Section 7.3/Flat Roof Snow Load
ASCE 7-22 Section 7.3 provides the flat roof snow load equation (pf = 0.7 Ce Ct Is pg) that converts the ground snow load to the design roof snow load.
The ground snow load (pg) must be determined from Figure 7.2-1 (ground snow load maps) or from site-specific case studies where the maps indicate CS (Case Study Required). In some mountainous regions, the ground snow load varies significantly over short distances and the maps cannot provide reliable values. The local building authority may also specify ground snow loads that supersede the map values.
Why this section exists
Snow loads on roofs are derived from the ground snow load using conversion factors in Section 7.3 and following sections. An incorrect ground snow load cascades through the entire roof design, affecting member sizes, connection forces, and foundation loads. In mountainous areas, the ground snow load can vary by a factor of three or more over a few miles due to elevation and terrain effects, making the map values unreliable and requiring site-specific data.
What plan reviewers look for
Plan reviewers check that the ground snow load is stated on the structural design criteria sheet and that the value matches the Figure 7.2-1 map for the project location. In regions marked CS on the map, the reviewer expects a reference to a site-specific study or local authority data. The reviewer also verifies that the ground snow load has been correctly converted to a flat roof snow load using the exposure, thermal, and importance factors.
Common violations
Related ASCE 7 requirements
Section 7.6 covers unbalanced snow loads on sloped roofs. Section 7.7 covers snow drifts on lower roofs. Section 7.8 covers sliding snow loads. Section 7.3-2 provides the flat roof snow load equation (pf = 0.7 Ce Ct Is pg) that converts the ground snow load to a design roof load.