Cool roofs like the on the Surgical Institute of Reading building in Wyomissing, Pa. refl ect and emit the sun’s heat back to the sky instead of transferring it to the building below. (Photo courtesy of McElroy Metal)
tural analysis, and multiple layers and systems. Some types of green roofs have more demand- ing structural standards, especially in seismic regions of the world. “For initial green roof construction, you are
going to have multiple layers and a lot more cost involved with insulation, as compared to metal roof- ing,” says Brian Partyka, president of Drexel Metals, Philadelphia. “It has to do with the depth of the roof and how many inches you’re going to create with the vegetative roof; there are a lot of variables here.” John Ferraro, general manager at Asphalt Roof-
ing Manufacturers Association, Washington, D.C., says the same principle applies to asphaltic roofi ng products. “Construction and installation costs for an asphaltic cool roof can vary greatly depending upon the desired cap sheet or surfacing material. A green roof on the same building could generally prove more cost intensive than a cool roof due to the labor, materials and drainage issues but, of course, each case is different.”
20 METAL CONSTRUCTION NEWS September 2012 It is important to remember that because a
green roof protects the waterproofi ng membrane from the elements, particularly UV light, the mem- brane’s life expectancy is doubled or even tripled, leading to recovered initial cost differentials.
Water runoff One of green roof’s best attributes and one of its biggest pluses over cool roofs is its ability to absorb, collect and reuse rainwater. Most buildings are designed to shed rain; they’re built with hard, impenetrable roofi ng surfaces. Rainwater bounces off them and collects as runoff, picking up impurities like infectious bacteria from animal waste as well as harmful pesticides and fertilizers on the way to mu- nicipal storm sewers. All of this eventually empties into local waterways. Green roofs decrease the total amount of runoff
and slow the runoff rate from the roof. They can retain up to 75 percent of rainwater, gradually releas- ing it back into the atmosphere via condensation
and evapotranspiration (a natural process that cools the air as water evaporates from plant leaves), while retaining pollutants in their soil. Correct drainage from a green roof’s vegetation
is critical. If excess water is not properly directed off the roof, or if the system is incorrectly designed and becomes clogged, this adds far more weight to the green roof than it is supposed to hold. If a green roof is incorrectly assessed for the weight it can bear, or if that weight limit is not heeded, the roof can be damaged or even collapse from the combined weight of plants, soil-holding water, drainage systems and other green roof construction materials. Also, the excess water can become stagnant
or drown the plant roots. Leaks in a green roof arise from poor design or inadequate root barriers. Root barriers are specifi c materials designed to inhibit the slow but inexorable dig of roots, which can break apart even sturdy roof materials in search of nutrients. Leaks also occur from holes or improper installation of the waterproof lining layer of a green