Looking for anyone that can explain why mold occurred on the bottom side of the roof deck overhang, between the rafter tails.
It is an open overhang, so no soffit. Black roof. The mold is on the back side of 3/4″ exterior plywood which replaced original 80 year old deck 11 years ago. This playwood was painted with oil based Sikkens Rubbol DEK. This mold is occurring on all orientations of the house. It is a gambrel roof, dutch colonial style. A detached garage with the same style overhang has the original wood deck and has no mold. I originally thought that the sun was driving water vapor slowly through the shingles, wood deck and it condensed on the back side.
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