Dark lines on ceiling

I was showing a home a few weeks ago when the buyers noticed the second floor ceilings had some faint to dark lines on them. It appeared to me that they we’re the same distance as the rafters should be above and my thought was that the nails or screws used could have bled throught the drywall. Other than that I had no advice on what it could be.  Since then I have found out what the problem is. The second floor was below an unheated attic and although the attic floor was insulated it probably wasn’t insulated enough. The reason for the dark lines on the ceiling is this. The attic is cold in the winter and that cold is transferred to the ceiling joists which make the ceiling directly under the joists cold as well. The second floor is heated and warm which makes the cold spots on the ceiling moist with humidity. Not enough to drip water but moist enough to catch small particles, lint and dirt which eventually add up enough to look like dark lines on the ceiling. Properly insulating your unheated attic will prevent this from happening.