Help!-need advise shed side roof construction-many pics
I'm not sure I buy the 2x12 at 24" o.c. for a 90 psf snow load (live load) and 10 psf dead load. You have to remember snow on the upper roof will slide off impacting the lower roof, plus with a wall above the shed roof you'll get snow drift (snow hitting the wall and piling up on the roof) so you'll have greater load adjacent to the wall of the barn. You are subject to greater loading than the 90 psf ground snow load in my opinion. You can build it as your describe and it will work, how long I can't say. I think you'd be better at 16" o.c., even better at 12" o.c. If your shed roof had something like a 10:12 or 12:12 pitch roof I might not be as concerned as the snow would slide off easier. I'd say you'll be looking at something like a 4:12 or 6:12 for your shed.
As far as attachment I see two ways of doing so. first is install a beam between the columns of the barn that will carry 1/2 the weight of the shed roof, or two build a 2x wall in between the columns and attach a ledger to it. The ledger would be secured to each stud in the wall with a couple of lag screws. It appears you have approximately an 8-foot span between the columns so you'd need something like a 3 ply 2x12 beam (see if your lumber yard can calculate it for you) if you went that route. you can through bolt the beam to the columns, but would need to add a 2x (width of column) to the exterior face of the column under the beam to support the beam load to follow prescriptive code, or could hire a professional engineer to design the bolt connection.
Whether you use a beam or a ledger use metal connectors to attach the rafters to the beam/ledger. Also use hurricane ties to attach the rafters to your header. Toe nails work great at resisting lateral and shear load (side to side and front to back) but do nothing for uplift.
Hope this helps. Good luck!
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