# Example 2.3: Find snow loads

Jonathan Ochshorn

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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.

Fig. 1. Typical roof types

Fig. 2. Magnitudes of unbalanced loads

 "" exposure terrain Boston, MA: 35 psf Chicago, IL: 25 psf Little Rock, AR: 5 psf Houston, TX: 0 psf Ithaca, NY: 35 psf Los Angeles, CA: 0 psf Miami, FL: 0 psf New York, NY: 30 psf Philadelphia, PA: 25 psf Phoenix, AZ: 0 psf Portland, ME: 50 psf Other (specify at right): full exposure partial exposure sheltered B: urban/suburban C: open terrain D: flat unobstructed windswept mountain Alaska-no trees normal: 1.0 cold, vented with R>25: 1.1 unheated structure: 1.2 heated greenhouse with R<2: 0.85 I=low hazard: 0.8 II=normal: 1.0 III=high hazard: 1.1 IV=essential: 1.2 roof type roof surface roof snow load hip or gable mono-slope or flat 1:12 = 4.76 deg 2:12 = 9.46 deg 3:12 = 14.04 deg 4:12 = 18.43 deg 5:12 = 22.62 deg 6:12 = 26.57 deg 7:12 = 30.26 deg 8:12 = 33.69 deg 9:12 = 36.87 deg 10:12 = 39.81 deg 11:12 = 42.51 deg 12:12 = 45.00 deg <= 5 degrees other (degrees) slippery non-slippery "" "" "" unbalanced snow loads: Checks: errors: exposure n/a W>0 angle >0 and < 90 ground snow load > 0