I have small sections of a room (about 4'x4') where some of the 1X8 3/4 thick planks got wet over time (hey were under toilet). They look iffy but they seem to be stable and hard with no warping, just look darker then the rest of the room since they got wet at one point. Should I replace the planks with similar planks or can I replace with a 3/4 plywood piece the size of the plank hole I will be removing (I would cut the hole from joist to joist so the plywood would have full support)?
After I fix it I will be installing new 1/2 plywood over the entire room and then hardwood if that makes a difference.

Thanks

 

If they are just stained, but solid, not warped, not rotted, then I'd probably just leave them be!

You have the eyes on the issue, we're just guessing!

 

If they are solid I wouldn't bother removing/replacing them.

Is this just a half bath? I like hardwood but a full bath isn't a good place for it.

 

I could probably just leave them but its already easily accessible and it would be a quick fix, I just wanted to know the plywood method would be OK since its a lot cheaper.
Also it wont be a bathroom any more so hardwood floor is fine.

 

Sounds like you want to replace the planks.

I replaced a number of 1 x 6 planks in my home when I redid the floors. I used 3/4 ply cut into 1 x 6 strips....but then I put 3/4 tongue and groove sheet over top of the current floor. It was a rental with a lower apt and I needed quiet floors.

3/4" plywood will work in your case, but if you cut it into 1 x 8 strips, it wont have as much rigidity as the same sized planks. You say you are then adding another 1/2" ply. I would suspect it will be fine. But make sure the ends of the plywood are on a joist and well screwed.