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I pulled up carpet today and revealed some blackening around the fireplace. At first I assumed soot build up due to the location, but I want to make sure that makes sense, and not that there’s some mold growth here. The carpet had a bit of black on the underside.

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    It looks like that could only be mold or mildew. If smoke or soot was drafting anywhere it would be higher, not at the floor level. Check the floor for moisture by taping a piece of plastic, 1 or 2 ft square to the floor for a few days or longer. If condensate shows up under the plastic, well that's not a good thing
    – Jack
    Apr 10 at 3:35
  • I'd suggest getting an inexpensive moisture meter and checking all around that area. I have a fireplace with a brick-clad chimney and the bricks were absorbing rain water like a sponge, leading to the sub-floor all around the fireplace rotting out. I've since sprayed a clear masonry sealer all over the outside of the chimney which has vastly improved the situation.
    – brhans
    Apr 10 at 13:29
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    We had this same issue with a number of units in my condo building. An inspection of the chimneys revealed that most of the chimney caps were rusted and leaking. In all the cases the rain didn't leak enough to be noticed until the flooring around the fireplace showed signs of water. Some with damp carpet, some had water weeping through the grout between tile. Some showed dark discoloration of wood flooring. Inspect your chimney
    – RMDman
    Apr 10 at 14:38

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With the closeups and seeing the mortar joint close up, then I realized the rest of the joints are done with black mortar. That is what is on the surrounding floor area.

Since it was a bare concrete floor and masonry can be messy, the workers cleaned up what was needed to get the floor flat, but not enough to clean the black residue that was left over. Black pigment can be difficult to get rid of completely, especially when it gets into the pores of the concrete floor.

Check for moisture anyway, always good to have a comfort margin.

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