Mold on Exterior Siding
Green or black streaking on the north side of a house, especially under trees, is most often algae (commonly Gloeocapsa magma on roofing, and various green algae on siding), not mold, and it is a cosmetic issue rather than a structural or health one. True mold on siding is less common outdoors because most siding materials are not the organic food source mold prefers, but it can grow where siding traps moisture against wood sheathing behind it.
The more relevant question for a homeowner is usually not the exterior appearance itself but what it implies about drainage and shade around the house: consistently damp, shaded siding is a sign that the same conditions could be pushing moisture toward the sheathing and framing behind it, particularly around poorly flashed windows, doors and utility penetrations.
If exterior staining lines up with an interior room that also has a musty smell or visible spotting on an interior wall, that is worth having looked at as a connected issue rather than two separate cosmetic problems.
Common questions
Does exterior mold or algae mean there is mold inside too?
Not automatically. It is worth checking the interior wall behind a badly affected exterior spot, especially if there is also a musty smell inside, but exterior growth by itself does not prove an interior problem.
Does pressure washing fix it?
It removes what is visible, but on porous surfaces like unpainted wood or stucco it can also drive water into the material if done aggressively, which can make things worse rather than better. A gentler wash with an appropriate cleaner is usually the safer approach.
Not sure what you are looking at?
Send a photo and rough measurements, or call and describe it. We will tell you whether it needs a professional.
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