Heavy Snow Load - Design Advice

[CabinNick]

Our property will be snowed in within the next couple weeks for the winter so time to transition from on the ground work to design/planning/permitting work. 

I was curious to get any lessons learned or suggestions from all of you for design considerations when building in a high snow load zone.  Our plan is to build the 20x30 1 ½ story with a full upper floor (i.e. no loft/open vaulted ceiling).   Only heat in the cabin will be wood stove as we will only be visiting it a few times in the winter and will be winterized the rest of the winter.   We will be building to code and have to get permit/inspection. Site is on a upper portion of a north slope (probably not helping my case....) but fairly protected by tall trees about 40' away in all directions.

Last year was an average snow year and we had 60" of packed snow on the ground in early February.  Using the snow load mapper predicts a snow load of 110/psf. 

So far my thoughts related to heavy snow:

•   12/12 pitch metal roof
•   Possibly one shed dormer but otherwise no other changes to roof line that would hold snow
•   Wood stove chimney would go high on the roof line to prevent snow build up and be "fortified" to keep sliding snow from ripping it off. 
•   Going back and forth on two options: 1) no deck on the long sides of the cabin where the snow will slide off; but I am worried about snow building up on the sides of the cabin and pressing on windows or 2) putting narrow covered deck on both long sides of the cabin to buffer the heavy snow build up from the sides of the cabin. 
•   Planning on having some good size overhangs.
•   Entrance door somewhere on the short side of the cabin where the snow from the roof does not build up.

I have not looked into trusses yet but I assume I will have to go with an engineered truss like a parallel chord or scissor truss.  Instead of engineered trusses, should I also be investigating the addition of a ridge beam and changes to foundation to support an internal load bearing post or two?  Cost and remoteness of location is a consideration for us when thinking about truss delivery, crane for a beam, etc.

Thank you all for any advice.

[ChugiakTinkerer]

@ CabinNick,

Here in Alaska the Talkeetna area has a ground snow load of 120 psf.  Snow coming off the eaves and piling up to the windows has been observed, even in cabins that are heated full time.  Snow build-up on the roof is more likely if the cabin is heated infrequently.  Designing and building for the potential roof stress is a straightforward thing.  Figuring out how to prevent damage from falling snow is quite another and is something that NathanS wrestled with for his house.

Solutions include extending the eaves to get the snow further from the wall, raising the foundation to get the building up higher, or putting in snow brakes to prevent or minimize slide-off events.  For my cabin which is on a 70 psf area, I'm opting for the first two strategies.

[NathanS]

Yeah 120psf is some serious weight.

Design guides, even for engineers, stop before analyzing the effects of impact. They concentrate more on how to adjust the snow load for a roof, or deck, below another roof.

Not looking at the guide right now, but it could be something like you would need to design your deck to carry 150-200psf snow load.

This doesn't account for the impact of the snow landing, which could be a big number. I will say I still think that 4000lb of snow falling on deck is lot different than a 4000lb truck.

A stone patio could be a good alternative to a deck.


2x12 #1 doug fir @ 16" OC could easily handle 120psf. I am totally going off memory here but I want to say my #1 SYP 2x10 @ 16" is rated for like 170psf.. then my shed roof with doubled 2x10 @ 16" is even crazier at something like 320psf... If you are going to balloon frame and your rafter ties are going to be a couple feet below the top plate, that will change the rafter thrust equation for the worse. This could be a problem and you'd need some kind of simpson tie to attach everything. At 7-12 pitch I had to put 9 or 10 16d nails into the rafter-joist connection. Not sure I would want to put many more in than that.

[CabinNick]

Thanks for the advice guys.  Looking forward to designing this building over the winter.