Snow Load Calculator (ASCE 7-22 Roof Snow Load)
Calculate flat roof and sloped roof snow loads using the ASCE 7-22 Chapter 7 method. Enter your ground snow load, exposure category, and roof slope to get a design snow load in psf, with minimum load checks and rain-on-snow surcharge flags built in.
Roof Snow Load Calculator
Based on ASCE 7-22 Equation 7.3-1 and Section 7.4 slope factor tables.
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View Chart →How the ASCE 7-22 Snow Load Method Works
Start with Ground Snow Load
Pull pg from your local building department or ASCE 7-22 ground snow load maps, Figures 7.2-1A to 7.2-1D.
Apply Exposure and Thermal Factors
Multiply by Ce (Table 7.3-1) for terrain exposure and Ct (Table 7.3-2) for roof heating condition.
Get the Flat Roof Load
ASCE 7-22 Eq. 7.3-1: pf = 0.7 x Ce x Ct x pg, then check the Section 7.3.4 minimum.
Convert to Sloped Roof Load
Apply the slope factor Cs from Figure 7.4-1 based on slope angle and surface type: ps = Cs x pf.
Exposure and Thermal Factor Lookup
These tables list the multipliers this calculator applies automatically based on your Step 2 selections. Use them to sanity-check the output or for manual calculations.
Exposure Factor (Ce) - ASCE 7-22 Table 7.3-1
| Terrain Category | Fully Exposed | Partially Exposed | Sheltered |
|---|---|---|---|
| B (urban/suburban) | 0.9 | 1.0 | 1.2 |
| C (open terrain) | 0.9 | 1.0 | 1.1 |
| D (flat, coastal) | 0.8 | 0.9 | 1.0 |
Thermal Factor (Ct) - ASCE 7-22 Table 7.3-2
| Roof Condition | Thermal Factor Ct | Typical Use Case |
|---|---|---|
| Heated structure | 1.0 | Occupied homes, offices |
| Heated, ventilated roof | 1.1 | Cold attic space above insulation |
| Unheated structure | 1.2 | Detached garages, barns |
| Open air structure | 1.2 | Carports, pavilions |
Minimum Roof Snow Load - ASCE 7-22 Section 7.3.4
| Ground Snow Load (pg) | Minimum Flat Roof Load | Applies To |
|---|---|---|
| pg ≤ 20 psf | pg (no reduction below ground value) | Low-slope roofs, monoslope < 15 degrees, hip/gable < 2.38 degrees |
| pg > 20 psf | 20 psf minimum | Same roof types as above |
Why Roof Snow Load Differs from Ground Snow Load
The 0.7 Reduction Factor
ASCE 7-22 Eq. 7.3-1 applies a baseline 0.7 multiplier because wind typically removes some snow from most roofs before it can accumulate to ground-level depths.
Snow Density Increase
Snow density in pcf increases with ground snow load, per the ASCE 7-22 drift and unbalanced load formulas, capped at 30 pcf.
Slope Factor Range
Cs ranges from 1.0 on flat roofs down to 0 near 70 degrees, since snow slides off steep slopes faster than it accumulates.
Rain-on-Snow Surcharge
Added per ASCE 7-22 Section 7.10 on low-slope roofs when pg is 20 psf or less, accounting for rain saturating snowpack and adding weight.
Who Uses This Snow Load Calculator
Structural Designers
Get a fast preliminary roof snow load before running full ASCE 7-22 load combinations in design software.
General Contractors
Verify that a proposed roof framing package meets the minimum snow load called out on permit drawings.
DIY Builders
Check whether a shed, carport, or deck roof addition needs a heavier structural design before buying lumber.
Building Officials
Cross-check a submitted snow load calculation against the ASCE 7-22 factors during plan review.
Sample Calculation Scenarios
Denver Suburb, Gable Roof
Ground snow load: 30 psf
Exposure: Category B, Partially Exposed (Ce = 1.0)
Roof: Heated home, 4:12 pitch (18.4°), asphalt shingles
Cs (non-slippery, Ct=1.0, 18.4°) ≈ 1.0
ps = 21.0 psf
At this slope and exposure, the sloped roof load equals the flat roof load since Cs stays near 1.0 until slopes exceed roughly 30 degrees on warm roofs.
Coastal Maine, Low-Slope Addition
Ground snow load: 60 psf
Exposure: Category D, Fully Exposed (Ce = 0.8)
Roof: Heated, 1° slope, membrane roofing
Minimum check (pg > 20): 20 psf floor, 33.6 psf governs
Rain-on-snow: pg = 60 > 20, surcharge not triggered
ps = 33.6 psf
Fully exposed coastal sites get a lower Ce, partially offsetting the high ground snow load common in this region.
Unheated Detached Garage, Vermont
Ground snow load: 50 psf
Exposure: Category B, Sheltered (Ce = 1.2)
Roof: Unheated, 6:12 pitch (26.6°), metal roofing
Cs (slippery, Ct=1.2, 26.6°) ≈ 0.85
ps ≈ 42.8 psf
Unheated structures use Ct = 1.2, raising the flat roof load, but slippery metal roofing partially offsets that with a lower Cs at moderate slopes.
Common Mistakes in Snow Load Calculations
Using an Outdated or Wrong Ground Snow Load
Local amendments to ASCE 7-22 sometimes require a higher pg than the published map value, especially at elevations above the map's stated limit. Confirm the required value with your building department rather than reading it directly off a national map.
Applying Is from ASCE 7-16 in an ASCE 7-22 Project
ASCE 7-22 removed the importance factor Is from Equation 7.3-1. Projects using ASCE 7-16 or earlier still need Is applied per Table 1.5-2 of that edition; mixing the two produces an incorrect flat roof load.
Forgetting the Minimum Snow Load Check
Section 7.3.4 sets a floor on flat roof snow load for low-slope roofs, monoslope roofs under 15 degrees, and hip/gable roofs under 2.38 degrees. Skipping this check can understate the load on very low-pitch designs.
Ignoring Unbalanced Loads on Hip and Gable Roofs
Per Section 7.6.1, hip and gable roofs between 1/2:12 and 7:12 pitch need an unbalanced load check since wind redistributes snow toward the leeward slope. This governs the design of many rafters and trusses even when the balanced load looks acceptable.
Overlooking Drift Loads Near Parapets and Adjacent Roofs
Section 7.7 and 7.8 require drift surcharge checks near roof steps and parapet walls. These localized loads can exceed the balanced snow load by a wide margin and are not captured by this calculator's simplified output.
Code Requirements and Permit Context
IBC 2024 Section 1608 adopts ASCE 7-22 by reference for snow load determination on all buildings within its scope. Most jurisdictions require the calculated roof snow load to appear on structural drawings submitted for permit, along with the ground snow load source and risk category assumed.
Truss manufacturers and engineered lumber suppliers size roof framing to the governing load combination, which includes snow load per ASCE 7-22 Chapter 2 combinations such as 1.2D + 1.6S. A snow load that looks acceptable in isolation can still fail once combined with dead load, wind, or seismic load per the applicable combination.
For additions, garages, and accessory structures, confirm whether your jurisdiction enforces the full ASCE 7-22 provisions or a locally amended snow load table. Some mountain counties and ski resort municipalities require ground snow loads well above the national map value due to microclimate effects.
Frequently Asked Questions
Ground snow load (pg) is measured on the ground and published in ASCE 7-22 Figures 7.2-1A to 7.2-1D. Roof snow load (pf) is calculated from pg using Equation 7.3-1: pf = 0.7 x Ce x Ct x pg, since wind removes some snow from most roof surfaces.
Local building departments typically list the required ground snow load in their permit checklists, since ASCE 7-22 map values vary by county and elevation per Table 7.2-1. Confirm the exact value before finalizing structural drawings, especially at elevations above the map's stated limit.
Cs, defined in ASCE 7-22 Section 7.4 and Figure 7.4-1, reduces the flat roof load as slope increases because snow slides off steeper surfaces. Warm, slippery roofs start reducing around 5 degrees and reach zero near 70 degrees. Cold or obstructed roofs use a less aggressive reduction curve.
Section 7.6.1 requires unbalanced load checks on hip and gable roofs with slopes between 2.38 degrees (1/2:12) and 30.2 degrees (7:12). Wind redistributes snow toward the leeward slope, which can govern the design of individual rafters or trusses even when the balanced load appears acceptable.
ASCE 7-22 removed the importance factor Is from the flat roof snow load equation, so it reads pf = 0.7 x Ce x Ct x pg instead of the ASCE 7-16 version that included Is. Risk category effects are now built into the updated ground snow load maps themselves rather than a separate multiplier.
Parts of the southern United States show 0 psf on ASCE 7-22 ground snow maps. Even then, most jurisdictions still require a minimum roof live load, often 20 psf on low-slope roofs, since flat roofs must support maintenance and construction loads regardless of snowfall history.
Section 7.10 requires a 5 psf surcharge added to flat roof snow load when pg is 20 psf or less and the slope is less than W/50, unless pf already exceeds pg. This mainly affects very low-slope roofs in regions with light-to-moderate snowfall and higher rain frequency.
Sources and Methodology
- ASCE/SEI 7-22, "Minimum Design Loads and Associated Criteria for Buildings and Other Structures," Chapter 7, Sections 7.2-7.10 (flat roof, sloped roof, and drift snow load provisions).
- International Building Code (IBC) 2024, Section 1608, "Snow Loads," which adopts ASCE 7-22 by reference.
- StructureMag, "Ground Snow Loads for ASCE 7-22" (2022), summarizing map revisions and the removal of the importance factor from Equation 7.3-1. structuremag.org
- American Wood Council, "Snow Provisions in ASCE 7-05" technical guide, referenced for historical thermal factor and slope factor context still consistent with current editions.
- Medeek Design Inc., residential snow load methodology summary, cross-referenced for the standard calculation sequence used by this tool. design.medeek.com
Last reviewed: July 2026. Formulas and factor tables checked against ASCE 7-22 Chapter 7 as published.
Engineering Disclaimer
This calculator provides estimates for planning purposes. For permitted structural work, foundations, multi-story construction, retaining walls over 4 feet, and commercial projects, calculations must be verified by a licensed structural engineer per IBC 2024 Section 1604. ConcreteCalculate.com is not liable for structural decisions made from these estimates.
Privacy Note
All calculations run in your browser session and are sent only to this tool's processing endpoint to compute results. No project data, ground snow load values, or location details are stored or shared with third parties.
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