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Drifting Snow Load Questions

Drifting Snow Load Questions

Drifting Snow Load Questions

Hi everyone,

When you have a chance, could you please take a look at my following questions and attached sketches (2 PDF files), and let me know your thought/ comments.

• Code: ASCE 7-10
• Ground snow load: 50 psf
• Ce = 1.0
• Ct = 1.0
• Is = 1.0
• Cs = 0.75 for 10:12 roof
• Cs = 1.0 for 7:10 roof
• Height difference between upper and lower roofs: 8’
• Rafter system roofs
• Sliding snow is not considered in this discussion/ calc

1. Lower roofs of Building 1, 2, 3 & 5 received same amount of drifting snow load regardless the pitch of upper or lower roofs. Does this make sense?

2. Building 3: Is it ok to use lu = 12’ (distance from ridge to eave) for drifting (leeward) snow load on lower roof (due to high pitch roof; >7:12)

3. Building 4: How one would combine drifting/ unbalanced SL for design of member (say beam/ header) at the common wall?

4. Building 6- Balanced snow load: Is balanced snow load (equal flat roof SL) within valley area ok?

5. Building 6 - Unbalanced snow load: Please comments

We have a copy of “Guide to the Snow Load Provisions of ASCE 7-05” by O’Rourke
Please point me to additional reference if you know.

Thank you,


RE: Drifting Snow Load Questions

1. Yes - the drifting is based on the horizontal width of the roofs either windward or leeward.
2. I'd say no, you can't discount the upper roof on the other side of the ridge per ASCE 7.

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RE: Drifting Snow Load Questions


Thank you for your comments/ answers.

1. I agree with you about how the drifting snow should be calculated per ASCE 7. However, I still have a hard time making sense of it when comparing the calculated drifting snow loads of building with very steep (say 12:12) roof and that with flat roof :)dazed

2. I would agree with you if we use ASCE 7-05, which requires unbalanced snow on leeward side of the gable regardless of roof pitch. in ASCE 7-10, unbalanced snow load is not required for roof with slope > 30.2deg (or 7:12)

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