Cathedral roof options
I have looked over many posts and can’t seem to find the answer for this roof issue. Contractor removed a tile roof and wants to install a new roof this week. The roof construction consists of exposed 1-1/2″ T&G decking, 5/8″ plywood, two layers of 1/2″ fiberboard, felt, and ceramic tile. Roof is approx. 5/12 pitch in a 4A climate zone (suburban Philadelphia area).
Owners moved in this summer, so have no history, but had some recent leaks. When the contractor removed the tile he found that much patching had been done in the past and some felt, fiberboard, and plywood were deteriorated, mostly near the bottom 1/4 of the roof. I think the leaks were partially due to tile issues and probably significant ice damming.
This is a single slope roof with a clerestory window at the top. Because of the window sill height, cost factors, and architectural factors, installing an ideal roof is not practical. I am thinking about the following, but have concerns:
Covering the T&G decking with plywood (preferred) or OSB. Either taping the seams or placing an air barrier above this (recommendations?). The two layers of 2″ polyiso. Then OSB roof sheathing, ice and water shield (entire roof) , and asphalt shingles.
My concerns include the lack of ability for the roof sheathing to dry outward or inward, roof sheathing attachment (beams are spaced approx. 8 feet o.c. oriented side to side), the potential for ice damming with an unvented roof and less than ideal level of insulation, and picture framing at panel joints. Giving up the ice an water shield may allow some drying, but maybe with a greater chance of water penetration, and then the ice and water shield at the eaves would trap the moisture. The roof is about 30 feet from eave to top and with the height limitation adding ventilation is out of the question without significantly reducing the amount of insulation. Changing the clerestory windows has been rejected.
Any advise or guidance would be greatly appreciated.
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