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BuyRoof Snow Load Guide: Ground Snow, Roof Snow, and Where to Look Up pg
Ground snow load is a mapped site value. Roof snow load is that value after exposure, heat, occupancy, and slope. They are not interchangeable
The roof snow load calculator runs ASCE 7-16 Equations 7.3-1 and 7.4-1. This page is what those symbols mean and where the ground number actually comes from. Rafters and beams that have to carry it: joist span calculator and deck beam span calculator.
Sag and cracking: troubleshooting. Why using pg as the roof load is a real mistake: comparison. Hub: roofing calculators.
Have a mapped pg and a pitch?Flat-roof pf, slope factor Cs, governing balanced psf.
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Ground Snow Load pg
Ground snow load is the mapped weight of snow on the ground at the site, in pounds per square foot. IRC Table R301.2 (Climatic and Geographic Design Criteria) is where most residential permits print it. ASCE 7-16 Figure 7.2-1 and Tables 7.2-1 through 7.2-8 are the engineering maps behind that cell. It is a site value, not a roof value, and it is not the same in two towns 20 miles apart in the mountains.
Case-study (CS) zones on the 7-16 map have no printed number because terrain changes the load too fast. Those jobs need local climatic data or a site-specific study. ASCE 7-22 replaced the single 50-year map with four risk-category maps at strength level. If your Hazard Tool output is 7-22, do not drop it into the 7-16 formula the calculator uses.
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Snow-Load Gear
The calculator returns a balanced roof snow load in psf. A roof rake is how you take snow off an existing deck from the ground. CSV rows 400–412 were nailers, vents, gutters, and ice barrier — not snow-removal tools — so these two SKUs are new.
Goplus 21 ft Aluminum Snow Roof Rake
21 ft extendable aluminum rake with wheels and oxford slide. Size the balanced roof snow load first; this is how you take snow off the deck from the ground — not a substitute for an engineer.
View on Amazon →
Garelick 89421 21-Foot Aluminum Snow Roof Rake
24 in blade with shingle-saver rollers so the edge never rides the asphalt. Relieving weight is not the same as designing for it — run the snow-load number before you argue with a sagging ridge.
View on Amazon →
Run the Balanced Snow Load
Look up pg, then Ce, Ct, Is, and pitch. Flat-roof pf, slope factor Cs, governing psf. Planning estimate only.
Open Roof Snow Load Calculator →
Joists Under That Load
IRC joist tables assume 40 psf live. A roof or covered deck in snow country is a different load path — size the snow first.
Open Joist Span Calculator →
Beams in Snow Country
Table R507.5(1) is 40/10. Tables R507.5(2)–(4) are snow rows this tool does not replace. Convert pg to roof snow, then talk to the table or an engineer.
Open Deck Beam Span Calculator →As an Amazon Associate, TestTalkHQ earns from qualifying purchases.
Roof Snow Load Is a Conversion
ASCE 7-16 Equation 7.3-1 converts ground snow to a flat-roof snow load:
pf = 0.7 × Ce × Ct × Is × pg
The 0.7 is the ground-to-roof conversion in that equation (wind erosion and ordinary heated buildings), not the ASD load-combination factor that also happens to be 0.7 in 7-22. Ce is exposure (Table 7.3-1). Ct is heat through the roof (Table 7.3-2). Is is occupancy (Table 1.5-2; 1.00 for Risk Category II houses).
Then Equation 7.4-1: ps = Cs × pf, applied on the horizontal projection of the slope. That is the balanced load the calculator prints (after the low-slope minimum when it applies).
How to Find Your Actual Ground Snow Load
- Ask the building department. The number on Table R301.2 for your jurisdiction is the one the inspector will use. Some counties publish it on the permit handout.
- ASCE 7 Hazard Tool at ascehazardtool.org — address lookup. Match the edition your code adopted (7-16 vs 7-22). 7-22 values are not drop-in replacements for this calculator.
- Do not Google a city average and call it done. Elevation, lake-effect, and CS zones will lie to you.
- Covered decks and porches still see snow. The 40 psf live-load cell in the joist and beam tools is not automatically conservative in snow country.
How Pitch Reduces Roof Snow Load
Steeper roofs shed. Figure 7.4-1 keeps Cs = 1.0 until a breakpoint that depends on thermal factor and whether the surface is unobstructed slippery, then falls linearly to 0 at 70° (a roof that is basically a wall). Asphalt shingles on a heated house use the “other” warm-roof curve: Cs = 1.0 until 30° (about 7/12), then down. A 6/12 (26.6°) does not get a slope reduction. A 12/12 (45°) does.
Worked 40 psf ground, heated house, partial exposure, 6/12Ce = 1.0, Ct = 1.0, Is = 1.0. pf = 0.7 × 40 = 28 psf. α = 26.6° < 30° so Cs = 1.0. ps = 28 psf. Same arithmetic the calculator prints. A 12/12 on the same house: Cs = (70−45)/(70−30) = 0.625, ps = 17.5 psf.
Snow that stays also dams.A cold eave with a warm attic is an ice-dam problem, not a psf problem. Membrane width: ice & water shield coverage calculator.
FAQ
Can I skip pg and type 30 because “that’s typical”?
No. The calculator leaves the field empty on purpose. Typical is how roofs get under-designed.
Does a steep roof mean zero snow load?
Only as Cs approaches 0 near 70°. A 12/12 still carries a real load. Unbalanced loading on gables is extra and is not in this tool.
Is this the number I put on a joist span table?
It is context. The IRC 40 psf live cells are a different assumption. Snow-country deck beams have their own tables. Talk to those tools and, outside prescriptive rows, an engineer.