I'm building a shed with a metal roof. What underlayment material should go between my OSB roof deck and the steel panels?
You need to always follow the metal roofing manufacturer’s recommendations for installation, but generally you can use 1-layer of 15# building paper if the slope is 4:12 or greater and it’s standing seam metal roof with concealed fasteners.
You need a minimum of 2-layers 15# building paper if it’s a slope between 2:12 and 4:12.
Below 2:12 slope you need to follow the manufacturer’s recommendation for installation. (They’ll probably recommend a peel-and-stick underlayment.)