We live in the Northwest and your home quite possibly has mold growing in the attic. Typically we see the mold growth and staining on the North side. It'll look dark and can cover large or small areas of what's called the attic sheathing. That's the wood decking you see nailed to the roof trusses, also the material your roofing material is nailed to.
When that old North Wind begins to blow in the wintertime it blows that cold air up the eave vents, also called bird hole vents, lower vents, or soffit vents. That cold air air meets the warm air which rises from the living space below. Since warm air holds more moisture than cooler air, when the two meet, condensation will occur on the closest surface which will be the roof sheathing. Other causes may be bathroom fans, not venting outside the attic space, roof leaks, or improper insulation. Sometimes those eave vents are simply blocked with insulation which won't allow for proper ventilation.
Whichever the reason, or possibly a combination of a few, AdvantaClean will discover it, provide an estimate for fixing the problem, and then we can get the mold cleaned as well.