Right now I'm working on removing the rotted plywood panels somebody put on the floor. Under the panels is a tarry paper type thing under which is galvanized steel. The paper stuff does not look like a carpenter thing.
The panel removed was rotted by seepage from the door on one side and by a leaky water heater on the other.
By the door will be the toilet. On the other side will be the bathroom. I think under those two areas, I might put plastic planks.