If your walls are turning black in a corner, it’s no accident: find out what it really means (and how to fix it)

Have you spotted a small black spot in a corner of the wall? At first, you think it’s probably a bit of dust or a forgotten mark… and then the days go by, and this area widens, darkens, and sometimes, a musty smell begins to be noticed. Nothing trivial about that. These marks are often a sign that something is going on behind the walls. But don’t panic: with a few good reflexes, you can take action, limit the damage, and restore a healthy interior.

Why do these black spots appear? (It’s not just a cleaning issue)

A wall that turns black, especially in a corner, is rarely just a matter of cleanliness. More often, it’s excess humidity that’s the cause.

In poorly ventilated rooms—a bathroom where steam gets trapped, a kitchen that heats up without openings, or even a bedroom that’s rarely aired—humid air can’t escape. It ends up condensing on the cold walls… and that’s where mold finds its favorite playground.

It can also be due to leaks from outside. An overflowing gutter, a damaged roof, a porous wall, or even water rising from the foundations… all of these can cause these infamous stains.