A.It is possible that the heads of the screws are corroding, but this is unlikely if you are using corrosion-resistant screws. Stainless steel screws are best.

The most probable cause of the problem is condensation on the screw heads. The screws are good conductors of heat and are sunk well into the cold studs on an exterior wall. Since they are insulated only by wood, not by the cavity insulation, the screws can become one of the coldest objects in the room, and a thin film of condensation may form on the walls over the screw heads. This moisture film will attract dirt, forming the dark spots. To check if dirt is sticking to the wall, wash the wall to see if the spots will come off.

While this may seem unusual, I once found a house where the head of every drywall nail was dark, with some other dark streaking. The pilot light on a gas log had been misadjusted and was producing carbon black. The fine carbon particles were attracted to any area that had a condensation film, and the house looked as though there had been a fire. A similar problem often appears in churches, due to the greasy carbon produced by candles.