Mold vs. Mildew: What Is Actually Different
Mildew is a flat, powdery fungal growth that lives on the surface of a material and usually wipes off with a damp cloth. Mold grows into whatever it is on, sends roots (hyphae) below the surface, and comes back after wiping because the part you removed was never the whole colony.
The distinction matters because the fix is different. Mildew responds to routine cleaning and better ventilation. Mold on porous material, once it has established, needs the affected material assessed and usually removed, not just cleaned, because cleaning the surface leaves the growth inside the material intact.
The most reliable field test is texture and persistence. Run a fingernail across it after it dries. Mildew is flat and comes off completely, leaving no shadow. Mold on drywall paper or wood grain tends to leave a stain even after the visible growth is gone, because it has already changed the material.
If the same spot keeps returning within days of cleaning, treat it as mold rather than mildew regardless of color, and have the moisture source identified before doing anything else.
Common questions
If it wipes off completely, is it definitely mildew?
Usually, yes, on a hard nonporous surface like tile or glass. On a porous surface like drywall or wood, something that wipes off the surface can still have growth underneath, so it is worth watching that spot over the next week.
Does bleach tell you which one it is?
No, and this is a common mistake. Bleach can lighten both, so a spot that looks gone after bleaching has not necessarily been identified or removed, only discolored.
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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