Want to replace 2 layers asphalt and 1 layer cedar roof with "pole barn" type steel on my house. I intend to put chipboard or plywood down first, then felt or other material, followed by roof.Other than exposed screws or nails, is there a downside to the plan? We have 3 out buildings roofed similarly, and seem to be fine. One roof is 50 yrs plus old. Want to stay away from standing seam, etc., because of cost. House has 6/12 pitch, many trees around it, and we get quite alot of snow. Also, currently have no ridge vent, want to go to full vent. Thanks