Snow Load Calculator
A planning estimate from the ASCE 7-22 balanced snow-load equation: flat-roof load = 0.7 × Ce × Ct × ground load, × the slope factor Cs. A 40 psf Risk Category II ground load on a heated, partly exposed 4-in-12 shingle roof gives 28 psf. Drift and unbalanced loads are not checked. The Roof Pitch Calculator gives pitch in degrees.
How it's calculated: pf = 0.7 · Ce · Ct · pg; ps = Cs · pf; below 15° the minimum pm = pg, capped at 25–40 psf by Risk Category (balanced load only — drift, sliding and unbalanced loads are not included).
Estimate only — this is not an engineering sign-off
This tool applies a published code equation or standard beam formula to the numbers you enter. It does not know your building, your local code amendments, your soil, your connections, or your load path. The result is a planning estimate for sizing conversations and material take-offs only.
- Every design value (loads, allowable stresses, factors) is your input — a wrong value in gives a wrong answer out.
- Balanced load from one equation only — drift, sliding, unbalanced and rain-on-snow loads are not calculated and often control.
- pg and all factors are your inputs from ASCE 7 / local code; a mis-read map value invalidates the result.
- No load path, connection, lateral, uplift, bearing, notch, hole, fire, or serviceability check beyond the one stated formula.
A licensed engineer or your local building official must review and stamp the final design before you build or order structural members.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Flat-roof snow load: pf = 0.7 · Ce · Ct · pg
Slope factor Cs from ASCE 7-22 Fig 7.4-1 (depends on slope, Ct and whether the surface is slippery)
Sloped-roof (balanced) snow load: ps = Cs · pf
Minimum (roof slope < 15°): pm = pg, but not more than 25 / 30 / 35 / 40 psf for Risk Category I / II / III / IV
Run the calculation to see each step with your own numbers.
pg = 40 psf (Risk Category II), partially exposed (Ce 1.0), heated (Ct 1.0), 4-in-12 (18.4°) asphalt-shingle roof.
pf = 0.7 × 1.0 × 1.0 × 40 = 28.0 psf
Slope 18.4° ≤ 30° cutoff (warm, non-slippery) → Cs = 1.0
ps = 1.0 × 28.0 = 28.0 psf; minimum does not apply (slope ≥ 15°) → design ≈ 28.0 psf balanced
- ASCE/SEI 7-22 Chapter 7, balanced snow load on the main roof only.
- Does NOT compute: drifting or sliding snow, unbalanced/gable loads, partial loading, rain-on-snow surcharge, or snow on lower roofs and against parapets — all of which can govern and are required by the code.
- pg must be the site-specific ground snow load for the building's Risk Category from the ASCE Hazard Tool or the locally adopted value — some regions have 'CS' (case study) zones with no mapped value. ASCE 7-22 has no separate snow importance factor.
- The Cs slope factor here follows Fig 7.4-1; verify the curve for your Ct and surface, and check whether ice-dam or cold-roof provisions apply.
- The low-slope minimum is checked for roof slopes under 15° only; ASCE 7-22 treats it as a separate uniform load case and also applies it to curved roofs rising less than 10° from eave to crown.
- Is this the only snow load I need to check?
- No. The balanced load from this equation is the starting point. Drift loads at roof steps and parapets, unbalanced loads on gable roofs, sliding snow from an upper roof, and rain-on-snow surcharge frequently govern the design and are all required by ASCE 7. An engineer must evaluate them.
- Where do I get the ground snow load?
- The ASCE 7 Hazard Tool (ascehazardtool.org) gives a site-specific value for each Risk Category, or your local building department publishes an adopted figure. Don't read it off an old national map — values were revised significantly in ASCE 7-22.
- How do you calculate roof snow load?
- Start from the site's ground snow load pg for the building's Risk Category, multiply by 0.7 and the exposure and thermal factors for the flat-roof load, then by the slope factor Cs. A 40 psf ground load with both factors at 1.0 gives 28 psf. An engineer must check drift and unbalanced loads.
- What is the difference between ground snow load and roof snow load?
- Ground snow load (pg) is the mapped load on the ground at your site, given separately for each Risk Category in ASCE 7-22. The roof load is usually lower: the calculator multiplies pg by 0.7 and the exposure and thermal factors, then by a slope factor.
Method
ASCE/SEI 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Chapter 7 (Snow Loads): Eq. 7.3-1 (pf = 0.7·Ce·Ct·pg), Table 7.3-1 (Ce), Table 7.3-2 (Ct), §7.3.3 and Table 7.3-4 (minimum snow load for low-slope roofs by Risk Category), §7.4 and Fig. 7.4-1 (Cs). Ground snow loads are mapped for each Risk Category, so ASCE 7-22 has no snow importance factor. Referenced by IBC §1608.
About the Snow Load Calculator
The Snow Load Calculator applies the ASCE 7-22 Chapter 7 method to the ground snow load and factors you enter, returning the flat-roof load, the slope-reduced load and the code minimum. It is a planning estimate for sizing conversations only — it computes the balanced load from one equation and does not evaluate drift, sliding, unbalanced or rain-on-snow loads, which frequently govern and are an engineer's responsibility.
What you enter
- Ground snow load pg (psf) — ASCE 7-22 maps a separate ground snow load for each Risk Category — use the value for the Risk Category selected below, from the ASCE Hazard Tool or your jurisdiction's adopted value. psf.
- Exposure factor Ce (Table 7.3-1) — Fully / partially exposed / sheltered — terrain B: 0.9 / 1.0 / 1.2; C: 0.9 / 1.0 / 1.1; D: 0.8 / 0.9 / 1.0; above the tree line or treeless Alaska: 0.7 / 0.8.
- Thermal factor Ct (Table 7.3-2)
- Risk Category (Table 1.5-1) — Caps the low-slope minimum load (Table 7.3-4). It does not multiply pf — in ASCE 7-22 the Risk Category is already built into the mapped pg.
- Roof slope (degrees) — Roof angle in degrees. A 4-in-12 pitch ≈ 18.4°.
- Roof surface
What you get back
- Flat-roof snow load pf — psf. pf = 0.7 · Ce · Ct · pg
- Roof slope factor Cs — the exact result of the calculation.
- Sloped-roof snow load ps — psf. ps = Cs · pf
- Minimum roof snow load — psf. Applies where roof slope < 15°. pm = pg, capped at 25 / 30 / 35 / 40 psf for Risk Category I / II / III / IV.
- Governing design roof snow load — psf. The larger of ps and (where it applies) the minimum. Balanced load only.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
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- Rafter Length Calculator — Common rafter length from the building span and roof pitch.
Part of the Roofing calculators.