Snow Load Calculator

Q: How do I find the ground snow load for my area?

Ground snow loads are published in ASCE 7 Figure 7.2-1 and vary by location. Your local building department can provide the exact value. As general guidance: southern states are 0-10 psf, mid-Atlantic 15-30 psf, northern states 30-60 psf, and mountain regions can exceed 100-300 psf. Many states also publish snow load maps specific to their jurisdictions.

Q: How much snow can my roof hold before it collapses?

Most residential roofs are designed for a live load of 20-40 psf (pounds per square foot). Fresh snow weighs about 5-10 psf per foot of depth, while packed or wet snow can weigh 20-30 psf per foot. As a rough guide: a roof rated for 30 psf can support about 3-6 feet of fresh fluffy snow, but only 1-1.5 feet of heavy wet snow. If snow load approaches your roof's capacity, consider professional snow removal.

Q: Does roof pitch affect snow load?

Yes. Steeper roofs shed snow more easily, reducing the design snow load. ASCE 7 uses a slope factor (Cs) that reduces load for pitches above about 30 degrees (7/12 pitch) for warm, slippery roofs. Below 15 degrees (3/12), the full flat-roof load applies. However, steep roofs can create dangerous slide zones at the eaves, and local codes may require snow guards.

Q: What is a snow drift surcharge and when does it apply?

A snow drift surcharge is the additional snow load that accumulates where a higher roof meets a lower roof (step-down), or at parapet walls. Wind blows snow off the higher roof and deposits it against the vertical surface. Drift loads can be 2-4 times the balanced roof snow load and often govern the structural design of the lower roof. ASCE 7 Section 7.7 and 7.8 provide the calculation methodology.

Q: When should I have snow removed from my roof?

Remove snow when it reaches about 60-70% of your roof's rated live load capacity. Warning signs include: visible sagging of the roof deck or ceiling, doors and windows that suddenly stick, cracking or popping sounds from the structure, and water stains indicating ice dam leaks. Always hire a professional for snow removal — walking on a snow-covered roof is extremely dangerous.

Calculate the snow load on your roof based on ground snow load, exposure, and thermal factors

Convert ground snow load to roof snow load per ASCE 7

Quick presets

Ground Snow Load (pg)

psf

Exposure Category

Trusses/Rafters Needed

24" spacing • 0.0 ft rafter length

PRO

Professional Calculator

Load calculations, truss spacing, and code compliance

Building Length

Building Width

Roof Pitch

Estimated Materials

53 bundles

Roof Area

1,581 sq ft

Squares

15.8

Detailed Breakdown

Roof Area1,581 sq ft

With Waste1,739 sq ft

Roofing Squares15.8

Bundles53

How to Use This Calculator

The Snow Load Calculator converts ground snow load into the design snow load on your roof, following ASCE 7 methodology used by structural engineers across the United States.

Ground to Roof Load tab: Enter your ground snow load (pg) in pounds per square foot, select your exposure category (how sheltered the building is), thermal condition, roof pitch, and risk category. The calculator applies the ASCE 7 flat-roof snow load formula (pf = 0.7 × Ce × Ct × Is × pg), then applies the slope factor (Cs) to give you the sloped-roof snow load (ps). This is the load your roof structure must be designed to resist.

Drift Analysis tab: If your building has a roof step-down (e.g., a two-story section adjacent to a one-story section), enter the upper roof length, height difference, and lower roof length. The calculator determines the drift surcharge height and load using ASCE 7 Section 7.7, showing both leeward and windward drift loads. The governing (larger) drift is highlighted. Structural Check tab: Enter the calculated snow load and your roof's rated live load capacity to see if your structure has adequate margin. The calculator flags load ratios above 80% as a caution and above 100% as a failure condition requiring immediate engineering review.

The Formula

The snow load calculation follows ASCE 7-22 (2022 edition):

Flat Roof Snow Load (pf) pf = 0.7 × Ce × Ct × Is × pg Where: - pg = ground snow load (psf) - Ce = exposure factor (0.8 sheltered to 1.2 exposed, varies by category) - Ct = thermal factor (0.85 to 1.3) - Is = importance factor (0.8 to 1.2)

Sloped Roof Snow Load (ps) ps = Cs × pf Where Cs = slope factor, which reduces load for steeper pitches: - Warm roof, slippery: Cs = 1.0 for slopes < 15°, decreasing to 0 at 70° - Cold roof, non-slippery: Cs = 1.0 for slopes < 30°, decreasing to 0 at 70°

Drift Surcharge Height (hd) hd = 0.43 × (lu)^(1/3) × (pg + 10)^(1/4) - 1.5 Where lu = upwind fetch distance in feet

Drift Load = gamma × hd (triangular surcharge) Where gamma = 0.13 × pg + 14 (pcf), 15 ≤ gamma ≤ 30

Example Calculation

Example: Home in Minneapolis, MN

The Johnson family lives in a two-story home with a single-story attached garage in Minneapolis.

Step 1: Ground to Roof Load
• Ground snow load (pg): 50 psf (Minneapolis area)
• Exposure: B (suburban neighborhood)
• Thermal factor: Ct = 1.0 (heated home)
• Importance: Is = 1.0 (residential)
• Exposure factor: Ce = 1.0 (Category B)
• Flat roof snow load: pf = 0.7 × 1.0 × 1.0 × 1.0 × 50 = 35 psf
• Roof pitch: 6/12 (26.6°) → Cs = 1.0 (below warm-roof threshold)
• Sloped roof snow load: ps = 1.0 × 35 = 35 psf

Step 2: Drift at Garage Roof
• Upper roof (2-story) length: 40 ft
• Step-down height: 8 ft
• Snow density: gamma = 0.13 × 50 + 14 = 20.5 pcf
• Balanced snow depth: hb = 35 / 20.5 = 1.71 ft
• Clear height: hc = 8 - 1.71 = 6.29 ft
• Drift height: hd = 0.43 × (40)^(1/3) × (50 + 10)^(1/4) - 1.5 = 2.6 ft
• Drift surcharge: 20.5 × 2.6 = 53.3 psf (triangular, at the step-down)
• Total at drift peak: 35 + 53.3 = 88.3 psf

Step 3: Structural Check
• Garage roof rated for: 40 psf live load
• Balanced load: 35 psf → OK (88% of capacity)
• Drift peak: 88.3 psf → EXCEEDS capacity — engineering review needed
• The garage may need reinforced trusses or headers near the step-down.

Frequently Asked Questions

How do I find the ground snow load for my area?

Ground snow loads are published in ASCE 7 Figure 7.2-1 and vary by location. Your local building department can provide the exact value. As general guidance: southern states are 0-10 psf, mid-Atlantic 15-30 psf, northern states 30-60 psf, and mountain regions can exceed 100-300 psf. Many states also publish snow load maps specific to their jurisdictions.

How much snow can my roof hold before it collapses?

Most residential roofs are designed for a live load of 20-40 psf (pounds per square foot). Fresh snow weighs about 5-10 psf per foot of depth, while packed or wet snow can weigh 20-30 psf per foot. As a rough guide: a roof rated for 30 psf can support about 3-6 feet of fresh fluffy snow, but only 1-1.5 feet of heavy wet snow. If snow load approaches your roof's capacity, consider professional snow removal.

Does roof pitch affect snow load?

Yes. Steeper roofs shed snow more easily, reducing the design snow load. ASCE 7 uses a slope factor (Cs) that reduces load for pitches above about 30 degrees (7/12 pitch) for warm, slippery roofs. Below 15 degrees (3/12), the full flat-roof load applies. However, steep roofs can create dangerous slide zones at the eaves, and local codes may require snow guards.

What is a snow drift surcharge and when does it apply?

A snow drift surcharge is the additional snow load that accumulates where a higher roof meets a lower roof (step-down), or at parapet walls. Wind blows snow off the higher roof and deposits it against the vertical surface. Drift loads can be 2-4 times the balanced roof snow load and often govern the structural design of the lower roof. ASCE 7 Section 7.7 and 7.8 provide the calculation methodology.

When should I have snow removed from my roof?

Remove snow when it reaches about 60-70% of your roof's rated live load capacity. Warning signs include: visible sagging of the roof deck or ceiling, doors and windows that suddenly stick, cracking or popping sounds from the structure, and water stains indicating ice dam leaks. Always hire a professional for snow removal — walking on a snow-covered roof is extremely dangerous.

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