SLIDING SNOW

[jeffhed]

I have an instance where the roof changes at the back rear wall of the home. The roof slope on the home is a 5:12 and then flattens out to a 3:12 pitch to maintain head height at the back of the covered deck. The deck roof is not "lower" than the main roof, but I have always designed a roof like the deck roof with sliding snow loads. My patio roof is not lower and I would really classify the patio roof and the main roof as just one roof, but I feel like the snow could slide off of the main roof and accumulate near the pitch break in the roof. However, I have repeatedly seen similar projects done by others where just the main roof snow load is used. Have I been over designing these types of roofs by applying the sliding snow load?

 

[jimstructures]

jeffhed,

Not in my opinion, but yes in my company's owner position. I agree that a sliding snow load will happen more often with a break in the roofs, but with a steep or slippery enough roof it will still slide when you don't want it too.

Jim

 

[WARose]

Have I been over designing these types of roofs by applying the sliding snow load?

I don't think so. The IRC says to consider it.....but doesn't really give you much guidance beyond that.

 

[Ron]

You should consider! Even though my practice is in Florida, where I don't have to consider such, I was a partner in a Canadian firm for 6 years and we did failure investigations of similar!

 

[jeffhed]

Thanks, everyone. I appreciate the input. The sliding snow section is pretty brief. The section mentions a couple times that the snow is sliding from a "higher" roof to a "lower" roof. In this case there really isn't a higher or a lower roof, it's a pitch break in a roof, so I wanted to find out what others are doing since I have seen other work where the sliding snow in this situation was not taken into account.

 

[MotorCity]

I can see snow accumulating in the pitch break in the roof, but I typically think of sliding snow as snow that falls from an abrupt drop in elevation. I would probably account for this buildup of snow using a snow drift approach (consider the steeper roof as an "obstruction").

 

[jeffhed]

MotorCity,
I also usually think of sliding snow as snow that falls from an upper roof to a lower roof like you said. But couldn't snow slide off of the steeper roof down to the lower flatter roof and accumulate there as well? I can also see how this could be approached as a windward drift load rather than a sliding snow load. May be prudent to look at both and use the worst case load, especially in higher snow load areas.

 

[dhengr]

Jeffhed:
It seems to me that there may be a little more to the problem than just looking at the two load conditions and then picking the worst one. The appropriate approach may be to sum the two conditions in some way, with some engineering experience and judgement involved in that determination. Consider the following:
1. Sliding snow is more likely from a steeper roof, and it is more likely from a metal roofing system than from most other systems, such as asphalt shingle roofs. The sliding snow comes from the leeward sloped side of the roof, onto the lower sloped (or flat) roof. Falling snow, as from a cornice drift build-up at a high roof edge is another animal altogether, an impacting load, at first.
2. Drifting snow usually comes from the windward side of the roof, and further reaches in that direction, over the ridge, and is dropped as a drift downwind of the ridge (or high point), likely around that roof slope change.
Thus, the two could be additive (use engineering judgement here) and are not (may not be) the same bulk or volume of snow.

Please keep this a secret discussion. If the ASCE7 people get wind of this (not the same wind causing drifting, but rather that which causes exponential code complexification) the code will grow another 73 pages. We will be considering the 68th derivative of the snow flake geometry, eccentric density pattern and flight or sliding characteristics, on a sunny day, right after a cold night, with a full moon. That is, another two days of calcs. and cogitation, all for some (a small?) change in a factored max. load, which should have been bumped up a little by inspection during the roof design process.