Aggregates Manager

February 2017

Aggregates Manager Digital Magazine

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Bill Langer is a consulting research geologist who spent 41 years with the U.S. Geological Survey before starting his own business. He can be reached at Bill_Langer@hotmail.com CARVED IN STONE 52 AGGREGATES MANAGER / February 2017 L ast summer, I installed a peel-and-stick underlayment topped with a white mineral rolled roofing cap sheet to the new roof on the extension my son-in-law, Rob, and I added to our ex- isting covered porch. When I asked my granddaughter, Delaney, (who was 11 years old at the time) to help me apply a white elastomeric coating on the original tan roof, she replied, "Cool. I'd love to help!" Cool, indeed. On a typical summer afternoon, a clean white roof that reflects 80 percent of sunlight will stay about 55°F cooler than a gray roof that reflects only 20 percent of sunlight. Putting a white coating on our roof probably gave it reflectance properties that would classify it as a cool roof. Actually, the first roofing project I ever undertook was about 55 years ago when my dad and I re-shingled the roof on our house with white three-tab asphalt shingles. I can personally testify that white materials have historically been a popular cool option for roofs for quite some time. But a cool roof does not necessarily have to be white. Sunshine includes ultraviolet, visible, and near-infrared light, and about half of the sun's energy is invisible near-infrared light. There are new cool-colored dark roofs that look like traditional dark roofs, but better reflect near-infrared light. So how does aggregate fit into the picture? The answer probably is obvious to many of you. Coarse sand, either natural or manufac- tured sand from crushed stone, serves as granules for roofing material including shingles, such as those on my childhood house, or rolled roofing, such as on our new porch roof. Some granules are coated; others are natural. But most start with coarse sand. Slightly larger aggregate is used on flat built-up roofs (BUR). Commonly referred to as 'tar and gravel' roofs, BUR are com- posed of alternating layers of bitumen and reinforcing fabrics. They are topped with a layer of pea stone, either natural or crushed. And, if the stone is white, it can be a cool roof. Stone-ballasted roof systems, which began appearing in the 1970s, are another way to cover flat roofs. Although stone-ballasted roofs appear similar to BUR, there are major differences. Ballasted roofs are 'loose-laid,' which means the membrane, thermal barrier, and rock are all laid down without fastening them to each other. Obviously, the membrane seams are sealed, and it is secured to the parapet and at roof penetrations, but it is not adhered to the roof deck or the layers be- neath it. The key to the entire system is the aggregate ballast that is placed on top of the membrane weighs down all the compo- nents to hold them in place. The stones in a ballasted roof are at least an inch in diameter and applied much more heavily than a BUR. The weight can vary from 10 pounds per square foot (the minimum allowed by code) to 25 pounds or more. Unless the aggregate is white, ballasted roofs do not meet the traditional requirements of reflectivity and, therefore, have not been officially recognized as a cool roof. However, the mass of the ballast acts as a heat sink and simulates the perfor- mance of a traditional cool-roof surface with high solar reflectance and high thermal emittance. So next time you are looking for an environmentally friendly application of aggregate, the answer might be lying right over your head. How cool is that! Aggregates can be part of a roofing system that keeps your home cool. Really Cool Roofing

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