One good way to check if this is going on is to inspect the nail heads in your attic.
Attic nail stains.
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After several years of this there may be.
The mold shown in the attic photographed above was identified as aspergillus sp.
Stains and rust around protruding nails are strong evidence that there is a high level of moisture in the cavity surfaces.
In the summer rust can be seen on the tip of the nails and sheathing around the nail protrusion.
Looks like either condensation encouraged mold growth or less likely the coating on the roofing nails migrating into the wood.
In a reasonably well insulated and vented attic stains may appear due to condensation.
This happens due to trapped damp air often in the late fall or early winter.
If enough of it forms and settles on the attic floor it will eventually migrate down to your ceiling.
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.
This is sure evidence of a condensation problem.
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If it goes away it is mold.
This usually occurs on the underside of the roofing structure.
When this humid relatively.
This is caused by moisture condensing on the roofing nails and dropping to the surface below.
On attic mold visible on pine tongue and groove roof sheathing near the building eaves.
During the winter moisture is present if there is frost on the nails.
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.