Expanding dark spot(s) on wood floor

Q: I have engineered wood floors that have been here since I moved in 8 years ago. There has always been one dark spot that kind of looks like a burn mark (as if someone placed something too hot on it). Now this spot has grown larger and there are two new spots (about 3 to 4 inches across).

All the spots are are equal distance from each other, in a line about 3 feet from each other. They are not moist and they are not soft. What causes dark spots on wood floors?

A: It sounds like water marking, but this would require a constantly present water source, and I would expect to see other signs of damage such as cupping if this were the case. Could it be mold? Again, it is my understanding that for mold to grow or spread it also needs the environment of wet and warm. I’ve posted a question about this on a forum but thus far nobody else has chimed in on the issue.

Follow-up Q: I thought about mold as well, but of course as noted the spots are not damp or spongy. One of my thoughts is that since they are in a a straight line pattern but about the same distance from one another that it may be water/mold from the where the water line runs under the concrete slab and may be possible these spots are where some joining or connection is made in the water line. Maybe there is a slow/low leak and very little or no water gets to the top surface, only the resultant mold. On the other hand, I am not sure of the path of the water line. I may call the water company to confirm this water line path to exclude this theory or take it to the next step. (Glad I purchased that water line insurance two years ago).

I also thought about pet stains, as the previous owner had a small dog that was left alone for long periods of time. If there were pet urine stains that were treated and the floor refinished, could they dark spots reappear a few years later if the pet stains were not correctly treated/cleaned?

A: I’ve sanded plenty of floors with pet stains and other water marks. They either can be sanded out or they can’t. BUT, they don’t grow and they don’t reappear after they have been sanded away. I think your first theory is likely more on track. If it is correct, I guess it will mean removing at least part of the floor and chopping up the concrete. The scenario you mention with the water line makes good sense to me. Have someone put a meter on those spots and reference them with other areas of the floor that don’t have spots. If there is water you will get a higher reading on those spots.

Webmaster’s note: we recently had a job where old pet stains were being kept damp by vent condensation from below, and it stunk to the high hills. We’d never seen old, WET pet stains before. It made me think of this Q&A.

Related Q: I had bamboo flooring installed five months ago and I just noticed dark spot coming from under the baseboards, it would appear to be moisture stains. How do I get rid of the dark marks?

A: The more important question you should be asking is “where is the water coming from”? You may have to sand or scrape off the black mark.

Related Q: I installed engineered wood floors in my living room in July 2007. The wood strips are 6 inches wide, Brazilian Timborana, bought at Lowe’s. They are glued down onto concrete. My living room is the first floor and the concrete is at least 6 inches above ground. Today, 10/15/07, I noticed black spots on the floor, and when I tapped on a couple of them, they sounded hollow. What problems am I facing? Do you think the wood was not properly treated?

A: Did you test for moisture before you installed this floor? Is there a possibility you have gotten a leak since installing the floor? Is there anything else that would indicate the wood has swelled?