Snow drifts against low roof parapet per ASCE 7-22

[Pratikmokal]

I have a situation with both a low and high roof, each with parapets. Upon reviewing the high roof parapet as a capture wall, I found its height insufficient to withstand the snow drift. According to ASCE 7-22, Figure C7.7-2, I am experiencing leeward snow drift; however, the code does not specifically address drift at the low roof parapet. Could you provide guidance on how to assess snow drift for the low roof parapet based on ASCE 7-22? Thank you.

 

[DaveAtkins]

I think you should use lu = 344 feet for the low roof parapet. I don't know of any literature or code that allows you to reduce the upwind length of roof due to interruption by a parapet.

 

[ANE91]

I think you might be mixing up certain provisions, here. The code certainly addresses WW/LW drifts at all steps/parapets.

Imagine your middle (left) parapet is infinitely tall. In that case, absolutely no snow from the left can “jump” over it, neither to accumulate as a LW drift on the right side of the step, nor to accumulate as a WW drift on the left side of the farther (right) parapet. Let’s now swap the wind direction — no snow from the right can jump over the “capture wall,” but you now have more space available for a WW drift to form on the right side of the step.

Practically, the middle (left) parapet is likely of a height to allow a windward ramp of snow to form, on the left, such that snow can “saltate” over the partial capture wall and contribute to both WW and LW drifts on the lower roof. The same could be true for right-to-left wind, with the added complication of potentially exacerbating WW drifts.

Refer to this recent thread for more guidance:

Thread 'Snow Drift Fetch Length for Stepped Roofs and Parapets'
https://www.eng-tips.com/threads/snow-drift-fetch-length-for-stepped-roofs-and-parapets.528073/

 

[Pratikmokal]

I think you should use lu = 344 feet for the low roof parapet. I don't know of any literature or code that allows you to reduce the upwind length of roof due to interruption by a parapet.

Yes, I couldn't find any code reference for this particular case. So I'll go with a conservative design and take max fetch length of the high roof + low roof. Thanks!

 

[Pratikmokal]

I think you might be mixing up certain provisions, here. The code certainly addresses WW/LW drifts at all steps/parapets.

Imagine your middle (left) parapet is infinitely tall. In that case, absolutely no snow from the left can “jump” over it, neither to accumulate as a LW drift on the right side of the step, nor to accumulate as a WW drift on the left side of the farther (right) parapet. Let’s now swap the wind direction — no snow from the right can jump over the “capture wall,” but you now have more space available for a WW drift to form on the right side of the step.

Practically, the middle (left) parapet is likely of a height to allow a windward ramp of snow to form, on the left, such that snow can “saltate” over the partial capture wall and contribute to both WW and LW drifts on the lower roof. The same could be true for right-to-left wind, with the added complication of potentially exacerbating WW drifts.

Refer to this recent thread for more guidance:

Thread 'Snow Drift Fetch Length for Stepped Roofs and Parapets'
https://www.eng-tips.com/threads/snow-drift-fetch-length-for-stepped-roofs-and-parapets.528073/

Thanks for the reply @ANE91. I guess my question was what fetch length I should be taking to calculate windward snow drift (wind left to right) at low roof parapet when my capture wall at high roof is not tall enough to capture all snow per capture wall commentary in ASCE 7-22, Figure C7.7-2. I believe there should be some reduction in fetch length from the high roof.

 

[ANE91]

Unless you really need the reduction in fetch, just go with 52'+292'=344'. You might be able to justify 15-25% off 292', if you have the patience to follow along with the trapping efficiencies described in this paper: https://ascelibrary.org/doi/abs/10.1061/(ASCE)ST.1943-541X.0002032.