Covered porch snow load

[shaneelliss]

I found a thread with this subject from a couple years ago but it didn't answer my question very well so I thought I would ask again.

I am in a heavy snow area with deck snow loads calculated at 230 psf plus drift. And I have a deck that is open on three sides but covered with a roof. At the peak of the roof, the open side is nearly 20 feet tall (from the deck surface to the roof) and on the two sides parallel to the roof peak the opening is about 9 feet tall (from the roof eave to the deck). The deck is 26' wide x 13' deep.

Do I design for full snow load, or do I assume that the roof will keep some of the snow off the deck? If the roof keeps some snow off, how much? I thought about just figuring that the drift snow load would be on the deck (without the base snow load) but am worried that might not be conservative enough. But I also feel that designing for the full snow load (plus drift) might be too conservative.

In reality, I expect to be too conservative and design for full snow load, but am curious about what everyone else thinks about it.

 

[MiketheEngineer]

Seems to me that full snow and drift should be used.

 

[shaneelliss]

Thanks Mike. Anyone else have an opinion?

 

[hokie66]

230 psf? Wow!

 

[shaneelliss]

It truly is incredible how much snow we get here in the mountains of Utah. And people like to build "cabins" way up there that are three times the size of my house with a lot of windows and decks. Needless to say, there are some pretty big beams involved.

 

[msquared48]

It's 400 psf at Stevens Pass Ski Area in Washington. Try designing wood beams for that - all shear critical.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RockEngineer]

Your Utah powdery snow is not as dense as the Cascade Sludge msquared48 and I get here in Washington so for the 230 psf you're probably talking about 12 to 15 ft of snow under your roof on the deck (assuming 15 to 20 pcf snow density which would be pretty heavy for the Utah powder) . If you have good overhangs and a little shelter from wind I wouldn't be afraid to design the covered porch for half the snow load (about 6 ft of snow). If you do this of course you are taking on additional liability and you don't want to design right to the limit. It's all engineering judgement. You want to document how and why you came up with what you did. You can take more leeway with deflection limited beams and girders than the shear and bending limited. Overdesign the connections and columns.

 

[shaneelliss]

Thanks for your 2 cents rockengineer. All this talk about 12-15 ft of powdery utah snow is making me want to get my snowboard out and ready for the coming season.

I think you are right to assume that I won't see full snow loads on the deck. It just doesn't physically seem reasonable. But taking on liability doesn't seem that great either. So it is the safe route for me and the costly route for the owner I guess.

 

[josta]

Here in Big Bear Lake (100 psf), it's a judgment call depending on how high the roof is above the deck itself. But for an average 8' height, 50% of the roof snow load is usually used for the covered deck to accomodate blowing snow.