New metal over existing low slope built-up roof

Brian Coldwell
I have an existing 8-yr old roof in good shape. Exposed beams 24" o.c., tongue-and-groove, topped with tar and gravel, slope is about 1:12. I need to get at least R-11 up there for energy code compliance to add central air. I found a metal panel with Miami-Dade NOA approval for this pitch which can be installed on purlins (Superlok by MBCI); I looked for weeks and this was the only one I could find. I have seen local installations that use two stacked 2x4's running perpendicular to the exposed beams, at 2'-4' o.c., serving as purlins. The stacked 2x4's will provide about 3" clearance between the existing roof and the underside of the panels. I plan to install rigid foam board between the purlins, which gives about R-15 and should provide some rigidity since it will be flush with the top of the purlins. The panels will then be attached to the purlins, which are attached to the beams. I read some previous posts that mention a 1" air space is required; I need to check if Florida's Building Code requires this because the remaining 2" for insulation is too small. But I am looking for feedback if this concept sounds legitimate. I have a slew of details and installation manuals from the manufacturer; but if anyone has a detail showing the purlin-to-existing-roof, that would be helpful. I included a pdf of my roof plan for reference. It shows the existing beams; the purlins would run the long direction. Thanks in advance for your feedback.
Brian Selig
Architectural Building Components Metal Roof and Wall Panels
The Superlok is a 2" mechanically seamed panel, like my 200DL. There are several manufacturers who provide the same panel that may have Dade approval. They offer clips that have up to a 1" offset to create that 1" air space that you discuss. The approvals are done as a system. You'll need to inquire with the manufacturer on the clips that are tested with that panel. Each building has it's own little characteristics that determine the structural attachment requirements. If I had a 1:12 slope with tongue and groove decking in Houston, I would remove the gravel. Once I had that done, I would secure the proper thickness of rigid insulation. After applying a waterproof membrane (peel-and-stick), I would install the metal roof panels attaching the clips into the deck used for the original roof. See if you can fight through the red tape. I wonder if there is any testing out there for that assembly.
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