Why is it a bad idea to put snow guards just over doorways?

Localizing snow guard systems over an entranceway or several of them rather than protecting the entire eave line of a standing seam metal roof can be done in limited cases, but extreme caution is advised. 

wrong way to install snow guards on a metal roof


Above is a picture of the WRONG WAY TO INSTALL SNOW GUARDS. 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 guard system up to the ridge. 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 being supported by the system -- not a small, rectangular-shaped bank. This is because the snowbank has cohesive strength within itself. The cohesive strength varies with the density and temperature of the snowbank, which always changes. Hence, the angle of shear is impossible to determine with any certainty, yet it if the correct tributary area is improperly anticipated the result may be overload and failure. See illustration below.


approximate tributary area to a localized snowguard


Click to read more about designing your snow guard system.

Metal Roof Design for Cold Climates is another highly informative article to read relating to snow retention design.