What is ground snow load (pg)?

Ground snow load is the weight of snow on the ground at a specific geographic location, expressed in psf or kPa. Values are published in ASCE 7 ground snow load maps and local building codes. For example, many parts of New England have pg = 40–60 psf.

How is flat-roof snow load (pf) calculated?

pf = 0.7 × Ce × Ct × Is × pg. The 0.7 factor converts ground snow to roof snow by accounting for wind scour and other reductions. Ce is the exposure factor, Ct is the thermal factor, and Is is the importance factor.

What is the slope reduction factor (Cs)?

Cs reduces the snow load on sloped roofs because snow slides off steeper surfaces. For warm roofs (Ct ≤ 1.0) Cs = 1.0 for slopes up to 30°, then linearly decreases to 0 at 70°. For cold roofs (Ct = 1.1) the threshold shifts to 37.5°, and for freezer roofs (Ct = 1.3) to 45°.

What is the minimum roof snow load?

ASCE 7 specifies that for slopes under 15° (roughly 3.2:12), the balanced roof snow load must not be less than Is × pg. This prevents under-design on nearly flat roofs where snow can accumulate heavily.

Which exposure factor should I use?

Use Ce = 0.9 for fully exposed roofs in windy areas with no obstructions, Ce = 1.0 for partially exposed roofs in typical suburban settings, and Ce = 1.2 for sheltered roofs surrounded by trees or taller structures that block wind.

Does this calculator handle drift or unbalanced loads?

No. This tool calculates balanced (uniform) roof snow load only. Drift loads at parapets, step roofs, and valleys, as well as unbalanced loads on gable and hip roofs, require additional analysis per ASCE 7 Sections 7.7–7.9 and are beyond the scope of this calculator.

What does 30 lb (or 40 lb) snow load mean?

A 30 psf or 40 psf snow load means the roof is designed to carry 30 or 40 pounds of snow per square foot of roof area. About 1 inch of fresh snow weighs roughly 1–2 psf, and packed or wet snow can reach 5–20 psf per inch, so 30 psf roughly equates to 5–15 inches of dense snowpack uniformly distributed across the roof.

Snow Load Calculator

Calculate the design roof snow load from ground snow load, roof slope, exposure, thermal condition, and occupancy factors per ASCE 7 methodology.

Snow Load Inputs

Ground Snow Load (pg) (psf)

From ASCE 7 ground snow load map or local code.

Exposure Factor (Ce)

Thermal Factor (Ct)

Risk/Importance Factor (Is)

Roof Slope (degrees)

Enter 0 for flat roofs. Use slope angle, not pitch ratio.

Results

Enter values and press Calculate

ASCE 7 Factor Reference

Factor	Value	Condition
Ce	0.9	Fully exposed
Ce	1.0	Partially exposed
Ce	1.2	Sheltered
Ct	1.0	Heated structure
Ct	1.1	Heated, well-insulated
Ct	1.2	Unheated structure
Ct	1.3	Cold storage / freezer
Is	0.80	Risk Category I
Is	1.00	Risk Category II
Is	1.10	Risk Category III
Is	1.20	Risk Category IV

Summary

This free online snow load calculator estimates the flat-roof snow load and sloped-roof design snow load for a structure based on ground snow load (pg), roof slope, exposure factor (Ce), thermal factor (Ct), and importance factor (Is). The calculation follows the ASCE 7 / IBC methodology widely used in structural engineering. Results help verify that roof framing is adequate for local snow accumulation.

How it works

Enter the ground snow load (pg) for your location in psf (pounds per square foot) or kPa.
Select the roof exposure category: fully exposed, partially exposed, or sheltered.
Select the thermal condition: heated building, unheated structure, or cold freezer roof.
Select the occupancy/risk category (I through IV) to determine the importance factor.
Enter the roof slope angle in degrees (0 for flat roofs).
Click Calculate to get the flat-roof snow load (pf) and the sloped-roof design snow load (ps).

Use cases

Verifying roof framing capacity before adding rooftop equipment in a snowy climate.
Estimating design loads for new residential or commercial construction.
Converting published ground snow load maps into a usable roof design load.
Comparing how slope and exposure affect the effective snow load on a roof.
Checking whether a low-slope roof meets minimum balanced-load requirements.
Educational reference for students learning structural load calculations.
Quick feasibility check during early schematic design of a building.

Frequently Asked Questions

Last updated: 2026-05-25 · Reviewed by Nham Vu