Built Up Felt Roofing: A Comprehensive Guide to BUR Systems

Built up felt roofing, commonly known as BUR (Built Up Roofing), is a durable, multilayer membrane designed for flat and low-slope roofs. This guide explains how BUR systems are constructed, their key materials, installation steps, and long-term maintenance. It highlights the benefits, typical applications in commercial and residential settings, and important considerations for selecting a BUR solution. Whether renovating an aging roof or specifying a new project, understanding BUR helps builders, property managers, and homeowners make informed decisions about performance, cost, and longevity.

What Is Built Up Felt Roofing?

Built up felt roofing is a composite membrane formed by alternating layers of roofing felt, bitumen (asphalt or coal tar pitch), and a surfacing material. The uppermost layer, or cap sheet, provides weather resistance and UV protection. Traditional BUR systems use multiple plies, typically three to five, bonded with hot or cold bitumen. Modern BUR may incorporate improved felt types and cool-applied asphalt blends, but the fundamental concept remains a layered, watertight assembly.

How BUR Roofing Works

The layers in a BUR system create redundancy and resilience against water intrusion. Bitumen acts as the primary waterproofing agent, sealing gaps between felts as it hardens. The felt plies provide tensile strength and crack resistance, while the cap sheet shields the membrane from UV damage and weathering. With each ply, the roof gains enhanced puncture resistance, hail tolerance, and wind uplift performance. The result is a robust, long-lasting membrane suitable for flat or low-slope roofs in a range of climates.

Materials And Layers

Key components of a BUR system include:

• Felt plies: Sawn or fiberglass-embeded felts form the core layers. Fiberglass felts offer higher tensile strength and dimensional stability than traditional organic felts.
• Bitumen: Hot-applied asphalt, coal tar pitch, or cold-applied bitumen binders coat and fuse the plies. Hot asphalt remains common for traditional BUR, while cold-process materials provide safer, faster installation.
• Cap sheet: A mineral-surfaced or smooth-cap layer that enhances weather resistance and UV protection. The cap sheet can be mineral-surfaced asphalt and often includes ballast options for wind uplift resistance.
• Primer and accessories: Surface primers improve adhesion; reinforcing fabrics, edge details, and flashings ensure perimeter water control and detailing around penetrations.

Industry practice varies by region and climate. In hot climates, asphalt blends with modifiers may improve flexibility at high temperatures. In colder regions, enhanced bitumen tackiness and reinforced felts reduce cracking. Some systems incorporate reflective or light-colored cap sheets to reduce heat gain in white-on-buildings or flat roofs with daylighting considerations.

Applications And Benefits

BUR roofing is widely used on commercial, multifamily, and institutional buildings, as well as some residential flat-roof designs. Its benefits include:

• Proven waterproofing: Multiple impermeable layers reduce the risk of leaks compared with single-ply membranes.
• Longevity: With proper installation and maintenance, BUR systems can last 20–40 years or more depending on climate and workmanship.
• Low-slope compatibility: Ideal for roofs with little slope where drainage and water runoff require robust waterproofing.
• Repairability: Localized repairs on BUR can extend service life without full replacement.
• Proven track record: BUR has decades of field performance in varied U.S. climates, from coastal to arid environments.

Drawbacks include heavier roof assemblies, longer installation times, and higher initial labor costs compared to some modern single-ply membranes. Proper detailing around penetrations and edges is critical to prevent leaks. BUR also requires periodic inspection and maintenance to address any edge delamination, ply wear, or cap sheet damage before it progresses.

Installation Considerations

Successful BUR installation hinges on substrate condition, weather, and correct material handling. Important steps include:

• Substrate preparation: The roof deck must be clean, dry, and structurally sound. Any prior moisture or debris can compromise adhesion.
• Surface priming: Primers improve the bond between felts and bitumen and help prevent edge lifting.
• Plies installation: Felts are laid with staggered joints and sealed with bitumen. The number of plies depends on the design specs, climate, and desired performance.
• Cap sheet and surfacing: The final layer provides weathering resistance and UV protection. On some projects, ballast like pavers or gravel is used for wind uplift protection.
• Edge detailing and penetrations: Metal flashings, curbs, penetrations, and drains require careful detailing to ensure continuous waterproofing.

Safety is essential during BUR installation due to hot asphalt handling and roof work at height. Contractors often use fall protection plans, proper PPE, and controlled hot asphalt procedures to minimize risks. Local codes and manufacturer guidelines should always be followed.

Maintenance And Longevity

Regular maintenance helps maximize BUR performance and lifespan. Key practices include:

• Inspections: Annual or post-storm inspections to identify blisters, ridges, delamination, or exposed cap sheet.
• Cleaning and debris removal: Keeping drains and scuppers clear prevents ponding and water backup that can deteriorate edges.
• Repair procedures: Prompt patching of punctures, minor delaminations, or lifted edges with compatible bitumen and felts.
• Ventilation and drainage: Adequate roof slope, scuppers, and internal drains reduce standing water and extend membrane life.

Repair costs for BUR can be lower than full replacement if addressed early. Significant damage, aging, or structural issues may necessitate partial or full replacement. Environmental exposure, climate, and roof traffic levels influence a BUR roof’s service life.

Safety And Environmental Considerations

BUR systems involve hot asphalt work in traditional installations, raising burn and fire hazards. Modern cold-applied products mitigate these risks, but proper handling remains essential. Environmental considerations include choosing low-VOC binders and responsibly sourcing surfacing materials. Proper disposal of obsolete felts and asphalt should follow local regulations. When evaluating options, consider both the environmental impact and the long-term energy performance benefits of reflective or white cap sheets that reduce cooling loads.

Design And Specification Tips

For a successful BUR project, keep these tips in mind:

• Match system to climate: Cold climates benefit from flexible, high-tack bitumen; hot climates benefit from heat-stable cap sheets.
• Specify ply count: More plies increase durability but add weight and cost. Common configurations range from three to five plies depending on performance goals.
• Detail around edges: Ensure precise flashing and perimeter detailing to prevent leaks at joints and penetrations.
• Quality control: Require manufacturer-approved accessories, proper primer application, and tested bonding for each ply.

Choosing the right BUR system involves balancing initial cost, expected service life, maintenance requirements, and the building’s exposure conditions. A well-designed BUR roof offers reliable waterproofing, established performance, and a proven track record for flat and low-slope structures in the United States.