Engineering · Civil Engineering · Load Analysis
Snow Load Calculator
Calculates the design snow load on a roof surface based on ground snow load, exposure, thermal conditions, and roof slope per ASCE 7 methodology.
Calculator
Formula
p_s is the sloped roof snow load (psf or kPa); 0.7 is the roof snow load factor accounting for natural redistribution; C_e is the exposure factor (0.7–1.3) reflecting wind exposure of the site; C_t is the thermal factor (1.0–1.3) based on building heat retention; I_s is the importance factor (0.80–1.20) based on occupancy category; p_g is the ground snow load (psf or kPa) from ASCE 7 Figure 7.2-1 or local codes; C_s is the slope factor (0–1.0) that reduces load for steeper or slippery roofs.
Source: ASCE 7-22: Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Chapter 7.
How it works
Snow loads on roofs differ from ground snow loads because wind, heat loss, and roof slope all redistribute or reduce accumulation. ASCE 7 codifies this relationship through a set of dimensionless factors applied to the mapped ground snow load (p_g), which is determined from regional snow maps or local jurisdiction data. The flat roof snow load (p_f) represents the load on a horizontal or near-horizontal surface before slope reduction is applied.
The full design formula is p_s = 0.7 × C_e × C_t × I_s × p_g × C_s. The constant 0.7 is an empirical conversion factor acknowledging that roofs inherently accumulate less snow than the ground due to wind and heat effects. The exposure factor C_e reflects wind conditions: fully exposed roofs in open terrain use C_e = 0.7, while sheltered roofs in dense forests or urban canyons use C_e = 1.2. The thermal factor C_t accounts for heat loss through the roof — unheated structures and freezer buildings retain snow longer, increasing C_t to 1.2–1.3. The importance factor I_s scales the load based on occupancy risk, ranging from 0.80 for low-risk storage buildings to 1.20 for essential facilities such as hospitals. The roof slope factor C_s reduces the load for steeper roofs: for slippery unobstructed surfaces, reduction begins above 15°, while for rough surfaces it begins above 30°, reaching zero at 70° for both.
Structural engineers use this output as a primary load case in roof beam, rafter, and truss design. The calculated p_s feeds directly into load combination equations (dead + snow, dead + live + snow, etc.) per ASCE 7 Chapter 2, and is used to size members, check deflections, and verify connections. Building officials reference the same methodology during plan review and permitting, making this calculation central to code compliance in snow regions.
Worked example
Consider a single-story office building in a partially exposed suburban area with a 30° metal panel roof (slippery surface). The jurisdiction specifies a ground snow load of p_g = 30 psf. The roof is heated, and the building is a standard Category II occupancy.
Step 1 — Select factors: C_e = 1.0 (partially exposed), C_t = 1.0 (heated), I_s = 1.00 (Category II).
Step 2 — Calculate flat roof snow load: p_f = 0.7 × 1.0 × 1.0 × 1.00 × 30 = 21.0 psf.
Step 3 — Determine slope factor C_s: The roof is slippery (metal panel, unobstructed). For slippery roofs, C_s = 1.0 at 15° and 0.0 at 70°. At 30°: C_s = 1.0 − (30 − 15) / 55 = 1.0 − 0.273 = 0.727.
Step 4 — Calculate design roof snow load: p_s = 21.0 × 0.727 = 15.27 psf (0.731 kPa).
This value would then be combined with dead loads and any applicable drift or unbalanced snow load provisions to complete the structural design. Had the same building used a rough-surfaced membrane roof, C_s would remain 1.0 at 30° (reduction only starts above 30°), giving p_s = 21.0 psf — a significantly more conservative result highlighting why surface type matters.
Limitations & notes
This calculator implements the basic balanced roof snow load equation from ASCE 7-22 and does not account for several important supplementary provisions. Unbalanced snow loads on sloped roofs, drift loads on lower roofs adjacent to taller structures, sliding snow from upper roofs, and rain-on-snow surcharges are all separate load cases required by ASCE 7 and must be evaluated independently. The minimum roof snow load requirement (p_f ≥ I_s × 20 psf where p_g > 20 psf, or p_f ≥ I_s × p_g where p_g ≤ 20 psf) is not enforced here and must be checked manually. Ground snow load values (p_g) must be obtained from the authority having jurisdiction — ASCE 7 Figure 7.2-1 values apply to the contiguous US, but many states and localities have adopted site-specific ground snow maps that supersede national figures. The slope factor relationships implemented here are the standard linearized approximations from ASCE 7 Table 7.4-1 for warm roofs; cold roof adjustments may differ. Always verify outputs against the full code document and consult a licensed structural engineer for final design decisions.
Frequently asked questions
What is the ground snow load and where do I find it?
The ground snow load (p_g) is the maximum weight of snow on the ground surface for a given return period (typically 50-year mean recurrence interval). In the United States, ASCE 7 provides national ground snow load maps, but many states — including Colorado, Oregon, and Washington — publish their own site-specific maps. Your local building department or a geotechnical/structural engineer familiar with the region can confirm the correct value for your project site.
Why is the roof snow load lower than the ground snow load?
Roofs accumulate less snow than the ground for several physical reasons: wind sweeps snow off elevated surfaces, heat conducted through the roof melts the bottom layer of the snowpack, and the geometry of sloped roofs promotes sliding. ASCE 7 captures these effects through the 0.7 factor and the C_e, C_t, and C_s multipliers, all of which are typically less than or equal to 1.0 for most common conditions.
When does the slope factor C_s equal zero?
The slope factor C_s reaches zero at 70° for both slippery and non-slippery roof surfaces under the ASCE 7 linearized model, meaning no balanced snow load is applied to very steep roofs. However, unbalanced and sliding snow loads may still apply to steep roofs and must be checked separately. A roof at exactly 90° (vertical wall) would clearly accumulate no snow in practice.
What is the importance factor and how do I choose it?
The importance factor I_s reflects the consequence of structural failure based on building occupancy. ASCE 7 Risk Category I (low-risk storage) uses 0.80, Category II (standard commercial and residential) uses 1.00, Category III (assembly, schools, high-occupancy) uses 1.10, and Category IV (essential facilities like hospitals and emergency shelters) uses 1.20. The risk category is assigned by the owner and engineer based on the building's function and is often stipulated in the building permit application.
Does this calculator cover drift loads and unbalanced snow loads?
No — this calculator covers only the basic balanced flat and sloped roof snow load per ASCE 7 Section 7.3 and 7.4. Drift loads (Section 7.7–7.8), unbalanced loads on gable roofs (Section 7.6), sliding snow (Section 7.9), and rain-on-snow surcharges (Section 7.10) are additional load cases that require separate calculations based on roof geometry, adjacent structure heights, and local precipitation data. For complex roof configurations, a licensed structural engineer should evaluate all applicable snow load provisions.
Last updated: 2025-01-15 · Formula verified against primary sources.