prevent moisture from within the building from migrating into the roofing system and causing blisters or other damage. Next, a layer of insulation is added to meet the building’s thermal performance requirements. The core of the BUR system is then applied: the piles. Each ply consists of a reinforcing fabric—traically made from organic or fiberglass materials—that is embedded in a hot layer of bitumen. The number of piles can vary, typically ranging from three to five, depending on the specific requirements for durability, weather exposure, and building codes. This multi-ply construction is the source of the system’s name and its renowned strength.
The advantages of choosing a built up roof are numerous and significant. The system’s primary benefit is its exceptional longevity; a properly installed and maintained BUR system can last 20 to 30 years, and often longer. This lifespan is a direct result of its redundant, multi-layer design. If one layer is compromised, the underlying layers continue to provide protection. Furthermore, built up roofs offer superior fire resistance. The layers of bitumen and the final surfacing of gravel create a Class A fire rating, the highest possible, which can lead to reductions in insurance premiums. The mass of the system also provides excellent resistance to foot traffic and mechanical damage from hail or falling debris. The finished surface, especially when topped with light-colored gravel, can also be highly reflective, contributing to energy efficiency by reducing the urban heat island effect and lowering cooling costs.
Despite its many strengths, a built up roof is not without its drawbacks. The installation process is labor-intensive and requires specialized equipment, including hot kettles to heat the bitumen to the precise application temperature. This can make the initial installation cost higher than some single-ply alternatives. The process also involves hot, potentially hazardous materials, requiring a highly skilled crew to ensure safety and quality. The finished roof system is also quite heavy, adding significant dead load to the building’s structure, which must be engineered to support it. Over time, a BUR membrane can become brittle, and if not properly maintained, the bitumen can dry out, leading to cracks and leaks. The smell during installation can be strong, and the system is generally not considered the most environmentally friendly option due to the petroleum-based products involved, though recycling options for old BUR materials are improving.
The performance and longevity of a built up roof are heavily influenced by the choice of materials. The two primary types of bitumen used are asphalt and coal tar pitch.
- Asphalt: More commonly used today, asphalt is versatile and adaptable to a wide range of climates. It remains flexible at lower temperatures compared to coal tar.
- Coal Tar Pitch: Known for its superior resistance to water ponding and chemical fumes, coal tar pitch is exceptionally durable but can become brittle in cold weather and has a stronger odor during application.
The reinforcing felts also play a critical role. Fiberglass felts offer high tensile strength and resistance to fire and rot, while organic felts, made from recycled wood and paper fibers, provide greater flexibility and absorption of the bitumen. The final surfacing is equally important. Aggregate surfacing (gravel or slag) provides superior protection from UV radiation and physical damage. Smooth surfacing, using a mineral-surfaced cap sheet, is lighter and can be easier to inspect and repair, but may be more susceptible to UV degradation over time.
Proper installation is paramount to the success of a built up roof. The process requires a dry substrate; applying bitumen to a wet surface will trap moisture, leading to future blisters and system failure. The mopping of the hot bitumen must be done evenly and at the correct temperature to ensure proper adhesion between plies. Each subsequent ply must be laid immediately while the bitumen is still hot to create a monolithic bond. Flashing details around roof penetrations, edges, and walls are critical areas that require meticulous attention to prevent water infiltration. Due to the complexity, it is essential to hire experienced, certified contractors who specialize in BUR systems.
Routine maintenance is the key to maximizing the service life of a built up roof. A proactive maintenance program should include:
- Biannual Inspections: Conduct thorough inspections in the spring and fall to identify any damage from winter weather or summer storms.
- Debris Removal: Keep the roof surface clear of leaves, branches, and other debris that can hold moisture against the membrane and clog drains.
- Drainage Check: Ensure all drains, scuppers, and gutters are free-flowing. Ponding water for more than 48 hours after a rainfall can significantly shorten the roof’s life.
- Flashing Inspection: Pay close attention to all flashing details for signs of cracking, separation, or deterioration.
- Prompt Repairs: Address any blisters, cracks, or punctures immediately to prevent minor issues from escalating into major leaks.
When compared to modern single-ply membranes like TPO, PVC, or EPDM, the built up roof holds its own. Single-ply systems are generally lighter, faster to install, and can be more reflective in their white variants. However, BUR systems often boast greater puncture resistance, better fire performance, and a proven long-term track record that some newer materials are still establishing. The choice between a BUR system and a single-ply membrane often comes down to specific project requirements, budget, structural considerations, and the preference for a time-tested versus a modern technological solution.
In conclusion, the built up roof remains a cornerstone of the commercial roofing industry. Its layered, redundant design provides a level of durability and resilience that is difficult to match. While the installation is complex and the material weight is a consideration, the benefits of a long service life, excellent fire resistance, and proven performance make it a compelling option for many building owners. For those prioritizing a robust, long-term roofing solution with a history of reliability, the built up roof system continues to be a wise and dependable investment. As with any major building component, success hinges on quality materials, expert installation, and a committed, ongoing maintenance regimen.