During the winter moisture is present if there is frost on the nails.
Attic nail stains.
After several years of this there may be.
This is sure evidence of a condensation problem.
In a reasonably well insulated and vented attic stains may appear due to condensation.
Also notice the condensation stains at the shingle nail more evidence of a history of attic moisture which was a factor in this mold growth.
If it goes away it is mold.
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Stains and rust around protruding nails are strong evidence that there is a high level of moisture in the cavity surfaces.
Get a garden sprayer hit it with a two cup to one gallon of water bleach solution on a few spots and see what happens.
When this humid relatively.
On attic mold visible on pine tongue and groove roof sheathing near the building eaves.
This is caused by moisture condensing on the roofing nails and dropping to the surface below.
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Random stains on your ceiling with no evidence of roof leakage usually mean that you ve got a condensation problem in your attic.
This latter phenomenon occurs when the house is too humid and the warm moist air seeps into the attic where it condenses and freezes on cold attic areas.
One good way to check if this is going on is to inspect the nail heads in your attic.
This happens due to trapped damp air often in the late fall or early winter.
Looks like either condensation encouraged mold growth or less likely the coating on the roofing nails migrating into the wood.
Even in warm or dry weather you often will see stains around roofing nails penetrating the roof sheathing or condensate drip spots on the attic flooring or insulation left photo below.
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If enough of it forms and settles on the attic floor it will eventually migrate down to your ceiling.