Rob, frankly there are too many unknown variables involved to directly answer your question: roof pitch; uniform vs. concentrated loading; depth & consistency of snow/ice; wind direction & speed, and maintenance. The construction on homes is controlled initially by the state & local building codes, and by the local building inspector, providing he/she is competent & inspects the construction at the specified construction milestones. I think you can get a good idea from the link Larry noted above, but I think the best person to answer your question would be your local building inspector, again providing he/she is competent.
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