Most so-called "flat" installs have a few degrees permanent tilt. This is necessary to drain water, but may not be enough to prevent snow accumulation.
Big 200W-250W panel mounted on 4 brackets at 45 deg (one bracket per corner) with a couple of #10 deck screws per bracket in 3/8" plywood CAN be ripped in heavy winds. The effort should be made to locate the rafters and drive screws into rafters, wherever possible. In plywood alone with 2 tilt legs at 45 deg I would use at least 4-6 screws per each of 2 North brackets, and 3-4 screws per each of 2 lower brackets (those at the South end).
