Designing for snow 'impact'?

Started by MushCreek, June 16, 2019, 05:01:09 AM

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MushCreek

I have a good-sized gambrel roof barn. I'd like to add a lean-to shed along the eaves for parking my tractor. My concern is the effect of snow sliding off of the roof. We don't get a lot of snow here, but can sometimes get a fair amount (6") of heavy wet snow. When it slides off of the tall roof, it makes quite a sound that you can hear inside the house 150' away. I'm worried about what that could do to a roof placed there. My plan is to make the framing fairly stout, maybe 2X10's on 16" centers for a 10' span, and put a solid 3/4" plywood deck in the impact zone so it doesn't damage the tin roof. The rest of the roof would just be 1X4's to fasten the tin to. Of course, the header and 2X10's would be very securely fastened to the barn wall.

The good news is that the snow wouldn't fall very far- maybe 2' instead of 12'. I've seen small snow fences that you attach to the roof. Do you suppose that they would be effective for this? It would be a lot better if the snow stayed put until it melted; usually only a day or two. The roof is gambrel, with the upper slope being 7/12, and the lower being 12/7. Once that snow starts to slide, it comes off of the roof fast. The potential snow would be coming off of 450 square feet and being added on to the snow already on the lean-to roof, another 300 square feet. To make things worse, it wouldn't be evenly distributed, but piled up right along the eave. Am I overthinking this? Thoughts?
Jay

I'm not poor- I'm financially underpowered.

ChugiakTinkerer

It's certainly something to think about - you're definitely not overthinking it yet! I would do both of the things you are considering.  Overbuild the shed roof and put in snow brakes to hold the snow in place.

An object falling a distance of 2' will have a speed of about 11 ft/s at impact.  The impact isn't instantaneous, it will take some amount of time for the snow to compress.  Without doing some calculus, I reckon it would be something like 0.1 seconds.  So if you have 50 pounds per square foot of snow on the roof, when it impacts you calculate the force using F = dP/dT, the change in momentum divided by the change in time.

The momentum would be 50 lbs * 11 ft/s = 550 ft-lbs/s
The force would be 5,500 lbs for each square foot of snow.

I think rather than attaching a ledger board to the barn wall, I would use posts anchored to footings.
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Dave Sparks

I would use the 3/4 plywood on the new roof. Not just the impact zone.
"we go where the power lines don't"

Don_P

For your snow load I'd say you are fine with your plan. Another thought is to eliminate the drop, tuck it up tight and do a pitch change rather than a drop, either way do count on a concentrated load right there which is more about the connection.


MushCreek

Yeah, I'll tuck it up under the eave, but the snow coming off of the upper roof is moving pretty fast once it gets on the lower pitch. That's why I was thinking the snow stops would help. They'll hold it on the upper roof longer so it can melt, and slow the momentum on the lower roof.
Jay

I'm not poor- I'm financially underpowered.