I wouldn't count on the snow sliding off. You might check but I don't imagine that snow-loading codes assume it's going to slide off either.

We have an unheated screen porch at our camp that has a steel roof and 2x6 rafters. It has a minimum pitch for steel roofs without going to low angle roofing. I don't know how that compares to a 12/12. I can say that the snow doesn't necessarily slide off and I rake it several times a winter. I think what happens is that the air on the inside of the porch melts the snow a bit on warm days. If the snow then doesn't slide off then it freezes to the roof. If that happens then it has to wait for an even warmer day and support any additional snow as well--good chance that making sugar would cause it to slide off with a little encourgagement from a rake though.

We have an old 'lean-to' type sugar shed a little larger than that which has stood more than 50-years with 'real 2x4's' for rafters. The walls were getting a bit splayed and the roof sway-backed though. I did some probably over-designed remedial work and it has been fine.

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