Sliding Snow vs. Drifting Snow

[medeek]

I have a roof layout as shown below:

My thinking was to cover all my bases by applying a sliding snow load to the lower portion of the roof (4:12 pitch portion). However, after reading the paragraph in section 7.9 of the ASCE 7-10 which states: "The sliding snow load shall not be further reduced unless a portion of the snow on the upper roof is blocked from sliding onto the lower roof by snow already on the lower roof.", I am wondering if a sliding snow load should even apply?

I am curious as to how others treat this sort of situation with a duopitch roof.



A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[racookpe1978]

Seems like the steeper upper roof would be subjected to the "dam" caused by the low-slope lower roof. It would be worse if the lower roof edge got an ice dam though.

 

[medeek]

Based on what I have found online with other examples for sliding snow the prevailing thought seems to include a vertical distance between the upper roof and the lower roof as shown in my diagram below and the calculator that I typically use for sliding snow loads:

The question is specifically how or should one apply the sliding snow load provisions when there is no vertical distance between the adjacent roofs. The second paragraph of section 7.9 of the ASCE 7-10 suggests some form of reduction where the lower roof snow blocks the upper roof snow from sliding however it offers no further guidance on how much of a reduction to take.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

This is actually a very interesting problem when you think about it (specifically for duopitch roofs). In the limiting case where the upper and lower roof pitches are equal there is no sliding snow load. However, as you increase the upper roof pitch relative to the lower roof pitch there will be an increased likelihood of snow piling up on the lower roof from the upper roof. Take for instance a typical mountain cabin in a high snow load area with a metal roof. The upper pitch is 12/12 and the porch roof is an 8 ft. span with a 2 ft. overhang and a 4/12 pitch. There is a real danger of snow piling up on the lower roof in my opinion and the engineer should take this into consideration.

Even my own real life experience growing up in Northern B.C. Canada lends some weight to this argument. Our barn (Dad's potato farm) was your typical gambrel roof but with large wings on each side that were added some years later to expand the square footage of the barn:

A couple of the years we had some really heavy snow falls and I remember climbing up into the attic of the lower roof and observing all of the broken rafters where the snow had piled up on the lower roof immediately below the upper roof eaves.

Unlike the current provisions of section 7.9 I think for duopitch roofs there needs to be an equation which not only includes the eave-to-ridge distance of the upper roof but also the pitches of both upper and lower roofs. I don't know what that equation will look like but some fairly basic research and case studies should shed some light on it, actually a great idea from someone's master's thesis. I am usually not in favor of increasing the size of the code or the ASCE 7-10 but I think this might be an area where I would suggest it.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com