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Mold – how to determine if it’s mold or just wood discolouration; if it’s mold, how do you treat it?

Mill_house | Posted in General Questions on

I’m remodelling my bathroom.  I just gutted it.  I’m looking up at the exposed ceiling.  I see the joists (8″ fir) and the floor of the second story.  I see that some of the wood has dark marks on it.  The wood is not wet/damp (visually or to touch).  I don’t have a moisture meter. 

How do you determine if it’s mold?  And if it is, how do you treat it and ensure it’s good prior to putting the room back together?  Not a lot of information on the internet about this topic.  I’m including a picture.  I did scrub the darker areas with soap and water and then used sand paper/steel wool.  The dark colors didn’t come out.  

And lastly, any advice on what to use / stay away from re: wall assemblies in bathrooms?  

Thank you.

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  1. GBA Editor
    Patrick Mccombe | | #1

    A drop or two of bleach will tell you immediately. If it's mold, it will go away under the drop of bleach. Dirt will not.

    1. Mill_house | | #3

      Thanks. I tried a few spots and the discolouration is still there. (Some of the discolouration came off on the bleach soaked q-tip. )
      I see the answer below advises use of a product called Concrubium. What do you recommend if I do come across any mold?

  2. Expert Member

    It doesn't matter if it's mold, or was mold sometime in the past, or is something else. As long as your renovation makes sure it doesn't provide the conditions for future mold growth in the bathroom, it's safe to simply ignore the discolouration on those boards. If it is going to bother you to leave it untreated, you can encapsulate it with Concrobium.

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