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Span tables for rafters asce3/18/2024 ![]() Figures 3 and 4 shows some examples of these conditions for roofs. This can cause uneven loading of the structure and can cause local areas of snow load that are significantly higher than the ground snow load. Obstructions and geometry transitions can provide areas that are “aerodynamically shaded,” and therefore accumulate more snow. For instance, on a simple gable roof, snow depth will typically be less on the windward side (side where the wind is coming from) than on the leeward side. Roofs and decks, with the exception of particularly large, flat examples, do not accumulate snow in such a uniform fashion. Ground snow load indicates the weight of snow per square foot that can be expected in an open, unobstructed flat ground situation. The following issues are worth considering however, especially at points during the snow season when excessive snow loading may become a concern. Generally, code requirements are intended to accommodate most foreseeable snow conditions and roof or deck characteristics. Engineered components, including trusses and manufactured buildings, should be designed to meet the local ground snow load requirements and should include documentation to verify compliance. In cases where ground snow load is more than 70 pounds per square foot per R301.2, or where the planned construction system is not covered explicitly in the code, an engineer’s stamped structural drawing will be required. These tables cover common lumber species, construction practices, and spans. ![]() Specific design guidance like span tables and bracing requirements can be found for roofs in IRC R802, and for decks in R507. Design Snow Loads Map for the United States, adapted from ASCE 7-10, Figure 7-1 (Source: Medeek Design 2015). See Figure 2 and also see the Climate tab of this guide. In the United States, maps of ground snow load can be found in IRC 2021, R301.2. It does not necessarily represent the weight of snow on a roof or deck, because of the multiple factors at play listed above. This is the typical weight of accumulated snow on open ground at a particular geographic location. temperature of the space below the roof (insulation levels).snow sliding onto a roof or deck from a roof above.cumulative snow amounts due to successive weather events.Heavy snow loads can exceed roof load limits risking roof collapse (Source: PNNL).Īnticipated roof and deck snow load, or the weight of snow sitting at a particular place on a roof or deck, is highly variable and depends on a number of factors including: Building codes such as the International Residential Code (IRC) provide basic guidance for building designers.įigure 1. For areas that experience snow in winter months, structural designs for roofs and decks need to accommodate additional loads placed on these structures by snow accumulation (Figure 1). ![]() In areas that see snowfall in winter months, accumulation of snow on roofs and decks can cause deflection of structural members and can even cause collapse of under-designed structures. Snow can weigh a lot, as much as 21 pounds per square foot ( FEMA P-957 2013). ![]() See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Single-Family New Homes, and Indoor airPLUS. Hire a snow removal contractor if danger is spotted.Watch for signs of excessive snow load:.In existing buildings, owners should monitor the status of roofs and decks during heavy snowfall events. For systems not covered in IRC 507 or where ground snow loads are above 70 psf, a structural engineer will be needed to verify compliance with ASCE 7-16 - Minimum Design Loads and Associated Criteria for Buildings and Other Structures.For covered structural systems and ground snow loads up to 70 psf, use the sizing and span tables in IRC R507, or approved span tables from engineered lumber products or composite decking products.Note that many engineered systems like roof truss packages will be engineered to these standards and should be provided with documentation for review by local code authorities. For systems not covered in IRC 802 or where ground snow loads are above 70 psf, a structural engineer will be needed to verify compliance with ASCE 7-16 - Minimum Design Loads and Associated Criteria for Buildings and Other Structures.For covered structural systems and ground snow loads up to 70 psf, use span tables in IRC R802, or approved span tables from engineered lumber products.Identify ground snow load at the proposed building site using IRC 2021, R301.2.Builders should follow these steps to avoid problems due to excessive loads caused by snow on roofs and decks.
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