Most of the metal roofs of this time period were mechanically fastened and the majority of snow guards were installed the same way. In the late 1970's metal roofing was so new to the industry, that not many installers had considered the dangers of sliding snow and ice on a metal roof. The result has oftentimes increased the population of snow guard assemblies with less certainty of success (and potentially higher cost) than if a continuous system was installed along the entirety of the eave line from the start.īecause of all the above complications (and others), we normally advise against the practice of localized snow retention.Our Mission: to be the most trusted provider of snow guards by offering the highest quality products and unmatched customer service. This presents a somewhat circular logic path to design the total system appropriately. The answer to this question once again depends on the strength of the cohesive bond within the snow bank on the metal roof, which once again is always changing. Because we are dealing with a wedge-shaped bank of snow, the question now arises, “How much should the length of the upper rows of assembly be increased compared to the lower row?” The answer to this question involves still more guesswork.Īnother question is, “Because of this wedge-shaped bank of snow, will the system be uniformly loaded, or will the ends of the assemblies experience a more concentrated load than the center area, and, if so, how much more load?” The result of a large tributary area and increased factor of safety is most often that additional rows of assembly are required above the first. When guesswork is involved, it is only prudent to increase the factor of safety. Because, as explained above, it cannot be known with certainty, some educated guesswork is involved. In order to do that, the “tributary area” (size of the snow bank on the metal roof) must be known. It is necessary to calculate the force of sliding snow on the snow guard system in order to engineer that system against failure.
The wrong way to use snow retention or snow guards Hence, the angle of shear is impossible to determine with any certainty. The cohesive strength varies with the density and temperature of the snow bank, which always changes. This is because the snow bank has cohesive strength within itself.
The angle of that line becomes more acute as the slope of the metal roof increases, resulting in a large, wedge-shaped bank of snow being supported by the system - not a small, rectangular shaped bank. Rather, it will shear in a diagonal line toward the ridge. It will not, however, shear in a straight line from end of the snow guard system (also known as snow retention) up to the ridge. It is very difficult to predict how snow banks will shear when only a portion of the bank is restrained. In limited cases this can be done, but extreme caution is advised. Often, we are asked to provide localized snow guards (snow retention systems) over an entranceway or several of them rather than protecting the entire eave line of a standing seam metal roof. Reposted with permissions – Original article can be found at Why is it a bad idea to put snow guards just over doorways?
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