Req'd Roof Obstruction Clearance to Prevent Snow Drift 1

[RPMG]

Problem: A duct run is to be placed on a roof of k-joists. The joists have very limited reserve capacity.

Given: The vertical depth of the ducts is 6'-0". Balanced snow height is 14".

Goal: Minimize the height of the duct run (for wind), but provide enough roof clearance that it is not subject to snow drift.

Question: What is the minimum bottom of duct elevation required to avoid snow drift?

I would ask for technical literature, but I don't believe it exists. I am asking for common practice. Guessing, I would say that:
[Air Space] = [6' Obstruction]/[2 T&B]
[Clearance] = [3' Air Space] + [1'-2" Balanced Snow Height]
But I would prefer to lower the elevation further.

 

[thaidavid40]

Excellent question! My first impression is that you would have to account for the balanced snow depth, plus the amount of snow that would fall off of the duct and land below (is it round, or rectangular?). Would it be fair to take a leeward drift depth, based on the width and height of the duct, and add that to the balanced snow depth? In your first equation above, what does the term "T&B" represent?
Dave

Thaidavid

 

[Once20036]

I typically use the balanced snow depth + 2'.
This is based on a webinar I participated in years ago hosted by a gentlemen who did a lot of research into snow loads and contributed to the ASCE snow provisions.
I`ll dig up his name if you want to do some more research into it.

 

[jayrod12]

Around our parts, it's typically 4 feet clear. We have a base snow depth of 22-23"

 

[Rex1100]

Once20036 - I would be interested if you can find that reference.

 

[Once20036]

The seminar was "Advanced Snow Loads" by Michael O`Rourke, PhD - through the Structural Engineers Coalition of CT, New Haven CT, Nov 14 2007.

Q1: What's the general standard on sharing webinar slides? Generally OK or generally discouraged?
Q2: If generally OK, how can I post a scan of the slides?

 

[RPMG]

Thanks everyone. This answers my question.

Unfortunately, I believe that you are not supposed to share webinar slides on the internet.