Gambrel Roof Snow Load Calculator: Upper and Lower Pitch Loading
ASCE 7 ground-to-roof snow load conversion for both gambrel slopes, plus a drift estimate at the knuckle.
Flat Roof Load pf
21.0 psf
Cs Lower
0.13
Cs Upper
1.00
Lower Design Load
2.6 psf
Upper Design Load
21.0 psf
Drift at Knuckle
10.5 psf
ASCE 7 Formulas
pf = 0.7 · Ce · Ct · I · pgps = Cs · pfpf is the flat-roof snow load; ps is the sloped-roof load for a given pitch. The slope factor Cs is 1.0 below 30° for warm roofs and drops linearly to 0 at 70°. Above 70° the surface is too steep to hold snow under any condition.
Drift Loading at the Knuckle
Snow sliding off the upper slope concentrates at the pitch transition. ASCE 7 Chapter 7 treats this as a leeward drift and requires the rafters within roughly 4 ft of the knuckle to be sized for the surcharge. A simplified drift estimate is half the flat-roof load; engineered designs require the full ASCE 7 calculation.
Design Tip
Do not just take the lower design load and call it done. Size the rafters within about 4 ft of the knuckle for the drift-loaded value, not the flat segment value, since that is where sliding snow from the upper slope actually piles up. Skipping the drift check is the most common under-design mistake on gambrel snow loading.
Frequently Asked Questions
›What is Cs in ASCE 7?
Cs is the roof slope factor, a multiplier that reduces ground snow load to roof snow load based on pitch. Steep slopes shed snow, so Cs decreases as pitch increases. For a warm roof Cs is 1.0 up to 30°, then drops linearly to 0 at 70°.
›Why is gambrel snow load split into upper and lower?
Because the two slopes shed differently. The steep lower slope (60°+) sheds nearly all snow; the shallow upper slope (15–30°) retains it. Design loads must be calculated separately for each segment.
›What is snow drift at the knuckle?
Snow from the upper slope slides and piles up at the pitch transition. ASCE 7 treats this as a drift surcharge and requires designing the rafters near the knuckle for the higher local load. Add roughly 30 psf of drift to the lower-slope design load within 4 ft of the knuckle in moderate-snow regions.