It is very common to find mould growing on attic sheathing. It is the area between the warmth of the house and cold of the outside in the winter and the opposite in the summer. These contradictory temperatures can create condensation in the attic, creating a condition for mould spores to begin growing.
The most common reason for the condensation is a lack of air movement. Air movement can be improved by increasing the vents on the roof of the home and ensuring that the soffits are clear of insulation or anything else blocking them.
We have also seen mould caused in an attic because a bathroom fan vent or dryer vent went directly into the attic and not through and out the roof, creating a nice warm, moist condition for mould growth. Certain moulds are bad for your health and should be removed, so this is one reason to ensure that there is no mould on the attic sheathing.
The other reason you don’t want mould on the ceiling of the attic is that it is a living organism and is literally ‘eating’ through the wood as its nourishment. So stick your head up into your attic a couple times each year. If you see something that could be mould removal professional to inspect the site, for your health and your home’s health.