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Standing seam metal roof over rigid foam board

side_oats_grama | Posted in Green Building Techniques on


Im in the process of design my new house. The roof assembly components goes as follow: 3/4” plywood over trusses, WRB,  two layers of 1” of polyiso rigid foam and this is where I’m stuck… do I need a second deck of plywood (as a base for metal roof) or can I instal a rain-screen strips on top of the polyiso and make them the base to attach the metal roof?

I live in North Texas (climate zone 3) and the plan is standing seam metal roof over the rigid foam. Is there a tried and true metal assembly with exterior roof insulation for my climate zone?

The roof slope will be 3:12

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  1. Expert Member
    Michael Maines | | #1

    I have not heard of metal roofing installed directly over foam. It's pretty common to install it over sleepers over foam. 1X sleepers might work but I usually see 2X or at least 5/4 material.

    Not every roofing manufacturer or roofer will allow installation over sleepers. It works best with thick-gauge steel (22, 24 or 26 ga) and with simple roofs. Trying to get valleys installed over sleepers is challenging.

    1. Expert Member
      Akos | | #2

      Some panels with clips can be installed over steel bearing plates right on the foam. The clips for the panels screw right into the metal plates which spread the load out. This is a bit of a specially product used in commercial world so the cost will be higher.

      For the OP. In your climate you only need R5 of exterior rigid for condensation control, so that is only 3/4" polyiso. The roof does not need any venting at all, so you can install strapping as Michael suggested or another layer of OSB. It might be simpler to start with nail base panel as it has both rigid and OSB as a single assembly.

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