A.Bill Rose, architect and building researcher at the University of Illinois in Urbana-Champaign, responds: I would expect the overall performance of a vented porch roof and a porch roof without vents to be about the same. I advise you to look at porch roofs in your area to see what the local practice is, and to check with building code inspectors and follow their direction.
The joint where the top of the porch roof meets the wall needs to be flashed well, and most of the vent details for that look to me like they might leak water under some conditions of rain and wind. Soffit vents have much less risk of water entry.
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