
Snow sliding has a lot to do with the sun as well -- not just gravity. Typically, the sun comes out and radiant heat passes through the snow load, strikes the metal, is reflected back out, and creates slipperiness to encourage sliding.
That said, I'd probably try to have at least a 6:12 pitch to try to avoid snow building up. Again, though, there are many factors at play here beyond just gravity and pitch. Weather conditions play a huge role.
I am having an awning done with metal roof for sun shade. Noone is there during the winter. At what slope can I be fairly confident snow will slide off ?