I did a home inspection yesterday. The ceiling in one bedroom always has mold at the corner only in the winter when furnae on. I checked the attic and shingles and it looks fine to me (mold only happened after roofing replacement in 2010). However, I found at the corner on the roofing missing underlayment as shown on the pictures.
The underlayment was installed all along the roofing except at the corner. This area was right above the mold area in the bedroom. It might be because this corner was colder than other area and once the furnace on, this area may accumulate condensation on the ceiling to cause mold (when warm air meets cold surface).
Any comments or suggestion?