Snow Load Calculations: Is Your Roof Built for Winter?

Snow accumulation on roofs creates substantial loads that many homeowners and builders underestimate. A cubic foot of fresh, dry snow weighs around 7 pounds, but wet, compacted snow can exceed 30 pounds per cubic foot. When snow accumulates to depths of 2-3 feet or more, the total weight can reach thousands of pounds, stressing roof framing to its limits or beyond.

Understanding how engineers calculate snow loads ensures your roof is designed to withstand winter weather without collapse. This guide covers ASCE 7 methodology, ground snow load determination, adjustment factors, drift and unbalanced loads, and when to call a structural engineer.

Why Snow Load Matters

Roof collapses from snow overload occur every winter, particularly during heavy, wet snowfall events or when multiple storms deposit layers without melting between events. Flat or low-slope roofs are especially vulnerable because they don't shed snow naturally like steep roofs.

The consequences of inadequate snow load design include:

Structural collapse causing property damage, injury, or death
Sagging roofs and ceiling damage from excessive deflection
Cracked framing members that may not fail immediately but are weakened
Insurance complications if the structure wasn't designed to local snow load requirements
Inability to obtain building permits without engineered designs in high-load areas

Proper snow load design isn't optional guesswork; it's a fundamental structural requirement codified in building codes based on decades of engineering research and historical weather data.

ASCE 7 Snow Load Methodology

The American Society of Civil Engineers publishes ASCE 7: Minimum Design Loads and Associated Criteria for Buildings and Other Structures, the authoritative standard referenced by the International Building Code (IBC) and International Residential Code (IRC).

Snow load calculations follow a systematic process:

Determine ground snow load (pg) for your location
Calculate flat roof snow load (pf) using adjustment factors
Apply slope reduction for pitched roofs (ps)
Calculate drift loads at discontinuities
Check for unbalanced load cases
Apply rain-on-snow surcharge where applicable
Verify minimum design loads are met

Let's examine each step in detail.

Ground Snow Load (pg): The Starting Point

Ground snow load represents the weight of accumulated snow on the ground, measured in pounds per square foot (psf). This value is determined from:

ASCE 7 Snow Load Maps: These maps provide pg values based on historical weather data and statistical analysis. The mapped values represent a 50-year mean recurrence interval (2% annual probability of exceedance).

Local Building Department Data: Many jurisdictions specify ground snow loads that supersede map values, particularly in areas with:

Complex terrain where localized conditions differ from regional averages
Lake-effect snow zones
High-elevation mountain areas
Locations where local experience indicates higher design values are prudent

Site-Specific Case Studies: For critical structures or unusual locations, engineers may perform site-specific analysis using local weather station data, extreme value statistical analysis, and terrain considerations.

Typical Ground Snow Load Ranges

Ground snow loads vary dramatically by region:

Mild/No Snow Climates (0-10 psf):

Southern states (Texas, Louisiana, Florida, Georgia, South Carolina)
Coastal California, Southern Arizona, New Mexico

Light Snow Regions (10-25 psf):

Mid-Atlantic coast (Virginia, Maryland, Delaware)
Pacific Northwest lowlands (Portland, Seattle areas)
Northern California, Arkansas, Tennessee

Moderate Snow Regions (25-50 psf):

New England (except mountains)
Upper Midwest (Michigan, Wisconsin, Minnesota lowlands)
Inland Pacific Northwest (Spokane area)
Northern Plains states

Heavy Snow Regions (50-100+ psf):

Rocky Mountain areas
Sierra Nevada mountains
Northern Maine, New Hampshire, Vermont mountains
Upper Michigan, northern Wisconsin, northern Minnesota
High-elevation areas throughout northern states

Extreme Snow Regions (100-300+ psf):

High Sierra Nevada (Lake Tahoe: 250+ psf)
Central Rockies high country (Aspen, Vail areas: 150-200+ psf)
Cascade mountain range
Locations with persistent lake-effect snow

Always verify the applicable pg for your specific location. A site 20 miles away at different elevation may have significantly different ground snow load requirements.

Flat Roof Snow Load Formula

For flat roofs (slopes ≤5°), calculate design snow load using:

pf = 0.7 × Ce × Ct × Is × pg

Each coefficient adjusts the ground snow load for specific building characteristics:

Exposure Factor (Ce)

The exposure factor accounts for wind effects on snow accumulation. Wind can scour snow from exposed roofs or deposit additional snow on sheltered roofs.

Fully Exposed (Ce = 0.9):

Roofs exposed to wind on all sides with no obstructions
In areas with sustained winds (open plains, coastlines)
No nearby structures, trees, or terrain blocking wind
Rare in most residential/commercial settings

Partially Exposed (Ce = 1.0):

Normal exposure condition for most buildings
Some nearby obstructions but not heavily sheltered
Typical suburban/urban settings
Most common Ce value used

Sheltered (Ce = 1.2):

Roofs in dense urban areas with tall surrounding buildings
In forested areas with tall trees protecting the roof from wind
In valleys or locations with substantial wind protection
Results in 20% increase in design load due to reduced wind scouring

Determining exposure requires engineering judgment based on site conditions, prevailing wind patterns, and proximity to obstructions. When in doubt, use Ce = 1.0 as a conservative middle value.

Thermal Factor (Ct)

The thermal factor accounts for heat loss through the roof affecting snow melting:

Heated Structures (Ct = 1.0):

Continuously heated buildings (residences, offices, most commercial)
Normal insulation levels
Roof temperatures above freezing reduce snow accumulation slightly
Standard design condition

Unheated Structures (Ct = 1.2):

Buildings not heated or only occasionally heated
Agricultural buildings, storage structures, some warehouses
Roof remains cold; snow doesn't melt from below
20% increase in design load

Structures Intentionally Kept Below Freezing (Ct = 1.1):

Cold storage, ice rinks, frozen food warehouses
Controlled temperature environments

Greenhouses and Similar (Ct = 0.85):

Continuously heated with minimal insulation
Significant heat loss through roof melts snow
Reduced design load (but must use Ct = 1.0 if heating can fail)

Always use Ct = 1.0 or greater if heating system failure could allow snow accumulation without melting.

Importance Factor (Is)

The importance factor adjusts design loads based on building risk category and consequences of failure:

Risk Category I (Is = 0.8):

Agricultural buildings
Minor storage facilities
Structures with low occupancy and minimal failure consequences
Rarely used for inhabited structures

Risk Category II (Is = 1.0):

Most residential and commercial buildings
Standard occupancy and failure risk
Default category for typical construction

Risk Category III (Is = 1.1):

Buildings representing substantial public hazard if failure occurs
Schools, daycare facilities (occupancy > 300)
Buildings with assembly uses (> 300 occupants)
Facilities with hazardous materials
Power plants, water treatment facilities
Jails and detention facilities

Risk Category IV (Is = 1.2):

Essential facilities that must remain operational
Hospitals and emergency medical facilities
Fire and police stations
Emergency shelters
Buildings containing extremely hazardous materials
20% increased design load for critical infrastructure

The 0.7 Coefficient

The 0.7 factor converts ground snow load to roof snow load under typical conditions, accounting for:

Wind redistribution (some snow blown off or redistributed)
Sublimation and evaporation
Melting and runoff during warm periods between storms
The fact that maximum roof accumulation rarely equals maximum ground accumulation

This coefficient is built into the formula and represents typical conditions for heated, partially exposed structures.

Flat Roof Snow Load Example

Scenario: Determine flat roof snow load for a heated retail building in northern Wisconsin

Given:

pg = 50 psf (from local building department)
Heated structure: Ct = 1.0
Suburban location: Ce = 1.0
Standard occupancy: Is = 1.0

pf = 0.7 × 1.0 × 1.0 × 1.0 × 50 = 35 psf

The roof structure must be designed to support 35 psf of snow load in addition to dead load (roof weight) and other applicable loads.

Sloped Roof Snow Load

Steep roofs shed snow more effectively than flat roofs. ASCE 7 provides slope reduction factors based on roof pitch and surface characteristics:

ps = Cs × pf

Where:

ps = sloped roof snow load
Cs = slope reduction factor (0.0 to 1.0)
pf = flat roof snow load

Slope Reduction Factor (Cs)

For slopes ≤ 15° (2.68:12 pitch): Cs = 1.0 (no reduction)

For slopes > 15° and ≤ 70°: Cs reduces linearly based on surface type

Warm Roof, Slippery Surface (metal, slate, smooth surfaces on heated structures):

15° slope: Cs = 1.0
30° slope: Cs = 0.67
45° slope: Cs = 0.33
70° slope: Cs = 0.0 (complete shedding assumed)

Cold Roof or Non-Slippery Surface (asphalt shingles, wood shakes, unheated structures):

15° slope: Cs = 1.0
30° slope: Cs = 0.83
45° slope: Cs = 0.67
70° slope: Cs = 0.33

For slopes > 70°: Cs = 0.0 (snow assumed to slide off immediately)

However, even for slopes > 70°, you cannot use zero snow load if the roof surface is obstructed (skylights, vents, solar panels) or if the roof is curved such that snow can accumulate in valleys or flatter sections.

Slippery vs. Non-Slippery Surfaces

Slippery surfaces allow snow to slide more readily:

Standing seam metal roofs
Smooth membrane roofing on steep slopes
Slate, ceramic tile
Glass (skylights, solar tubes)

Non-slippery surfaces retain snow better:

Asphalt composition shingles
Wood shakes and shingles
Rough-textured surfaces
Surfaces with obstructions (ribs, battens, step flashings)

Example: Sloped Roof Calculation

Scenario: Residential roof, 6:12 pitch (26.6° slope), asphalt shingles, pf = 35 psf

6:12 pitch = 26.6°. This is between 15° and 70°, so slope reduction applies.

For cold roof (asphalt shingles), Cs interpolates between:

At 15°: Cs = 1.0
At 70°: Cs = 0.33

Linear interpolation for 26.6°: Cs = 1.0 - [(26.6 - 15) / (70 - 15)] × (1.0 - 0.33) = 1.0 - [11.6 / 55] × 0.67 = 1.0 - 0.14 = 0.86

ps = 0.86 × 35 = 30 psf

The sloped roof load is reduced to 30 psf (from 35 psf flat roof value) due to partial shedding on the moderately sloped asphalt shingle roof.

Drift Loads: When Snow Piles Up

Wind causes snow to drift and accumulate against obstructions, creating localized high loads that can exceed flat roof design loads by 2-3 times or more.

Drift loads must be calculated for:

Roof projections above adjacent lower roofs
Parapets and walls extending above the roof
Rooftop equipment, penthouses, mechanical units
Elevation changes on the same roof

Leeward Drift

Leeward drift forms on the downwind side of obstructions as wind-blown snow deposits in the sheltered area.

Drift geometry:

Triangular load distribution
Height (hd) depends on fetch length and ground snow load
Width typically 4 × drift height

Windward Drift

Windward drift can form on the upwind side of tall obstructions (parapets > 3 feet high) as wind pushes snow up against the vertical surface.

Drift Load Calculation

ASCE 7 provides detailed formulas for drift height based on:

Ground snow load (pg)
Fetch length (distance wind travels across roof before reaching obstruction)
Roof geometry
Thermal conditions

Simplified concept:

Longer fetch = more snow available to drift = taller drift
Higher ground snow load = denser, heavier snow = taller drift
Drift height typically ranges from 1-6 feet for common building geometries

Design implications:

Roof framing near obstructions must be reinforced to handle drift surcharge
Drift loads can govern structural design even when base snow load is moderate
Lower roofs adjacent to taller sections are particularly vulnerable

Example Scenario

A single-story office building (roof elevation 14 feet) is adjacent to a two-story office building (roof elevation 28 feet). The 14-foot height difference creates a vertical obstruction. Snow drifts from the tall roof and piles against the wall, creating a triangular drift load on the lower roof.

If drift height calculates to 4 feet with snow density 20 pcf:

Drift load = 4 ft × 20 pcf = 80 psf at the peak
This is more than double the typical flat roof snow load
Structural members within the drift zone require larger sizing

Unbalanced Loads on Sloped Roofs

Wind can remove snow from one side of a gable roof while the other side remains fully loaded, creating asymmetric loading that produces different stresses than balanced loading.

Unbalanced load cases required for:

Gable roofs
Hip roofs
Curved roofs (arches, domes)
Sawtooth roofs

Design approach:

Assume full balanced load on one side
Assume partial or zero load on the windward side (wind scours snow away)
Results in torsional and bending stresses different from uniform load case

Structural analysis must check both balanced and unbalanced load patterns to ensure framing is adequate for all scenarios.

Rain-on-Snow Surcharge

In some regions (typically with ground snow loads ≤ 20 psf), rain falling on existing snowpack creates a surcharge load. Rain saturates the snow, dramatically increasing density and weight without increasing depth.

When required: Ground snow loads ≤ 20 psf (temperate climates with occasional snow)

Surcharge value: 5 psf added to design snow load

Rationale: These areas experience rain during winter that can saturate snow rather than melting it completely. The 5 psf surcharge accounts for this worst-case scenario.

Sliding Snow Loads

Snow sliding from an upper roof onto a lower roof creates impact and accumulated load.

Design requirements:

Lower roof must support its own snow load PLUS sliding snow from above
Sliding snow load = 0.4 × pf per lineal foot of eave (for slippery surfaces) or reduced values for non-slippery surfaces
Critical for multi-level roofs, roof decks over entries, and sunrooms below main roofs

Warning: Sliding snow presents hazard to occupants and objects below eave lines. Consider snow guards, wider overhangs, or awnings to protect entry areas.

Snow Density

Snow density varies based on age, moisture content, and temperature:

Fresh, Dry Snow: 3-7 pcf

Immediately after light, cold snowfall
Fluffy, low-density crystals
Rarely governs design (compacts quickly)

Settled Snow: 10-20 pcf

Typical mid-winter snowpack
Moderate compaction
Common design assumption

Wet, Compacted Snow: 20-30 pcf

Rain-saturated snow
Spring conditions, freeze-thaw cycles
Dense, heavy snow near melting point
Often governs design in temperate zones

Ice: 57 pcf

Frozen solid (rare except in ice dams)

ASCE 7 formulas implicitly account for typical density based on ground snow load, but understanding density helps explain why wet spring snow causes more collapses than deeper but lighter mid-winter snow.

Minimum Snow Loads

Even in low-snow areas, codes require minimum design snow loads to account for occasional unusual weather:

IBC and IRC minimum: Typically not less than the design rain load or other specified minimums

For regions with pg = 0 (no mapped snow load), some jurisdictions still require minimum 10-15 psf snow load based on historical anomalies or conservative design practice.

Prescriptive Residential Codes vs. Engineered Design

When Prescriptive Code Tables Are Sufficient

The International Residential Code (IRC) provides prescriptive span tables for roof framing (rafters and trusses) based on species, grade, spacing, and snow load. These tables are appropriate for:

Simple gable or hip roofs
Standard residential construction (one or two stories)
Snow loads within table limits (typically up to 70 psf)
No unusual roof features (large openings, complex geometry, heavy roof coverings)
No significant drift or unbalanced load concerns

Builders can use IRC prescriptive tables without engineering calculations, provided all conditions and limitations are met.

When Engineering Is Required

Professional structural engineering is necessary for:

High Snow Loads: Ground snow loads exceeding 70 psf typically exceed prescriptive table limits

Complex Roof Geometry:

Multiple roof levels with drift potential
Large valleys or areas where snow accumulates
Curved, arched, or non-standard roof shapes
Roofs with significant cantilevers

Long Spans: Roof spans exceeding prescriptive table limits (typically > 20-24 feet for rafters, though engineered trusses can span much farther)

Commercial and Public Buildings: Non-residential structures, Risk Category III and IV buildings

Unusual Loading Scenarios: Rooftop equipment, solar panel arrays (adding dead load and potentially obstructing snow shedding), green roofs

Existing Structure Evaluation: Assessing whether an old roof can handle snow loads to current code standards, or evaluating for additions/renovations

Evaluating Existing Roof Framing

If you're unsure whether an existing roof is adequate for local snow loads:

Determine required design snow load using ASCE 7 or local code
Identify existing framing: Species, grade, size, spacing (measure in attic)
Calculate existing capacity: Use IRC span tables or structural formulas
Compare required load to capacity

If capacity exceeds required load: Structure is adequate

If capacity is deficient: Options include:

Add intermediate supports (posts/beams in attic)
Sister existing rafters with additional members
Install engineered trusses or structural upgrades
Remove snow when accumulation exceeds safe levels

Never assume an older structure meets current snow load requirements. Building codes have been updated over decades as engineering knowledge improved and historical snow events demonstrated higher loads.

Signs of Roof Overloading

During heavy snow accumulation, watch for warning signs:

Interior Signs:

Sagging ceiling (visible deflection)
Cracks in drywall at ceiling/wall joints
Doors and windows sticking or becoming difficult to operate
Popping, cracking, or creaking sounds from roof structure
Roof framing visibly bending or deflecting when viewed from attic

Exterior Signs:

Visible sagging of ridge line or roof planes
Bowing or leaning walls (snow load pushing outward on rafters)
Cracks in masonry walls
Roof appears lower than normal

If you observe these signs:

Evacuate the building immediately if severe deflection or cracking is evident
Remove snow load from roof (carefully, working from roof edges inward)
Contact a structural engineer to assess damage and required repairs

When and How to Remove Snow

Remove snow when:

Accumulation exceeds design load (know your roof's capacity)
Multiple storms without melting create layered accumulation
Ice dams form (indicate potential for water intrusion and added load)
Visible deflection or other warning signs appear

Snow removal best practices:

Work from eaves toward ridge (don't trap yourself)
Remove in layers, don't dig down to roof surface (leave 2-3 inches to protect roofing)
Use plastic shovels (metal damages roofing)
Don't use ice chippers or sharp tools
Consider hiring professionals with proper equipment and safety gear
Never remove all snow from one section while leaving adjacent sections fully loaded (creates unbalanced load)

Case Studies: Historical Roof Collapses

Major snow load failures have occurred throughout history, driving improvements in building codes:

Notable incidents (general references without specific dates to avoid copyright issues):

Civic center collapses from heavy snow in the Northeast
Warehouse and big-box store failures from prolonged snow accumulation
Arena roof collapses during winter storm events
Agricultural building failures in lake-effect snow regions

Common factors in failures:

Flat or low-slope roofs where snow accumulated without shedding
Inadequate design for drift loads at parapets or roof level changes
Long-span structures with marginal framing
Wet, heavy snow events that exceeded design assumptions
Lack of engineering for non-residential structures built using residential standards

These incidents underscore the importance of proper snow load design and the consequences of inadequate structural capacity.

Design Snow Load Summary Table

| Ground Snow Load (pg) | Typical Flat Roof (pf) | Comments | |-----------------------|------------------------|----------| | 0-10 psf | 0-7 psf | Mild climate; rain-on-snow may govern | | 20 psf | 14 psf | Light snow areas; rain-on-snow surcharge applies | | 30 psf | 21 psf | Moderate snow; typical mid-Atlantic, Pacific NW | | 50 psf | 35 psf | Heavy snow; northern states, elevated areas | | 70 psf | 49 psf | Very heavy snow; exceeds most prescriptive tables | | 100 psf | 70 psf | Extreme snow; mountain regions, engineering required | | 150+ psf | 105+ psf | High-elevation mountain areas; specialist engineering |

Assumes typical values: Ce = 1.0, Ct = 1.0, Is = 1.0. Actual values vary based on specific conditions.

Interaction with Other Loads

Snow load is one of several loads in structural design:

Dead Load: Weight of roof structure, sheathing, roofing material, insulation, ceiling. Permanent, always present.

Live Load: Maintenance workers, equipment on roof. For residential, typically 20 psf (but snow load can replace live load; use greater of the two, not both simultaneously).

Wind Load: Uplift and lateral forces from wind. Design must resist wind uplift while also supporting snow load (but full wind and full snow typically don't occur simultaneously; load combinations per ASCE 7 Chapter 2).

Seismic Load: Not typically combined with snow load (low probability of simultaneous occurrence).

Structural design uses load combinations per ASCE 7 to determine critical load cases. The most common combinations involving snow are:

1.2D + 1.6S (dead load + snow)
1.2D + S + 0.5W (dead load + snow + partial wind)

Practical Takeaways

Always use local ground snow load data from your building department, not generic map estimates
Flat roofs are most vulnerable because they don't shed snow naturally
Drift loads at roof level changes and parapets often govern design, not the uniform snow load
Prescriptive code tables work for simple residential roofs in moderate snow areas, but engineering is required for complex situations
Cold roof or unheated structures require 20% higher design loads than heated buildings
Monitor snow accumulation during heavy winters and remove snow before reaching design limits
Wet spring snow is heavier than deep mid-winter snow; don't be fooled by reduced depth
For commercial buildings, Risk Category III/IV structures, or ground snow loads over 50 psf, hire a structural engineer

Snow load design isn't guesswork or overkill; it's essential structural engineering based on physics, historical weather data, and lessons learned from failures. A properly designed roof supports winter snow loads for decades without distress, while an inadequate roof risks catastrophic collapse. When in doubt, consult a licensed structural engineer familiar with local snow conditions and building code requirements. Use our snow load calculator as a preliminary estimation tool, but always verify with engineering analysis and local code compliance before construction.