I had to take off part of a roof that we put on about 10 yrs ago, and
found a fair number (but not all) of the nails were showing
considerable rust. It's a hip roof, about a 40-degree pitch, and the
areas I looked at were around a dormer valley and along the dormer
Roof is 3-tab shingles (4.5" exposure) over felt paper. The valley is
'open', with generous roofing cement under the exposed shingle edges.
Sheathing is 1/2" plywood over 3/4" slats. We get some snow (Syracuse
NY), a fair number of freeze-melt cycles, and it can be pretty humid.
I don't see any other evidence of leakage or water penetration.
So, I'm wondering why the nails are rusty. A roofer (a relative) said
that electro-galvanized nails just rust in this climate. Someone else
suggested that it's evidence of inadequate ventilation, where moist
air is leaking from the house through the nail holes.
Any speculation would be appreciated.