Snow Load Calculator — ASCE 7 Roof Design
Calculate roof snow loads per ASCE 7 for structural design. Flat roof: pf = 0.7×Ce×Ct×Is×pg. Sloped: ps = Cs×pf. Includes exposure, thermal, importance factors; drift, unbalanced, and rain-on-snow loads. Required for permit and engineering.
Why This Construction Metric Matters
Why: Snow load failures cause roof collapses and fatalities. ASCE 7 is the referenced standard in IBC; structural engineers and building departments require snow load calculations for permits. Ground snow load varies by region (10–300+ psf).
How: Flat roof: pf = 0.7×Ce×Ct×Is×pg. Sloped: ps = Cs×pf (Cs from slope and thermal). Ce = exposure, Ct = thermal, Is = importance. Add drift loads at steps/parapets, unbalanced loads on gables, rain-on-snow surcharge where applicable.
- ●pf = 0.7×Ce×Ct×Is×pg for flat roof; ps = Cs×pf for sloped
- ●Ce exposure (sheltered to fully exposed); Ct thermal (heated vs unheated)
- ●Drift loads at steps, parapets; unbalanced on gable roofs
- ●Rain-on-snow surcharge in certain regions per ASCE 7
Sample Projects - Click to Load
Heated Residential
Moderate snow, heated home
Unheated Garage
Heavy snow, unheated structure
Commercial Building
Moderate snow, commercial structure
Mountain Cabin
Very heavy snow, steep roof
Agricultural Building
Heavy snow, unheated barn
Critical Facility
Moderate snow, essential facility
Selected: Moderate Snow Region - Moderate annual snowfall
Typical locations: Mid-Atlantic, Southeast, Pacific Northwest Coast
🏠 Roof Characteristics
Two-slope roof with ridge
📊 Exposure & Factors
Exposure
Open terrain with scattered obstructions
Thermal
Structure maintained above freezing
Importance
Standard occupancy
🏔️ Drift & Sliding Loads
📊 Results
Flat Roof Load
17.50 psf
Sloped Roof Load
17.50 psf
Total Design Load
31.84 psf
Load Category
Moderate
Risk: Moderate
Special Loads
Unbalanced Load
31.84 psf
⚠️ Structural Requirements
- • Requires structural engineer review
- • Verify roof framing capacity
- • Design for unbalanced loads on gable/hip roofs
💡 Recommendations
- • Verify local building codes for specific requirements
- • Consider periodic snow removal for heavy loads
Step-by-Step Calculation
=== Snow Load Calculation (ASCE 7) === Ground Snow Load (pg): 25.0 psf Exposure Category: Exposure C (Open) (Ce = 1) Thermal Factor: Heated Structure (Ct = 1) Importance Factor: Risk Category II (Is = 1) Roof Type: Gable Roof === Flat Roof Load (pf) === pf = 0.7 × Ce × Ct × Is × pg pf = 0.7 × 1 × 1 × 1 × 25.0 pf = 17.50 psf === Slope Reduction Factor (Cs) === Roof Slope: 30° Slope Reduction Factor (Cs): 1.000 === Sloped Roof Load (ps) === ps = Cs × pf ps = 1.000 × 17.50 ps = 17.50 psf === Unbalanced Load === Unbalanced Load = 31.84 psf (Applies to one side of gable/hip roof) === Total Design Load === Design Load = Maximum of: - Balanced Load: 17.50 psf - Unbalanced Load: 31.84 psf Total Design Load = 31.84 psf
📈 Visualizations
📊 Load Comparison
📏 Factor Comparison
Planning estimates only. Verify with a licensed engineer or contractor before construction.
📐 Construction Industry Facts
pf = 0.7×Ce×Ct×Is×pg; sloped ps = Cs×pf
— ASCE 7
Ce exposure; Ct thermal; Is importance factor
— ASCE 7
Ground snow load 10–300+ psf by region
— ASCE 7 maps
Drift, unbalanced, rain-on-snow add to base load
— ASCE 7
📋 Key Takeaways
- • Flat roof | pf = 0.7 × Ce × Ct × Is × pg (ASCE 7)
- • Sloped | ps = Cs × pf; Cs from slope & thermal
- • Factors | Ce exposure; Ct thermal; Is importance
- • Special | Drift, unbalanced, sliding, rain-on-snow
Did You Know?
ASCE 7 is the US standard for snow load design.
Source: Standard
pf = 0.7 × Ce × Ct × Is × pg for flat roof.
Source: Formula
Drift loads at steps; unbalanced on gables.
Source: Loads
Unheated structures use Ct = 1.1.
Source: Thermal
Importance factor: I = 1.0–1.2 by occupancy.
Source: Importance
Exposure B 0.9; C 1.0; D 1.1.
Source: Exposure
What is a Snow Load Calculator?
A snow load calculator determines design snow load on roofs per ASCE 7. Uses ground snow load, roof slope, exposure, thermal and importance factors. Includes drift, unbalanced, sliding, and rain-on-snow loads for structural design.
🔬 Key Concepts
pf
Flat roof snow load
ps
Sloped roof = Cs × pf
Ce, Ct, Is
Exposure, thermal, importance
pg
Ground snow load (psf)
How to Calculate Snow Load
- 1
Ground load
pg from region/map
- 2
Flat load
pf = 0.7 × Ce × Ct × Is × pg
- 3
Sloped load
ps = Cs × pf
- 4
Special loads
Add drift, unbalanced, sliding as applicable
When to Use Snow Load Calculations
Flat Roof
Slope ≤ 5° (≤ 1:12)
Monoslope Roof
Single-slope roof
Gable Roof
Two-slope roof with ridge
Hip Roof
Four-slope roof
Shed Roof
Single-slope lean-to
Curved/Arched Roof
Curved or arched structure
Snow Load Formulas
ASCE 7 Factors Reference
| Factor | Value | Description |
|---|---|---|
| Exposure B (Ce) | 0.9 | Sheltered |
| Exposure C (Ce) | 1.0 | Open |
| Exposure D (Ce) | 1.1 | Unobstructed |
| Heated (Ct) | 1.0 | Normal |
| Unheated (Ct) | 1.1 | Not heated |
Important Considerations
⚠️ Avoid
- • Ignoring drift at steps/parapets
- • Wrong exposure for site
- • Skipping engineer for critical structures
✓ Best Practices
- • Use ASCE 7 ground snow map
- • Consider drift, unbalanced, sliding
- • Verify with local building official
⚠️ Disclaimer: For planning. Structural design must be performed by a licensed engineer. Local codes may differ.
Related Calculators
Birdsmouth Cut Calculator - Rafter Seat Cut Dimensions
Calculate birdsmouth cut dimensions for rafters including seat depth, heel height, and plumb cut angle. Essential for proper rafter-to-plate connections in...
ConstructionGambrel Roof Calculator - Barn Roof Design Dimensions
Design gambrel (barn-style) roof dimensions with dual pitch angles. Calculate rafter lengths, angles, and material requirements for upper and lower roof...
ConstructionMetal Roof Cost Calculator - Metal Roofing Estimate
Estimate metal roofing costs including panels, underlayment, fasteners, and installation. Supports standing seam, corrugated, and metal shingle systems with...
ConstructionRafter Length Calculator - Roof Framing Dimensions
Calculate rafter length, run, rise, and overhang dimensions for roof framing. Supports common, hip, valley, and jack rafters with birdsmouth cut calculations.
ConstructionRoofing Calculator - Materials & Cost Estimator
Estimate roofing materials, shingles, underlayment, nails, and total cost for your roof project. Supports gable, hip, flat, and complex roof types with...
ConstructionRoof Pitch Calculator - Slope, Angle & Area Multiplier
Calculate roof pitch, slope ratio, angle in degrees, and area multipliers. Convert between rise/run, pitch ratio, and angle. Get material suitability...
Construction