Hi, a few years ago I posed this question-
https://www.greenbuildingadvisor.com/question/ventilating-shed-roof-overhang

This guest house was taken to drywall and then put on hold as there was a change in my clients financial situation.  The roof frame was filled with spray foam, sheeted and roofed with 1 1/2″ standing seam.  The roofer used ice and water up about 5′ and then completed the deck with a synthetic underlayment.  There is zero overhang at the sides and eave.

We are now back on the project and framing the main house which is the same design as the guest house.  Instead of using spray foam we are installing polyurethane sip’s.  In researching the roof install I’ve come across varying thoughts 0n the proper roofing underlayment schedule.  Some say at 2.2/12 I must use a self adhered membrane,  while others advise against on a hot roof
 because of the possibility of trapping moisture at the plywood.  Having said that, I’m not too sure how permeable the synthetic underlayment is.   The roof manufacture is fine with a synthetic underlayment with a 50% lap due to the use of the mechanical seam. The sip manufacture says to follow the roof manufacture guidelines and suggests any of these products can be used.

Any other opinions out there?
Thanks, Bill