Snow Load Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
Flat roof snow load is pf = 0.7 × Ce × Ct × Is × pg. With a 40 psf ground load, standard exposure and a heated building, pf is 28 psf. Multiply by the slope factor Cs to get the sloped roof load, which drops as pitch increases.
Your measurements
Enter the values you know — results update live.
From your local building department or the ASCE 7 hazard tool. Do not guess this value.
Steeper roofs shed snow, which reduces the design load.
1.0 for partially exposed, 0.9 fully exposed, 1.2 sheltered by trees or taller buildings.
1.0 for a heated building, 1.1 unheated with insulation, 1.2 for an unheated structure.
1.0 for houses (Risk Category II), 1.1 for assembly buildings, 1.2 for essential facilities.
Analysis
Live results from your measurements.
First-pass check only. Confirm the ground snow load with your building department before designing anything.
Show the working
- Flat roof load:
0.7 × 1 × 1 × 1 × 40= 28 psf - Roof angle:
arctan(6 ÷ 12)= 26.57° - Slope factor:
Cs for 26.6° warm roof= 1 - Sloped load:
28 × 1= 28 psf - Total weight:
28 × 1,600= 44,800 lb
What this calculator does
Snow load is the downward pressure snow puts on a roof, measured in pounds per square foot. ASCE 7 starts from a mapped ground snow load for your location, then adjusts it for how exposed the roof is to wind, whether the building is heated, and how critical the structure is. The result, pf, is the flat roof load. A slope factor then reduces it on steeper roofs, because snow slides off rather than accumulating. Drifting against walls, dormers and parapets can push local loads far above these numbers and is handled separately in the code.
How to use it
- Look up the ground snow load (pg) for your exact address from the ASCE 7 hazard tool or your building department.
- Choose the exposure factor Ce based on how wind-swept the site is.
- Choose the thermal factor Ct based on whether the building below is heated.
- Choose the importance factor Is from the building's risk category.
- Multiply 0.7 × Ce × Ct × Is × pg to get the flat roof snow load pf.
- Apply the slope factor Cs for your roof angle to get the sloped roof load ps.
The formula
Flat roof snow load
pf = 0.7 × Ce × Ct × Is × pg
ASCE 7 Equation 7.3-1. The 0.7 accounts for snow blowing off a roof relative to open ground.
Sloped roof snow load
ps = Cs × pf
Cs is 1.0 up to 30 degrees for a warm roof, then reduces linearly to zero at 70 degrees.
Total roof weight
total weight = ps × roof plan area
Snow load acts on the horizontal projection, so use plan area rather than sloped area.
Worked example
A heated house in a 50 psf ground snow load zone, partially exposed, 8:12 roof, 1,800 sq ft plan area.
- pf = 0.7 × 1.0 × 1.0 × 1.0 × 50 = 35 psf
- Roof angle: arctan(8 ÷ 12) = 33.69°
- Cs = 1 − (33.69 − 30) ÷ 40 = 0.908
- ps = 35 × 0.908 = 31.8 psf
- Total: 31.8 × 1,800 = 57,200 lb
About 31.8 psf on the sloped roof — some 28.6 tons of snow across the whole roof at design load.
Reference chart
| Pitch | Angle | Slope factor Cs | ps at 35 psf pf |
|---|---|---|---|
| 2:12 | 9.46° | 1 | 35 psf |
| 4:12 | 18.43° | 1 | 35 psf |
| 6:12 | 26.57° | 1 | 35 psf |
| 8:12 | 33.69° | 0.908 | 31.8 psf |
| 10:12 | 39.81° | 0.755 | 26.4 psf |
| 12:12 | 45° | 0.625 | 21.9 psf |
| 14:12 | 49.4° | 0.515 | 18 psf |
| 18:12 | 56.31° | 0.342 | 12 psf |
| 24:12 | 63.43° | 0.164 | 5.7 psf |
Snow density — depth needed to reach a given load
| Snow type | Density | Depth for 30 psf |
|---|---|---|
| Fresh dry snow | 5–8 lb per cu ft | 45–72 in |
| Settled snow | 12–18 lb per cu ft | 20–30 in |
| Wet packed snow | 20–30 lb per cu ft | 12–18 in |
| Ice | 57 lb per cu ft | 6.3 in |
Common mistakes
- Using a state-wide ground snow load. Values change sharply with elevation — always use the address-specific figure.
- Ignoring drift loads against walls, parapets and dormers, which can be several times the balanced load.
- Applying the load to sloped area. Snow load acts on the horizontal plan projection.
- Assuming a steep roof sheds all snow. The slope reduction only begins at 30 degrees and needs an unobstructed slippery surface.
- Forgetting rain-on-snow surcharge, which the code adds in low-slope, low-ground-load regions.
- Treating this calculation as a structural design rather than a first-pass check.
Assumptions and limits
- Warm roof slope factor curve (Ct ≤ 1.0), unobstructed surface, no snow guards.
- Balanced snow load only — drift, sliding and unbalanced cases are not included.
- No rain-on-snow surcharge applied.
- The ground snow load you enter is the governing local value; the tool does not supply one.
Frequently asked questions
How do I calculate roof snow load?
Multiply the ground snow load by 0.7 and by the exposure, thermal and importance factors to get the flat roof load. Then apply the slope factor for your roof angle to get the sloped roof load.
How much does snow weigh per square foot?
Fresh dry snow weighs about 0.5 pounds per square foot per inch of depth. Wet packed snow can reach 1.7 pounds per inch, so 12 inches of wet snow is roughly 20 pounds per square foot.
What roof pitch sheds snow best?
Snow sheds reliably above about 6:12, and the ASCE 7 slope reduction begins at 30 degrees, which is roughly 7:12. Metal surfaces shed at lower pitches than shingles because they are slipperier.
Where do I find my ground snow load?
Use the ASCE 7 Hazard Tool for your exact address, or ask your local building department. Many jurisdictions publish a locally amended value that overrides the national map.
What is a drift load?
Snow blown against a wall, parapet or roof step piles far deeper than the balanced load. ASCE 7 Chapter 7 has a separate procedure for it, and it often governs the design near those features.
Should I rake snow off my roof?
Only if accumulation approaches the design load or ice dams are forming. Rake from the ground with a roof rake — climbing an icy roof is far more dangerous than the snow itself.
Last updated 2026-08-05. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.