Snow Load Is a Significant Structural Consideration in Much of the Continental United States
For architects practicing in northern states, mountain regions, or anywhere with meaningful winter snowfall, snow load is one of the most important design loads affecting roof structure design. A heavy snow event can impose loads on a roof that exceed the dead load of the roof structure itself, and in some high-snow regions, the snow load is the governing design load for the roof structure. Architects do not perform structural calculations, but understanding how snow loads are determined - and the key factors that affect them - is part of the structural systems knowledge that the ARE PPD exam tests. It enables architects to recognize when a roof design might be particularly susceptible to snow loading issues and to coordinate effectively with structural engineers.
Ground Snow Load (Pg)
Snow load analysis begins with the ground snow load (Pg), defined as the weight of snow on the ground at the project location. ASCE 7 Figure 7.2-1 provides mapped ground snow load values in psf across the continental United States, derived from historical snowfall data. Values range from less than 10 psf in most of the South and Pacific Coast to over 100 psf in parts of the Sierra Nevada, Cascades, and northern Rockies. In mountainous regions, ground snow loads can reach 200+ psf at high elevations. The ground snow load is the starting point for determining the design snow load for the roof structure.
Flat Roof Snow Load (Pf)
The design flat roof snow load (Pf) is calculated from the ground snow load using several factors that account for how roofs accumulate and shed snow differently than the ground. The basic formula in ASCE 7 is Pf = 0.7 × Ce × Ct × Is × Pg, where: Ce is the exposure factor (sheltered roofs accumulate more snow; windswept roofs accumulate less; typically 0.7–1.3); Ct is the thermal factor (heated buildings shed snow; cold or unheated buildings retain it; 1.0 for heated, 1.1–1.3 for cold or refrigerated); and Is is the importance factor (higher occupancy and essential facilities use a factor greater than 1.0). The 0.7 factor accounts for the general tendency of roof snow loads to be somewhat less than ground snow loads due to wind effects and heat loss from the roof.
Drift Loads
Snow drift loads are an additional design consideration that can significantly exceed flat roof snow loads at specific locations. Drifts form when wind moves snow from a higher roof surface and deposits it against a vertical obstruction on a lower roof - creating a wedge of snow much deeper than the flat roof snow load would predict. Common drift scenarios: snow blowing from a higher roof and drifting against the parapet of an adjacent lower roof; snow accumulating against mechanical equipment curbs and penthouses; snow drifting against skylights or other roof projections. ASCE 7 Chapter 7 provides drift load calculation procedures that structural engineers use to determine the additional snow load at drift locations, which can be several times the flat roof snow load value.
Unbalanced Snow Loads on Sloped Roofs
Sloped roofs experience unbalanced snow loads when snow slides from one side of a gabled roof but remains on the other, creating an asymmetric loading condition that must be considered in structural design. Unbalanced loads can induce lateral thrust in roof framing that balanced loads would not produce. ASCE 7 provides specific methods for calculating unbalanced snow loads on gable and hip roofs as a function of roof slope and ground snow load.
Key Exam Points
- Ground snow load (Pg): from ASCE 7 mapped values; starting point for roof snow load analysis.
- Flat roof snow load: Pf = 0.7 × Ce × Ct × Is × Pg; accounts for exposure, thermal, and importance factors.
- Drift loads: windward and leeward drifts at roof level changes and obstructions; can significantly exceed flat roof values.
- Unbalanced loads: asymmetric loads on sloped roofs due to wind-driven snow accumulation on one side.
- Cold roofs (unheated) have higher Ct (retain more snow); windswept roofs have lower Ce (shed more snow).
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