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Directions: Enter values for ground snow load, exposure, terrain, thermal factor, importance, roof type and slope, and surface characteristics (i.e., slippery or nonslippery surface). Typical roof types are illustrated in Fig. 1 below, with the distance, W, measured from eave to ridge in gable- or hip-type roofs.
Note that this calculates only the basic roof snow load (and unbalanced loads for hip and gable roofs), and does not take other aspects of roof geometry into account (for example, surcharges due to drifting snow that could accumulate against walls, parapets, or other obstructions).
In addition to regular (balanced) loads, unbalanced loads are computed for hip and gable roofs with roof angles between 1/2 in 12 (2.38°) and 7 in 12 (30.26°). See Fig. 2 for magnitudes. Values for "b" and "c" are zero for ridge-to-eave distances, W, of 20 ft or less. Both the balanced and unbalanced load cases must be checked.
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More detailed explanations and examples can be found in my text.