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Garage roof assembly

Allweatherfun | Posted in Building Code Questions on

I have constructed a shop on the back of my main house.  It will NOT be heated or cooled.  The ceiling is a cathedral-style ceiling with 2X6 rafters supported by an engineered beam at the center and by outer walls of 2X6 construction.  The 3.5/12 roof construction is 7/16″ osb with 30# roofing paper and shingle.  The soffits are only a couple of inches wide, but right now they are open, so I can put a continuous vent cover along each side of the shop.  So far I have not installed a ridge vent.  I now want to insulate and sheetrock the interior.  I am seeing lots of discussion on venting, hot roofs, etc. and it is confusing.  I live in a climate zone 5 area in the high desert, so the climate is not humid, but it does snow and freeze in the winter.  It seems that one option is to simply use R19 batting between the rafters and close up with sheetrock.  The risk may be undetected water damage should something ever leak.  Alternatively, I can install a ridge vent and insulate between the rafters with a 2″ or 3″ polyiso or similar board fixed flush with the lower edge of the rafters thus leaving a plenum that vents each cavity from the soffit to the ridge vent, expensive, but I’m not sure that’s the best option either.  There are probably other approaches as well.  Any recommendations for achieving a long lasting, dependable roof that will also get me a green tag from the inspector?

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