Improper exhausting of bathroom fans or dryer vents.
Mold in attic roof.
Wooden beams and plywood may be affected and it can be difficult to remove all traces of mold from wooden surfaces.
Blocked or insufficient ventilation.
The photo at the left was identified as a toxic mold that probably should be removed although the ease of movement generally tearing off a roof and roof sheathing and perhaps roof framing to remove mold would not be sensible nor cost justified.
Mold growth on attic roof sheathing is a common issue in cool climates such as the pacific northwest.
Dryer exhaust vents kitchen exhaust fans and bathroom exhaust fans.
Insulation is often affected.
This occurs when the temperature of the sheathing drops below the dew point creating a thin layer of moisture on the substrate.
Heating ventilation and air conditioning ducts may also.
In the vast majority of cases the mold growth is caused by condensation.