What Is Roof Snow Load?
Roof snow load is the downward force exerted on a roof structure by accumulated snow and ice, measured in pounds per square foot (psf). It is a primary structural design consideration in all climates that receive meaningful snowfall. Unlike wind or earthquake loads, snow load accumulates gradually and can persist for days or weeks — sustaining continuous stress on rafters, trusses, connections, and bearing walls. A typical residential roof in a moderate snow zone is designed to carry 20–40 psf; some mountain regions require structural capacity for 100 psf or more.
Snow weight varies dramatically with its condition. Fresh light powder weighs approximately 3 pounds per cubic foot. As snow settles and partially melts it compacts to 8–20 pounds per cubic foot. Rain-saturated snow or sleet can reach 50–60 pounds per cubic foot. A one-foot accumulation of wet heavy snow can weigh six times as much as the same depth of fresh powder — which is why late-season wet snowstorms are particularly dangerous on low-slope roofs.
Ground Snow Load vs. Roof Snow Load — How ASCE 7 Works
Structural engineers do not design roofs to carry the full ground snow load. The ground snow load (Pg) is mapped across the United States in ASCE 7 Figure 7.2 and represents the 50-year return-period ground accumulation for each location. Local building departments provide Pg values for any jurisdiction.
The design roof snow load is always less than Pg because roofs shed snow more readily than flat ground. ASCE 7 applies reduction factors: the slope factor Cs (based on pitch and thermal condition), the exposure factor Ce (based on wind exposure), the thermal factor Ct, and the importance factor Is. This calculator displays Cs values for educational reference only. A complete structural calculation requires all five factors and must be performed by a licensed structural engineer.
The ASCE 7 Slope Factor (Cs) — How Pitch Reduces Snow Load
The slope factor Cs ranges from 0.0 to 1.0. It represents what fraction of the design ground snow load applies to the roof surface. A flat roof has Cs = 1.0, retaining the full load. As pitch increases Cs decreases, because steeper slopes shed snow through sliding. For warm roofs (typical heated residential), ASCE 7 begins reducing Cs at pitches above roughly 5:12 (22.6°). By 12:12 (45°), Cs approaches zero — the roof sheds snow so rapidly that nearly no accumulated load applies.
Cold roofs — where the deck stays near outdoor temperature due to extreme insulation or an unheated space below — retain snow longer. ASCE 7 provides separate Cs curves for warm and cold conditions, reflecting the different shedding behavior based on whether heat from the building interior warms the roof surface.
Snow Types and Their Weight
The weight of snow on a roof depends entirely on its density. Understanding snow density helps interpret depth measurements and assess actual structural load.
- Fresh powder snow: approximately 3 lb/cu ft — a 12-inch depth adds only about 0.25 psf
- Settled snow (24–48 hours old): approximately 6–8 lb/cu ft — common during extended winter accumulation
- Wet compacted snow: approximately 12–20 lb/cu ft — occurs during thaw-freeze cycles and rain-on-snow events
- Ice (fully frozen meltwater): approximately 57 lb/cu ft — even one inch can add nearly 5 psf
Warning Signs of Excessive Snow Load
If any of these signs appear during a snow event, evacuate the building until the structure has been evaluated by a professional.
- Snow depth exceeding 2–3 feet on a low-slope or flat roof, especially wet or icy accumulation
- Cracking, creaking, or popping sounds from roof framing or the ceiling below
- Doors or windows that suddenly stick or fail to latch — a sign of structural deflection
- Visible sagging between rafters or trusses when viewed from inside the attic
- Ice dam formation at the eaves combined with heavy accumulation above
Educational Purpose — Professional Structural Design Required
The Cs values in this calculator are approximations intended to illustrate how roof pitch influences snow load reduction. They are not a substitute for a complete structural snow load analysis. Actual structural design requires the full ASCE 7 procedure: ground snow load from official maps, all four reduction factors, drift load calculations for adjacent structures, and compliance with local building codes. This analysis must be performed by a licensed structural engineer. Never rely on simplified online estimates for load-bearing structural design decisions.
Use the estimate as a way to ask better questions, not as permission to leave snow in place or alter a roof. Record the pitch, roof shape, framing type, thermal condition, snow depth, and whether wind has created drifts. Snow guards, parapets, valleys, dormers, and lower roofs can create local accumulations that a simple slope factor does not capture. If conditions change quickly, keep people away from the affected area and arrange an inspection before attempting removal.
Snow can shift suddenly during thawing, rain, or vibration, creating hazards at eaves and entrances even when the main roof appears stable. Do not climb onto a snow-covered roof or attempt removal without suitable training, fall protection, and a plan for falling snow and ice. Keep children, visitors, and vehicles clear of the drop zone, and contact emergency services if you observe severe sagging, structural movement, or collapse.