If the deck is open air and not conditioned, underlayment for the purposes of vapor control is not required.
You will need to check with the panel/metal maker to see if you require a slip layer based on what the metal interface is with the decking.
I may not be visualizing what you have here to work with so take my recommendations with a grain of salt until I can picture it or you can help me understand.
Eric Novotny
2014-03-07 10:13:51.000000
We have a building under construction that has steel roof trusses with a structural metal deck attached as a diaphragm (1-1/2" B type deck with ribs and grooves) The attached structural deck was necessary for seismic purposes. It runs perpendicular to the slope of the roof which is 3:12. The attic is a ventilated cold attic. We want to install a standing seam metal roof over the structural deck. Our questions are:
1. Can the standing seam roofing be installed directly over the structural deck without providing any membrane under it?
2. Do we have to be concerned with condensation forming in the grooves of the structural deck if we install the standing seam over since we have a cold attic?
3. If we cannot install the standing seam directly oover the structural deck, what do we need to provide to have a properly installed roof?
