Hello,
slow unseen leak in bathroom did what one would expect to particle board underlayment. (And thanks for the advice elsewhere in this forum about how to remove p board !).

The 5/8 plyboard subfloor appears sound from underneath: visually and from within bathroom (tapping w hammer, probing for rot). However, from within the bathroom the plywood is mostly black. This does not appear to be mold and it is not wet. I am thinking that when the house was built (1968 - North Carolina) that some sort of waterproofing was applied before the p board underlayment was installed.

1. Does this sound likely / reasonable ?
2. Regardless, what is the best way to confirm the plywood subfloor does not need to be removed ?

many thanks,

or perhaps the black stuff is/was glue ? I

I think it's more likely to be some sort of glue/tar. That was often used under VCT (or VAT) or other types of vinyl.

@floorgirl
thanks for your response

to be clear the layering is

vinyl
p board
unknown stuff
plywood

Yes, my guess would still be adhesive left over. Then, when they put the next level of vinyl on top, they couldn't get it off and put plywood on top.

But, this is purely a guess. I can't see it...I'm not there. I'm simply saying this because it's a coating. If it were mold, it is more likely to be in the wood rather than on the wood. Again, just guesses. Regardless, you may want to remove it, if you can. The better the subfloor, the better your next installation.

Just to report back
the 'coating' is actually paper. One person I showed it to said it was like almost like roofing paper.

I don't think we can we really help you since we can't see what's there. Your best bet is talk to a local contractor. We are just going off of the info you are providing.

but, if there is roofing paper or material or whatever, I would just remove it.

generally, you would install solid hardwood on top of 3/4" plywood (w/ tar paper in between).

Debbie Gartner, aka The Flooring Girl
http://TheFlooringGirl.com
Westchester County, New York