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Metal Panel Manufacturer is recommending a 16GA minimum support plate to go above the purlins, under gambrel transition areas. Is this really necessary? It doesn't appear that it would provide much support, and seems like it would be an avoidable expense?
Unfortunately what you attached is not opening up properly. I am struggling visualizing what's going on but this does not sound like a typical residential detail. For residential metal roofs, I always recommend solid decking. In any event, I would never recommend anything less than what the metal panel manufacturer calls for.
Here is a jpeg of the detail.
So you do not recommend purlins, but rather solid deck (i.e. plywood sheeting) below corrugated exposed fastener metal roof panels? This is surprising, as purlin support is by far the most common installation method used in my area (upper midwest).
If you use this detail, I would stick with the 16 gauge. The reason I prefer to not see metal roofs on open framing on residential roofs is the high chance of condensation. This typically is of greatest risk in northern climates in fact. So, you're seeing this be done in your area on residences and no condensation issues? What is being done as far as insulation, ventilation, and vapor barrier? Those things, to my way of thinking, would all have to be spot-on in order to avoid condensation.
With a properly vented attic space using vented soffits and roof vents, the temperature delta from one side of the roof (whether it be metal or asphalt shingle) to the other is minimized, thereby minimizing the risk of condensation at the roof. Attics in the upper midwest typically have R40+ of blown celluose, and poly sheeting beneath that (here's the vapor barrier), then your ceiling material (sheetrock, wood, etc). In cold months when the inside of the home is being heated, humidity is controlled inside the home using an air exchanger.
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