Section 7.2/Ground Snow Loads
ASCE 7-22 Section 7.3.2 covers determining the ground snow load (pg) from the snow load maps, site-specific studies, or the local authority.
The ground snow load (pg) is determined from Figure 7.2-1 for the project location. For sites in case study (CS) areas where the map cannot be read accurately, the local authority having jurisdiction must be consulted. The flat roof snow load (pf) equals 0.7 times Ce times Ct times Is times pg, where Ce is the exposure factor (Table 7.3-1), Ct is the thermal factor (Table 7.3-2), and Is is the importance factor. The minimum roof load is 20 psf times Is for low- slope roofs when pg exceeds 20 psf.
Why this section exists
Snow loads can be substantial in northern climates, with ground snow loads exceeding 100 psf in mountainous areas. The conversion from ground to roof snow load accounts for the fact that roofs generally accumulate less snow than the ground (wind removes snow from roofs) but also that some conditions increase roof loads (sheltered roofs, cold buildings, unheated structures). Drifting snow on lower roofs adjacent to higher structures can produce loads several times the flat roof value.
What plan reviewers look for
Plan reviewers check the structural general notes for the ground snow load, exposure factor, thermal factor, and the resulting flat roof snow load. They verify the ground snow load against the ASCE 7 map for the project location. They check for snow drift loads on lower roofs adjacent to higher building elements (parapets, upper stories, adjacent buildings) per Section 7.7.
Common violations
Related ASCE 7 requirements
Section 7.3 (already covered) provides the flat roof snow load equation. Section 7.6 covers unbalanced snow loads on gable and hip roofs. Section 7.7 covers snow drifts on lower roofs. Section 7.9 covers sliding snow from upper roofs. Section 7.12 covers rain-on- snow surcharge.