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