How much snow load can a roof handle?

Most residential roofs are designed for 20–40 pounds per square foot (psf) of snow load. A typical well-built roof can handle 2–4 feet of fresh snow (about 1 inch of snow = 0.5–3 psf depending on density). Wet, heavy snow is 3–5x heavier than dry powder. If you can see structural deflection (sagging) in rafters or trusses, remove snow immediately.

How do you calculate snow load on a roof?

Roof Snow Load (S) = 0.7 × Ce × Ct × Is × Pg. Where: Ce = exposure factor (0.8–1.2 based on terrain), Ct = thermal factor (1.0 for heated, 1.2 for unheated), Is = importance factor (1.0 residential, 1.5 essential facilities), Pg = ground snow load from ASCE 7 map. This calculator implements this ASCE 7 formula.

How heavy is snow on a roof?

Fresh dry snow: 1–3 psf per foot of depth. Settled snow: 3–5 psf per foot. Packed/wet snow: 5–8 psf per foot. Ice: 12–15 psf per foot. One foot of wet heavy snow can weigh over 20 lbs per square foot — enough to approach structural limits on older or poorly built roofs.

When should I remove snow from my roof?

Remove snow when accumulation exceeds 2 feet of dry snow or 1 foot of wet/heavy snow. Always remove snow after unusual precipitation events. Warning signs requiring immediate action: doors/windows suddenly sticking (building is deflecting), cracking or popping sounds from the structure, visible sagging in the roof.

What is the ground snow load in my area?

Ground snow load (Pg) varies by location from 0 psf (deep South) to over 300 psf (mountain regions). ASCE 7 provides a ground snow load map for the US. Common values: Minneapolis 50 psf, Denver 20 psf, Boston 35 psf, Seattle 10 psf. Check your local building department or ASCE 7 for your specific location.

Snow Load Calculator

Calculate design roof snow load using the ASCE 7 formula — enter your ground snow load, roof geometry, and exposure conditions to get structural load in pounds per square foot.

Snow & Roof Parameters

Ground Snow Load (Region)?

Ground Snow Load (psf)?

psf

Roof Pitch (rise:12)?

:12

Roof Area (sq ft)?

sq ft

Thermal Factor (Ct)?

Importance Factor (Is)?

Exposure Factor (Ce)?

Design Roof Snow Load

21.0 psfModerate

Flat Roof Load (pf)21.0 psf

Slope Factor (Cs)1.000

Sloped Roof Load (ps)21.0 psf

Pitch Angle26.6°

Total Weight42,000 lbs

Weight in Tons21.0 tons

ASCE 7 Calculation

pf = 0.7 × Ce × Ct × Is × Pg

pf = 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21.0 psf

ps = Cs × pf

ps = 1.000 × 21.0 = 21.0 psf

Snow Depth Equivalent

Fresh snow (20 pcf)12.6" deep

Packed snow (30 pcf)8.4" deep

Total weight on 2,000 sq ft roof42,000 lbs (21.0 tons)

Based on ASCE 7-22 Section 7. Always verify with local building codes and a licensed structural engineer for construction purposes.

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How to Use This Calculator

Snow load calculations follow ASCE 7-22 (Minimum Design Loads for Buildings). Enter:

Ground Snow Load (Pg) — From ASCE 7 Figure 7.2-1 or your local building code. Enter 0 for locations below the CS (case study) line on the ASCE 7 map. Typical values: Northern states 20–60 psf, mountain areas 60–300+ psf.
Roof Pitch — Slope factor (Cs) reduces design load for sloped roofs. Unobstructed metal roofs get additional reduction.
Exposure Category — Fully exposed (Ce=0.8), partially exposed (Ce=1.0), sheltered by nearby trees or buildings (Ce=1.2).
Thermal Category — Heated structure (Ct=1.0), cold structure (Ct=1.2), continuously heated greenhouse (Ct=0.85).
Importance Factor — Residential (Is=1.0), commercial (Is=1.1), essential facilities (Is=1.5).

Output: flat roof snow load (pf), sloped roof snow load (ps), total load in pounds on your entire roof area, and maximum snow accumulation depth before structural risk.

The Formula (ASCE 7)

Flat Roof Snow Load: pf = 0.7 × Ce × Ct × Is × Pg
Slope Factor Cs: based on pitch (per ASCE 7 Fig. 7.4-1)
Sloped Roof Load: ps = Cs × pf

Where:
Ce = Exposure Factor (0.8 exposed → 1.2 sheltered)
Ct = Thermal Factor (0.85 heated greenhouse → 1.3 unheated)
Is = Importance Factor (1.0 residential → 1.5 essential)
Pg = Ground Snow Load (from ASCE 7 map, psf)

Total Roof Load (lbs) = ps × Roof Area (sq ft)

Example: Minneapolis House, 6/12 Pitch

Example: Minneapolis area (Pg = 50 psf), 6/12 pitch, partially exposed, heated, residential

Ground Snow Load (Pg)50 psf

Exposure Factor (Ce)1.0 (partially exposed)

Thermal Factor (Ct)1.0 (heated)

Importance (Is)1.0 (residential)

Flat Roof Load (pf)0.7 × 1.0 × 1.0 × 1.0 × 50 = 35 psf

Slope Factor (6/12)Cs = 1.0 (warm roof, 6/12 is below sliding threshold)

Design Roof Snow Load35 psf

1,500 sq ft roof total load35 × 1,500 = 52,500 lbs (26.25 tons)

This 35 psf design load means your roof structure must be designed to support at least 35 lbs per square foot of roof area from snow accumulation. This is why Minneapolis homes have heavier framing than Florida homes.

Frequently Asked Questions

Most residential roofs are designed for 20–40 psf snow load. This equals roughly 2–4 feet of fresh dry snow. Wet heavy snow can exceed structural limits at 1–2 feet. If you're unsure of your roof's design load, consult a structural engineer — never guess on this.

Yes. Per ASCE 7, steep roofs get a reduction via the slope factor (Cs). For unobstructed smooth metal or glass surfaces, Cs reaches 0 at about 70° (which means no snow load). For typical shingle roofs, meaningful reduction starts above 5/12 pitch. The calculator applies the correct Cs for your roof type and pitch.

Ground snow load (Pg) is the 50-year recurrence snow accumulation on the ground at your location. Roof snow load is less than ground snow load because roofs lose heat (melting), are exposed to wind (drift redistribution), and have slopes that allow snow to slide. ASCE 7 uses a 0.7 conversion factor from ground to flat roof load.

Yes. Unheated structures (garages, barns, unheated storage) use a thermal factor of Ct = 1.2, which increases the design snow load by 20% compared to a heated building. Continuously cold buildings (freezer buildings, certain greenhouses) use Ct = 1.3.

Roof snow removal is dangerous. Use a roof rake from the ground when possible. If you must go on the roof, wait for daytime, never work alone, use non-slip boots and a safety harness, and be aware that loosened snow can slide suddenly. Consider hiring a professional for steep roofs or heavy accumulations.

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Sources & References

The formulas, code requirements, and reference data used in this calculator are sourced from the following authoritative organizations and publications. Local codes vary — always verify with your jurisdiction's building department.

International Residential Code (IRC) 2024 — Chapter 9: Roof Assemblies — ICC
NRCA Roofing Manual (Steep-Slope & Low-Slope) — National Roofing Contractors Association
Asphalt Shingle Installation Manual — ARMA (Asphalt Roofing Manufacturers Association)
ASCE 7-22 — Minimum Design Loads & Associated Criteria — American Society of Civil Engineers
AWC Span Tables for Joists & Rafters — American Wood Council
IRC Chapter 8 — Roof-Ceiling Construction — ICC

Disclaimer: Calculator results are estimates intended for planning and educational purposes only. They do not replace professional engineering analysis, building permit review, or contractor estimates. Always consult a licensed roofing contractor or structural engineer for final specifications.