Whoich bit are you refering to? None of it looks like mold. It's chimney creosote.

Wipe some fresh bread on it and see if it takes, but I agree with your sweep.

What else could it be? It can be wiped off with a piece of cloth. I'll try with bread.

..... If you leave a dirty peice of bread out, something will take. You have to taste it to be sure.

You could always just build a fire in your fireplace. Even if it was mold, I suspect that that would handle the issue pretty easily.

OP, I was going to say that I don’t think it’s mold, because I don’t think mold can grow on concrete, but apparently it can! And it does kinda look like that when it does. Search for leaks. Check to see if you have a dead valley at your chimney. If so, you need to install cricket flashing.

Seems like a non issue to me

[removed]

• You are asking about a project that someone else did.
• You are not asking a question relating to something DIY.
• You are not asking a question at all.
• You are submitting a "DIY Tips" or "Tricks" post.
• You are trying to submit a survey / fundraiser.
• You are asking what an item is called - try /r/whatisthisthing.
• /r/DIY allows only two types of posts - finished projects & help requests.
• If your submission is neither of these - it will be removed.

Please read our guidelines before resubmitting.

If you believe this was a mistake, please message the moderators.

It used to have a leak, not sure for how long as I bought the place last year. The crown was leaking. The flashing was okay.

The discoloration may be permanent even if spores are gone

Wouldn't mold smell? I have a very strong sense of smell but there's no musty smell around this.

I don't really care about that. More of a health concern

Mold needs water to thrive. If there was a water issue in past, the dormant spores could be visible.

Only way to remove is warm soapy water and elbow grease. Bleach just shocks them into dormant state but could come back with more water/humidity. Mold doesn't always smell.

I would add one of those humidity checkers to the area and check how it compares to.other part of the house. If it's damp in there probably still leaking. GL

Makes sense. I wonder how far up the chimney it runs though. Also it doesn't seem to be spreading, if that means anything. It's been like a this for a few months now

Hygrometer on Amazon. I think they are two for $12.

leave one in there and pay attention to the humidity level during rainy days and in humid summer.

Maybe scrub off what you can reach with soap.

If you are worried about it still. Either spray bleach water in there from the top down and hope to shock the mold.spores that could float around the house. Or get ozone blasted in there to kill the spores. Unless you have a cooky friend with an ozone machine like me you probably need a pro for that.

There's also one non chemical option . It's called benefect and it's mostly essential oils. I hired a pro to apply it in a clients house once. Way safer than the hard stuff. GL

There are many different types of mold. But I think for most of them, the smell (and definitely the biohazard) comes from the byproducts not the spores. So the smell could be gone but the discoloration remains.