Asphalt emulsions and cut-backs have their place in the low-slope roofing market, but where does plain asphalt stand?

While hot-applied built-up roofing (BUR) has lost considerable market share to single-ply and modified bituminous systems, it remains attractive to building owners who are re-covering or upgrading their roof systems. In some markets, such as the Southwest, the labor force is more familiar with hot-applied BUR, helping asphalt maintain its regional popularity.

This system has been around since the 1870s, according to this historical perspective on BUR by Carter Slusher. Hot-applied BUR has evolved considerably in its 140-year history, achieving these 10 favorable qualities.

1) Availability. Asphalt is a residual of petroleum distillation, with roughly 75% of produced asphalt used in paving. Roofing grades of asphalt can be packaged in cartons or delivered as a hot liquid using bulk tankers. As the costs of petroleum rise, all related construction products follow suit, including single ply, modified bituminous roofing, and conventional BUR. However, this well-understood system remains widely available.

2) Use of recycled materials. Asphalt paving is widely accepted with 45-50% recycled material content. Great progress has also been achieved with recycling of asphalt shingles because these are not contaminated with adhered underlayments and any fasteners can be separated magnetically. Recycling commercial BUR has also progressed considerably, but contamination with adhered insulation, mechanical fasteners, asbestos fibers, and some surfacings remains more problematic.

3) Maintainability. This is a major strength of BUR. Asphalt is easily soluble in mineral spirits, so cold patches or tie-ins to the existing membrane are very simple and reliable. This repairability is possible throughout the BUR’s life because cross-linking or loss of plasticizer does not occur. Modified bituminous materials are also compatible with BUR so patches can be made by torch or cut-backs. Temporary patches, repairs, and even replacement are possible under a variety of weather conditions.

4) Durability. This BUR quality is also well established thanks to over a century on the market. Many BUR roof systems have been in place for 30 years or more. Reinforcements have transitioned from mainly organic (rag or paper) felts to glass fiber felts, which are less likely to blister, split, shrink, or rot as a result.

5) Versatility. These systems can be applied from dead level to vertical and anywhere in between, making BUR a viable choice for any roof slope. They also offer an appealing versatility during moisture testing for wet thermal insulation – the moisture survey is conducted using infrared, nuclear, capacitance, or vector examinations, and the roofing system will permit partial replacement as long as leakage and insulation wetting is limited.

6) Low VOC content. Low-fuming asphalts are now available for situations where odor is objectionable. Pair them with kettles that use an afterburner to consume volatiles released during the heating cycle.

7) Unlimited storage. Asphalt does not change while it’s stored in cartons and is not affected by cold or heat. When it’s directly exposed to the weather, the thin coat of applied asphalt can sustain some dehydrogenation (also known as mud cracking), which is why opaque surfacings such as aggregates or coatings are recommended.

8) Near-immediate bonding without cure time. Asphalt is the original thermoplastic. Heated to its application temperature, it solidifies in seconds and becomes traffic- and weather-resistant in minutes.

9) Compatibility. While asphalt-treated felts and fabrics are the main reinforcements for BUR, hot asphalt is both a weather-proofer and a construction adhesive. Hot asphalt provides adhesion to facers on cover boards, thermal insulation boards, primed structural concrete, and oil-free metals.

Applied reflective coatings such as aluminum roof coatings and acrylics work well to screen UV but may have adhesion problems where water ponds. However, they’ll still adhere to weathered asphalt and asphalt emulsions. Asphalt pairs well with many surfacing options, from glaze coats, fibrated roof coatings, emulsions, aluminum, and acrylic coatings to the traditional mineral-surfaced cap sheets, flood coat, gravel, or slag.

10) Code and regulation compliance. BUR systems meet UL Class A, B, or C depending upon surfacings, layers of reinforcement, slope, and substrate. As a broad generality, aggregate-surfaced BURs are typically Class A and mineral-surfaced cap sheet systems Class B. Comprehensive listings are available in UL directories, FM Global listings, and reports from other testing facilities. Hail resistance and wind uplift resistance are also well-established.

The Disadvantages of Hot BUR Systems
Though built-up roofing remains popular, it does come with some disadvantages that require careful consideration of your project’s requirements. It requires labor-intensive application, with multiple layers necessary to provide durability, strength, and puncture resistance. Hot asphalt can burn skin and melt polystyrene foam insulation, which require extra attention to safety and roofing product choice.

BUR systems are constructed in the field, whereas most single-ply systems are produced in a factory under stringent quality control and merely joined together in the field. Combined with a tendency to use phased construction, in which only the base sheet is applied to close in a building and remaining layers are added later, this can invite malfunction if not applied properly. Phased construction in particular invites moisture, which leads to undesirable inter-ply blistering when it’s trapped in the membrane.

Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems and continues to participate in seminars for the University of Wisconsin and RCI Inc., the Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, the William C. Correll award from RCI, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.

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