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Component Manufacturing dverti$ dverti $ er
Don’t Forget! You Saw it in the
July 2023 #15288 Page #96
Adverti$$er
Ground Snow vs. Flat Roof Snow By MiTek Staff
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iTek Engineering software gives you two options to input snow load – Ground Snow and Roof Snow (flat roof snow). What is the difference between ground snow and flat roof snow?
Ground snow load, pg, is obtained directly from the American Society of Civil Engineers Standard (ASCE 7) Figure 7-1 for the contiguous United States and Table 7-1 for Alaska. There are areas on the map listed as “CS” (Case Study) – they are regions that experience abnormal weather patterns and therefore ground snow cannot be directly found on the map provided in ASCE 7. Ground snow within the CS areas shall be approved by the authority having jurisdiction. Flat roof snow, pf, is ground snow multiplied by factors that account for roof snow loss – some snow is blown off the roof and some is melted off by heat escaped from the space below the roof. The flat roof snow load (roof with slope equal to or less than 5 degrees) is calculated using formula 7.3-1 of ASCE 7: Where: Ce – Exposure factor Ct – Thermal factor Is – Importance factor Pg – Ground Snow load
𝑝f = 0.7𝐶e𝐶t𝐼s𝑝g
Exposure factor, Ce, shall be determined from Table 7-2 in ASCE 7 and is based on the Terrain exposure category (Surface Roughness category in ASCE 7-16) and Roof exposure category. Terrain exposure category (Surface Roughness category in ASCE 7-16) is dealing with the obstructions to wind within a 2600 feet distance of the structure or 1500 feet for structures with a mean roof height of 30 feet or less. Exposure B refers to urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. Exposure C is for open terrain with scattered obstructions having heights generally less than 30 feet. This category includes flat open country and grasslands. Exposure D is for flat unobstructed areas and water surfaces outside hurricane prone regions, it involves a structure a close distance (typically within 600 feet) from an “open waterway” one mile or more across. This category is readily distinguishable, where the locally enforced code very likely has considered this in their requirements. The Roof exposure category deals with obstructions right next to the structure. Sheltered roofs are those that are tight in among conifers and fully exposed roofs are those that have no shelter from terrain, higher structures, or trees. Partially exposed roofs are all of those that do not meet one of the first two definitions. Continued next page
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