Built-Up Roofing (BUR)
Built-Up Roofing (BUR), often called 'tar and gravel' roofing, is one of the oldest and most reliable low-slope roofing systems, consisting of alternating layers of bitumen (asphalt or coal tar pitch) and reinforcing fabrics that create a finished membrane. With a history spanning over 150 years, BUR systems continue to provide exceptional waterproofing, durability, and multi-ply redundancy for commercial, institutional, and industrial buildings.
Composition
Built-Up Roofing systems consist of alternating layers of bituminous material and reinforcing fabrics that create a finished membrane. The primary components include: 1) Bitumen - either asphalt (derived from petroleum refining) or coal tar pitch (derived from coal processing), with asphalt being more common in modern applications; 2) Reinforcing Fabrics - traditionally organic felt (cellulose fibers saturated with asphalt), though fiberglass felts and polyester mats are now more common for their superior strength and moisture resistance; 3) Surfacing - typically aggregate (gravel, slag, or mineral granules), reflective coatings, or cap sheets with factory-applied mineral surface. A typical BUR system includes a base sheet mechanically fastened or adhered to the substrate, followed by 3-5 interply sheets fully adhered with hot bitumen, and finished with a flood coat of bitumen and aggregate surfacing or a cap sheet.
Properties
Thickness
3-ply: 270-360 mil, 4-ply: 360-480 mil
System thickness depends on the number of plies, type of felts, and surfacing. A 3-ply system typically ranges from 270-360 mil (0.27"-0.36"), while a 4-ply system ranges from 360-480 mil (0.36"-0.48"). Aggregate surfacing adds approximately 1/4" to 3/8" to the total thickness.
Weight
5-7 lbs/ft² (with aggregate)
BUR systems are among the heaviest roofing options. A typical 4-ply system with gravel surfacing weighs approximately 5-7 lbs/ft², requiring substantial structural support. Systems with cap sheet surfacing instead of aggregate are lighter, typically 3-4 lbs/ft².
Fire Rating
Class A (ASTM E108)
Aggregate-surfaced BUR systems typically achieve Class A fire ratings due to the non-combustible nature of the gravel or slag surfacing. Cap sheet systems may achieve Class A, B, or C ratings depending on the specific materials and assembly.
Puncture Resistance
Excellent (with aggregate)
Aggregate-surfaced BUR systems offer superior puncture resistance due to multiple reinforcing plies and the protective layer of gravel or slag. This makes them particularly suitable for roofs with mechanical equipment or regular maintenance traffic.
Wind Uplift Resistance
60-120 psf
Wind uplift resistance varies based on attachment method, number of plies, and surfacing. Properly installed BUR systems with aggregate surfacing typically achieve excellent wind uplift ratings due to the weight of the system and the interlocking nature of the aggregate.
Solar Reflectance Index (SRI)
Dark gravel: 5-20, Light gravel: 30-40, Reflective coating: 60-85
Traditional dark aggregate surfacing has low solar reflectance. Light-colored aggregate or reflective coatings significantly improve SRI values and energy performance in hot climates.
Applications
Commercial Buildings
Widely used on office buildings, retail centers, warehouses, and other commercial structures with low-slope or flat roofs, particularly where proven long-term performance and durability are priorities.
Institutional Facilities
Common on schools, universities, hospitals, and government buildings where longevity, reliability, and minimal maintenance are critical considerations and justify the higher initial investment.
Industrial Buildings
Preferred for manufacturing facilities, processing plants, and other industrial structures where resistance to chemicals, pollutants, and harsh environmental conditions is required.
High-Traffic Roof Areas
Ideal for roofs that require regular access for equipment maintenance, window washing systems, or other services, as the aggregate surfacing and multi-ply construction provide excellent resistance to foot traffic and punctures.
Plaza Decks and Protected Membrane Assemblies
Used in inverted roof membrane assemblies (IRMA) and plaza deck applications where the membrane is installed below insulation and/or pavers, protected from UV exposure and physical damage.
Regions with Extreme Weather Conditions
Well-suited to areas with severe weather including hail, high winds, and extreme temperature fluctuations, as the robust construction and protective surfacing provide exceptional resistance to environmental stresses.
Advantages
- Exceptional durability with documented performance of 30+ years
- Multi-ply redundancy reducing the risk of leaks from single-point damage
- Superior puncture and tear resistance, particularly with aggregate surfacing
- Excellent resistance to foot traffic and mechanical damage
- Self-healing properties for minor cuts and abrasions
- No seams in the field of the roof, only at perimeters and penetrations
- Proven performance in extreme weather conditions
- Excellent chemical resistance, particularly coal tar pitch systems
- High thermal mass contributing to building energy stability
Limitations
- Labor-intensive installation requiring specialized equipment
- Hot application methods involve fumes, odors, and fire risks
- Significant weight requiring substantial structural support
- Higher initial cost than many single-ply options
- Traditional dark surfaces absorb heat, potentially increasing cooling loads
- Difficult to identify and repair leak sources when problems occur
- Limited cold-weather installation capabilities
- Petroleum-based product with associated environmental considerations
- Limited recyclability at end of service life
Sustainability Profile
Moderate sustainability profile with significant variables. BUR systems are petroleum-based products with associated environmental impacts from extraction and processing. Hot application methods generate VOCs and fumes during installation, though cold-applied alternatives are available with improved environmental profiles. The exceptional durability with service lives of 30-40+ years creates a favorable lifecycle assessment compared to materials requiring frequent replacement. The multi-ply redundancy provides protection against premature failure, reducing waste generation. Traditional dark-surfaced BUR absorbs solar energy, potentially increasing cooling loads, though reflective surfacing options are available that significantly improve energy performance. The high thermal mass of BUR systems can contribute to building energy stability by moderating temperature fluctuations. End-of-life considerations remain challenging, as recycling infrastructure for asphalt roofing products is limited in many regions. The weight of BUR systems increases transportation impacts and embodied carbon. Overall, the primary sustainability advantage is the exceptional durability and proven long-term performance.