Just guessing but....
I suspect the cause is why a cold glass of beer sweats on a hot summer
day. The nail is much colder than the warmer attic air. When dewpoint gets
close to temperature
you get visible water vapor. Try a small fan to get the air moving. Again,
only a guess.

"Gntry" wrote in message
news:HQwzf.2167$pq5.1585@trndny02...
I live in western NY and the weather here for the last week or so has been
around low 30"s, I was in the attic the other day and the nails coming
through the roof are dripping water....I have a full ridge vent, full
soffit on the west side of the house(and they aren't blocked by any
insulation), and gable vents. It is definately cold in the attic and there
is about R25 on floor w/ barrier down towards living space, the roof was
replaced about 10 yrs ago....what is causing this and should I be
concerned?