Classic asphalt still reigns as a cost-effective roofing material. But this is a new century, so why in the world would we still want to use such an old product? We have such a wide variety of alternative materials and methods, most of which claim to be new, improved, and perhaps safer. We see roofing warranties for these new systems that now are offered for 35 or 45 years, or even a lifetime. So how come we don’t pave our roads with PVC, TPO, or EPDM?
Lots of reasons:
- Asphalt, also known as bitumen, is the bottom of the petroleum barrel – what is left after the gasoline, heating and motor oil, jet fuel, diesel, kerosene, and middle distillates have been extracted, making it a cost-effective material.
- The bitumen can receive great quantities of fillers or extenders, frequently materials that are indigenous to a region, such as gravel or even oyster shells.
- The material is inherently water-resistant.
- Asphalt is heated for mixing and application and is put into service as soon as it cools down. It is a 100% solid material with no waiting for evaporation of solvents or moisture curing.
- It consists of recycled material and is typically 100% recyclable at the end of its life.
- The material is easily repaired. Potholes represent a substrate failure, not a bitumen defect.
- Asphalt has been around for more than a century, evolving to incorporate new materials and technologies. Patents on polymer-modified bitumen appeared as early as 1843.
- The material is versatile. It can be emulsified, dissolved in simple hydrocarbon solvents, used as a hot mix, packed with mineral fillers to make fire-rated coated base sheets and asphalt shingles, utilized as a saturant to make No. 15 and 30 shingle underlayment, laminated with glass fiber ply felts to make built-up roofing membranes, and aluminized for reflectivity and UV protection.
- When coated with polymeric white coatings, BUR can meet reflectivity (albedo) and emissivity requirements of cool roofs.
- It is a hot-melt adhesive. Many single-ply membranes have an asphalt-compatible version, usually achieved with a non-woven fleece back serving as a separator from the polymeric sheet.
- Asphalt roof systems meet fire and wind codes.
Does it really date back more than a century? Would you believe that Egyptians used asphalt to preserve mummies, that bitumen is said to have waterproofed Noah’s ark and the basket that the carried the infant Moses to safety, and that the Hanging Gardens of Babylon used asphalt mortar with mud bricks? Had it not been for an earthquake in the second century B.C., the gardens might still be functioning today (and termed a vegetative roof system).
Consider these items:
Q: What would you use to embed metal gravel stops in NRCA’s Embedded Metal detail, above?
A: Asphalt primer to enhance adhesion and penetrate any roll-forming residual oil, then asphalt mastic in which to set the metal.
Q: What material works best to reattach asphalt shingles that have lifted from wind action?
A: Asphalt tabbing cement.
Q: What material available at local hardware stores should you use to refill pitch pockets?
A: Asphalt mastic – commonly called pitch pockets, though asphalt is generally used today rather than coal tar pitch. Pourable sealers with moisture-curing urethane asphalt blends are popular today with single ply systems.
Q: To construct a durable flashing system, what would you use?
A: Early flashing systems might have used asphalt mastic and layers of burlap, also known as Osnaburg fabric. Then asphalt-saturated asbestos felts with a mineral-surfaced cap sheet were employed. More recently, scrim-reinforced fabrics and now scrim reinforcement with polymer-modified bitumen are used. While we may think that the polymers such as APP or SBS are the key to success, the blend is usually 80-90% asphalt and only 10-20% polymer.
Q: If asphalt is so great, what are the downsides?
A: Asphalt is a derivative of petroleum. So are gasoline, oils, greases, waxes, and other hydrocarbons. This results in a bad situation in industrial environments where such hydrocarbons are exhausted onto a roof surface. A similar bituminous system using coal tar pitch that is relatively impervious to the usual process hydrocarbons would be preferred. In a restaurant, installing a grease trap on the roof might confine the damage to small areas that could be serviced regularly.
Q: How about ultraviolet degradation?
A: Attack by UV, combined with evaporation of lighter ends of asphalt, results in “alligatoring” or “mud-cracking.” The solution is to use opaque surfacing, such as aggregate, aluminized roof coatings, clay-stabilized asphalt emulsions, or reflective roof coatings. A flood-coat and aggregate surface is good for the life of the roof, while the others would require recoating every five to seven years.
Q: How about skilled labor?
A: Installing a built-up roof is demanding. There are fumes, heavy rolls of felt, hot (twice as hot as boiling water) asphalt at 450 to 500 degrees F., and the logistics of getting the heated asphalt from the kettle to the point of application while maintaining a proper application temperature to consider. Cold process roofing may use pumps and spray guns to transport adhesive to the point of application, and use of polyester scrim that certainly weighs less than rolls of felt and cap sheets is preferable. Modified bitumen felts applied with a torch simplifies the kettle-to-lugger-to-mop sequence, but comes with an increased risk of an on-roof fire. Yet all of these are asphaltic systems, meeting today’s challenges in numerous ways. A built-up roof is just that, a membrane manufactured under field conditions.
Long Live the King!
So is the king dead?
The traditional built-up roof has a smaller piece of the roofing pie than in years past. When we realize it is an excellent adhesive for installing roof insulation and cover boards, a good air and vapor barrier, the heart of a modified bitumen system, an adhesive for fleece-backed single plies, the basis of one of the most cost-effective steep roofing covers, and the best way to use the petroleum distillation bottoms, the odds are that asphalt in roofing and paving will still be here a century from now.
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, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.
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