Snow Loads Around Mech Units

[KarlT]

Something that really bothers me about the Canadian Code is when it comes to snow drifts around rooftop mechanical units. The code states it isn't necessary to consider extra snow load if the width of the obstruction is less than (3.0*Ground snow load)/Unit Wt. of Snow.

Here in Winnipeg, this translates into a maximum obstruction width of about 5'-6". Once you get over that width, then the snow load suddenly jumps up to 75 psf max (from 33 psf basic snow load) and the length of drift goes out a minimum of approx. 10'-0" or twice the unit height.

When it comes to replacing rooftop mechanical units on renovation jobs we often end up getting into beefing up the existing structure to handle these drift loads. Then I get the "look" from the architect and mechanical guys like I'm from another planet.

Some questions.....

1. What if one side of the unit is less than 5'-6", but the other side is say 6'-0". Would you only conside drifting on two sides?

2. What about if you had a unit that was 5'-6" by 5'-6". If the wind blows at the corner of the unit then the projected width is now greater than the minimum. Would you only get drifts at the four corners or can you ignore drifting all together?

3. When checking roof design, I usually account for the dead load of the unit plus the basic snow load on top of the unit. Is that right, or would you reduce the snow on top of the unit due to wind scouring?

4. What would the actual drift shape around a mechanical unit look like, with the prevailing winter winds from the northwest? I always assume the full drift load all around the unit but is there any way to reduce it?

 

[onteng]

I share your frustration in dealing with the snow pileup issue around mechanical units. There is no logical answer to give an owner except to show him the code requirements.
As for your queries, this is how I handle those issues.
1. We use the pileup extent on the side required and extend pileup on the short side a distance equal to the height of the unit.
2. We look at the actual lengths. The code requirements are already ridiculous.
3. We usually include minimum live load of 1 KPa on the unit.
4. We also use drift on four sides. There does not seem to be a reference to allow for prevailing winds in the code.

One interesting idea I have heard of was to raise the units above the roof 2 to 3 feet to allow for the wind to travel without depositing the drift. I have not found any reference to this so I have been hesitant to use it.
Hope this helps.