Roof Snow Load Calculator
Enter your ground snow load and building parameters to calculate the flat roof and sloped roof design snow loads per ASCE 7-22.
ASCE 7-22 Snow Load Formula Explained
The American Society of Civil Engineers standard ASCE 7-22 (Minimum Design Loads for Buildings) defines how to calculate roof snow loads from ground snow data. The process involves two key equations:
pf = 0.7 × Ce × Ct × Is × pg
ps = Cs × pf
| Variable | Name | Description |
|---|---|---|
| pg | Ground snow load | 50-year return period ground snow load from ASCE 7 maps or local data (psf) |
| Ce | Exposure factor | Accounts for wind exposure that removes snow from the roof (0.9 to 1.2) |
| Ct | Thermal factor | Accounts for heat loss through the roof that melts snow (1.0 to 1.3) |
| Is | Importance factor | Higher for essential/critical buildings; 1.0 for standard residential |
| pf | Flat roof snow load | Design snow load for a flat (or low-slope) roof |
| Cs | Slope factor | Reduces load for steeper pitches where snow slides off |
| ps | Sloped roof snow load | Final design snow load for your pitched roof |
The 0.7 factor accounts for the statistical relationship between ground-measured snow and what actually accumulates on roofs. Wind, solar radiation, and heat loss all reduce roof accumulation compared to the ground.
Important minimum: ASCE 7-22 Section 7.3.4 specifies that the flat roof snow load pf must not be less than the following: if pg ≤ 20 psf, then pf ≥ Is × pg. This minimum governs in low-snow areas with unfavorable exposure/thermal conditions.
50-State Ground Snow Load Reference
Representative ground snow loads (pg) for major cities across all 50 states. Values are from ASCE 7-22 and may vary within a state based on elevation and microclimate. Always verify with your local building department.
| State | City | pg (psf) | Zone |
|---|---|---|---|
| Alabama | Birmingham | 5 | Low |
| Alaska | Anchorage | 50 | Very High |
| Arizona | Phoenix | 0 | None |
| Arizona | Flagstaff | 40 | High |
| Arkansas | Little Rock | 10 | Low |
| California | Los Angeles | 0 | None |
| California | Sacramento | 0 | None |
| Colorado | Denver | 25 | Moderate |
| Colorado | Vail | 80 | Very High |
| Connecticut | Hartford | 30 | Moderate |
| Delaware | Wilmington | 20 | Moderate |
| Florida | Miami | 0 | None |
| Georgia | Atlanta | 5 | Low |
| Hawaii | Honolulu | 0 | None |
| Idaho | Boise | 20 | Moderate |
| Illinois | Chicago | 25 | Moderate |
| Indiana | Indianapolis | 20 | Moderate |
| Iowa | Des Moines | 30 | Moderate |
| Kansas | Wichita | 15 | Low |
| Kentucky | Louisville | 15 | Low |
| Louisiana | New Orleans | 0 | None |
| Maine | Portland | 50 | Very High |
| Maryland | Baltimore | 20 | Moderate |
| Massachusetts | Boston | 35 | High |
| Michigan | Detroit | 25 | Moderate |
| Minnesota | Minneapolis | 50 | Very High |
| Minnesota | Duluth | 60 | Very High |
| Mississippi | Jackson | 5 | Low |
| Missouri | St. Louis | 15 | Low |
| Montana | Billings | 30 | Moderate |
| Nebraska | Omaha | 25 | Moderate |
| Nevada | Las Vegas | 0 | None |
| Nevada | Reno | 20 | Moderate |
| New Hampshire | Concord | 50 | Very High |
| New Jersey | Newark | 25 | Moderate |
| New Mexico | Albuquerque | 10 | Low |
| New York | New York City | 20 | Moderate |
| New York | Buffalo | 50 | Very High |
| New York | Syracuse | 55 | Very High |
| North Carolina | Charlotte | 10 | Low |
| North Dakota | Bismarck | 35 | High |
| Ohio | Columbus | 20 | Moderate |
| Oklahoma | Oklahoma City | 10 | Low |
| Oregon | Portland | 15 | Low |
| Pennsylvania | Philadelphia | 20 | Moderate |
| Pennsylvania | Pittsburgh | 25 | Moderate |
| Rhode Island | Providence | 30 | Moderate |
| South Carolina | Charleston | 5 | Low |
| South Dakota | Sioux Falls | 35 | High |
| Tennessee | Nashville | 10 | Low |
| Texas | Dallas | 5 | Low |
| Texas | Houston | 0 | None |
| Utah | Salt Lake City | 25 | Moderate |
| Vermont | Burlington | 50 | Very High |
| Virginia | Richmond | 15 | Low |
| Washington | Seattle | 15 | Low |
| Washington, D.C. | Washington | 15 | Low |
| West Virginia | Charleston | 20 | Moderate |
| Wisconsin | Milwaukee | 30 | Moderate |
| Wyoming | Cheyenne | 25 | Moderate |
Zone Key: None = 0 psf | Low = 1-15 psf | Moderate = 16-30 psf | High = 31-45 psf | Very High = 46+ psf. Mountain areas within any state may have significantly higher values. Site-specific studies may be required for locations above 4,000 ft elevation.
Snow Weight by Type
Not all snow is created equal. The density and weight per cubic foot varies dramatically depending on age, temperature, and moisture content. This is critical for estimating actual loads on your roof.
Light Snow Types
Heavy Snow Types
Real-World Example
A 2,000 sq ft roof with 2 feet of settled snow (12 lb/ft³ average):
2,000 × 2 × 12 = 48,000 lbs (24 tons)
If rain falls on this snow and saturates it to 25 lb/ft³, the load jumps to 100,000 lbs (50 tons) — potentially exceeding the roof's design capacity.
Exposure & Thermal Factors
Exposure Factor (Ce) — ASCE 7-22 Table 7.3-1
The exposure factor accounts for how wind affects snow accumulation. Windswept roofs accumulate less snow; sheltered roofs accumulate more.
| Terrain Category | Fully Exposed | Partially Exposed | Sheltered |
|---|---|---|---|
| B — Urban / Suburban | 0.9 | 1.0 | 1.2 |
| C — Open terrain | 0.9 | 1.0 | 1.1 |
| D — Coastal / flat | 0.8 | 0.9 | 1.0 |
| Above treeline in mountains | 0.7 | 0.8 | N/A |
Thermal Factor (Ct) — ASCE 7-22 Table 7.3-2
Buildings that lose heat through the roof melt snow faster. Unheated or well-insulated structures retain more snow.
| Condition | Ct | Examples |
|---|---|---|
| Heated structure | 1.0 | Most homes, offices, stores |
| Unheated / ventilated | 1.1 | Garages, barns, warehouses |
| Open-air structure | 1.2 | Carports, open pavilions |
| Below-freezing interior | 1.3 | Freezer buildings, cold storage |
| Continuously heated greenhouse | 0.85 | Greenhouses with heating system |
Roof Pitch Slope Factor (Cs)
Steeper roofs shed snow more effectively, reducing the design load. The slope factor Cs depends on the roof pitch, surface type, and whether the roof is heated (warm) or unheated (cold). For standard heated residential roofs with rough surfaces (asphalt shingles):
| Pitch | Slope (degrees) | Cs (warm roof) | Cs (cold roof) | Effect |
|---|---|---|---|---|
| Flat-2:12 | 0-9.5° | 1.0 | 1.0 | No reduction — snow sits |
| 3:12 | 14° | 1.0 | 1.0 | Minimal — threshold of reduction |
| 4:12 | 18.4° | 0.94 | 1.0 | Slight reduction begins (warm) |
| 6:12 | 26.6° | 0.79 | 0.93 | Moderate reduction |
| 8:12 | 33.7° | 0.66 | 0.82 | Significant reduction |
| 10:12 | 39.8° | 0.52 | 0.70 | Major reduction |
| 12:12 | 45° | 0.38 | 0.58 | Snow mostly slides off |
| 14:12+ | 49.4°+ | 0.24 | 0.46 | Very steep — minimal retention |
Slippery surface bonus: Metal roofs and membrane surfaces have lower Cs values (snow slides off sooner) compared to rough surfaces like asphalt shingles. If you have a metal roof, the reduction begins at lower pitches and is more aggressive. However, you must install snow guards to prevent dangerous avalanches.
Warning Signs of Snow Overload
Roof collapses from snow rarely happen without warning. Know these signs and act early to prevent structural failure.
- Visible bowing or sagging of the roof ridge line
- Cracking or splitting sounds from roof trusses or rafters
- Sudden cracking in interior walls, especially near load-bearing walls
- Ceiling drywall separating from the framing
- Doors or windows that suddenly stick or won't close
- New cracks in drywall around door frames
- Popping or creaking sounds from the structure
- Sprinkler heads pushing below ceiling tiles (commercial buildings)
- Snow accumulation exceeding 50% of design load
- Visible drifting on the leeward side of the roof
- Rain forecast on top of existing snow load
- Ice dams forming at the eaves (trapping meltwater)
- New water stains on ceilings from structural flex
Drift & Unbalanced Snow Loads
Wind doesn't deposit snow evenly. ASCE 7-22 requires checking for drift loads at roof steps, parapets, and projections. These localized loads often control the structural design.
Windward Drift
Forms when wind carries snow up and over a roof step, depositing it against the higher wall. Typically smaller than leeward drifts.
Leeward Drift
Forms on the downwind side of a roof step or obstruction. Can be 2-3 times the balanced snow depth. This is usually the controlling drift case.
Unbalanced Gable Roof Load
On gable roofs with slopes between 2.38 degrees and 70 degrees, wind can strip snow from the windward side and deposit it on the leeward side. ASCE 7-22 requires checking this unbalanced condition: the leeward side gets an increased load while the windward side is reduced.
This is why many codes require structural analysis beyond simple uniform load calculations for gable roofs in heavy snow areas.
Common Drift Problem Areas
- Two-level roofs: Snow drifts against the upper wall from the lower roof
- Parapets: Snow accumulates against parapet walls on all sides
- Roof-mounted equipment: HVAC units, skylights, and solar panels create drift zones
- Adjacent buildings: Taller neighbors can cause drifts on your lower roof
- Valleys: Intersecting roof planes concentrate snow
Snow Removal Guidelines
Safe Methods
- Roof rake from ground: Telescoping aluminum rakes (16-24 ft reach). Best option for residential.
- Calcium chloride ice melt: Fill pantyhose and lay on ice dams to create melt channels.
- Professional removal: Hire insured contractors for commercial or steep roofs.
- Heated cables: Preventive system installed before winter along eaves and valleys.
Never Do This
- Never climb onto a snow-loaded roof — collapse risk is highest during removal.
- Never use a metal shovel — damages shingles and creates leak points.
- Never use salt (NaCl) — corrodes metal flashing and gutters, damages shingles.
- Never remove all snow — leave 1-2 inches to protect the roof surface.
When to Remove Snow
Remove snow when accumulation exceeds 50-60% of your roof's design snow load. Use this quick rule: if your design load is 30 psf and settled snow weighs ~12 lb/ft³, the critical depth is roughly 30 ÷ 12 = 2.5 feet. Start removing at about 1.5 feet of settled snow. Remove evenly from both sides to avoid unbalanced loads.