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BuyGround Snow Load vs Roof Snow Load: Why These Are Different Numbers
pg is mapped on the ground. ps is what the rafters see after 0.7, exposure, heat, occupancy, and slope. Using ground snow directly for structural decisions is a common, real mistake
Size the conversion with the roof snow load calculator first. This page is why the two numbers exist and what goes wrong when they get swapped. How to look up pg: guide. Sag vs creep: troubleshooting.
Spans that still assume 40 psf live: joist span calculator, deck beam span calculator. Hub: roofing calculators.
Have pg from the permit?Do not paste it onto a rafter as the roof load. Convert it.
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Why Ground and Roof Are Different
Snow on the ground sits in the wind shadow of the earth. Snow on a roof sees wind, heat from below, and a slope that can shed. ASCE 7-16 therefore starts with mapped pg and applies Equation 7.3-1 (0.7, Ce, Ct, Is) then Equation 7.4-1 (Cs). A heated, partially exposed 6/12 house at 40 psf ground sees 28 psf balanced on the roof — not 40, and not 0.7 × 40 with the other factors ignored.
The other direction matters too. An unheated, sheltered barn (Ct = 1.2, Ce = 1.2) at the same 40 psf: pf = 0.7 × 1.2 × 1.2 × 1.0 × 40 = 40.3 psf. Using 0.7 × 40 = 28 there understates the roof. Using 40 “because that’s the ground” accidentally lands close — and still misses slope, the minimum, and drift.
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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.
The Common, Real Mistake
Someone reads 40 psf on the permit (ground), then sizes rafters or a covered-deck beam as 40 psf roof live load. Two failures:
- Wrong physical quantity. The permit cell is pg. The member sees ps (plus drift where it applies).
- Wrong table. IRC joist/beam dropdowns in the joist and beam calculators are 40 psf live / 10 psf dead. Snow-country deck beam tables are separate rows. Pasting pg into that conversation without converting it, and without checking whether 40/10 still applies, is how a covered porch gets a summer-house beam.
ASCE 7-22 makes the swap even easier to botch: mapped pg is strength-level and risk-category-specific, and Is is gone from pf. Mixing a 7-22 Hazard Tool number with the 7-16 formula is a third edition of the same mistake.
Worked Numbers (Same pg = 40 psf, Is = 1.0)
| Condition | pf | Cs / slope | Governing ps |
|---|---|---|---|
| Heated, partial, shingles, 6/12 | 28 psf | 1.00 at 26.6° | 28 psf |
| Heated, partial, shingles, 12/12 | 28 psf | 0.625 at 45° | 17.5 psf |
| Heated, fully exposed, 6/12 (Ce = 0.9) | 25.2 psf | 1.00 | 25.2 psf |
| Unheated, sheltered, 6/12 | 40.3 psf | 1.00 | 40.3 psf |
| Heated, partial, 2/12, pg = 15 (minimum check) | 9.45 psf | 1.00; pm = 15 | 15 psf (minimum governs) |
That last row is why favorable Ce cannot shrink a low-slope roof below the §7.3.4 minimum. This cluster has no standalone chart page: pg is location-specific, Cs is a continuous function of pitch, and the factor tables already live as radios in the calculator.
FAQ
Is roof snow load always less than ground snow load?
Usually on a heated, exposed house. Not always on an unheated, sheltered, low-slope roof, and never once you add a drift the balanced formula ignores.
Can I use 0.7 × pg and skip the factors?
That is Ce = Ct = Is = 1.0 with no slope reduction. It is a common shortcut for a heated house on a low slope. It is wrong for unheated, sheltered, steep, or low-slope-with-minimum cases. Use the factors.
Does ice barrier change the snow load?
No. Membrane manages water that ponds. Load is still ps. Ice-barrier calculator.