Often, we are asked to provide localized snow guard systems (snow retention systems) over an entranceway or several of them rather than protecting the entire eave line of a standing seam metal roof. In limited cases this can be done, but extreme caution is advised.
It is very difficult to predict how snow banks will shear when only a portion of the bank is restrained. It will not, however, shear in a straight line from the end of the snow retention system (also known as snow guard) up to the ridge. This is a certainty. Rather, it will shear in a diagonal line toward the ridge. 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 supported by the system -- not a small, rectangular shaped bank. This is because the snow bank has cohesive strength within itself. The cohesive strength varies with the density and temperature of the snow bank, which always changes. Hence, the angle of shear is impossible to determine with any certainty.
It is necessary to calculate the force of sliding snow on the snow guard system in order to engineer that system against failure. In order to do that, the “tributary area” (size of the snow bank on the metal roof) must be known. Because, as explained above, it cannot be known with certainty; some educated guesswork is involved. When guesswork is involved, it is only prudent to increase the factor of safety. Most often, the result of a large tributary area and increased factor of safety requires additional rows of assembly above the first row. 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.