OK, I have pretty much decided on 3/4" quarter sawn oak stapled to 3/4" plywood, floated over a concrete subfloor.

There are two rooms that have about 350 square foot in each, and my wife really dislikes the idea of a threshold or expansion joint between them.

I know that the proper appoach would be to put in a "T" moulding or proper threshold that allows the two floors to move independently. But that would really mess up the lines -- even I agree.

So the question is -- this will be a very strong floated floor, with 3/4" flooring stapled down to good quality subfloor over a nice slick vapor barrier -- can I just give the floors generous expansion joints all around, INCLUDING the door jam being radically undercut? I think I can get as much as 1 1/4" undercut under the jamb and then I would run the flooring about 1/2" under the jamb. That would allow quite a lot of lateral movement, before the "connection" between the two floors would begin to bind on the sides.

I will remove the door trim completely and use a full sheet of plywood under the floor in the door way, just "notching" it for the walls. This would further strengthen the floor in the critical door area, and help tie the two rooms together.

Am I doomed to failure? I know full well it MIGHT fail, but I am willing to take a bit of chance. Worst case is a buckled floor that I need to remedy by sawing a chunk out between the two rooms and putting in a threshold. Or a crack I suppose, but I doubt that. My concern is that one room may essentially try to "twist" which would then cause the whole floated floor unit to lift off the floor somewhere, as the floors deform.

The rooms are all part of one large concrete slab and the slab is very stable, with no cracks and no seams between the two rooms, so the slab itself is not going to move.

What do you think?

Oh -- I know one alternative is to screw down the whole subfloor. Lot of work for one little doorway.

Floating a subfloor and installing a finished floating floor are two totally different things. But you do have to watch the doorways. That is why the plywood is either cut into 2'x8' or 1'4"x8' planks and run the opposite direction you want the floor to lay. This means running the subfloor planks through the door, with staggered ends.

Perry's absolutely correct on this. You can forget the t-mold (and I would if it were me) but you must run your subfloor panels at 90 degrees to the direction of the wood flooring. I like the idea of 2x8 panels or even 16" x 8' for the floating subfloor. Stagger the end butts as Perry mentioned. And there cannot be any subfloor/underlayment joints in that doorway Here's an article by NOFMA:
https://www.nofma.org/Portals/0/Publica ... 010_04.pdf

"Floating a subfloor and installing a finished floating floor are two totally different things."

Well aware of that. But what exactly do you mean?

Would you suggest that floating a subfloor as I have described is less likely to buckle or more likely, as compared to just a "surface float"?

I want to run the floor "through" the door -- which means that the 2x8 strips of subfloor ought to run parrallel to the door (90 deg to the flooring). I plan to use doorway as my reference point and make the whole floor a "system" with the greatest strength exactly at that doorway.

I am less concerned that the floor will crack there, but I am worried it will lift/buckle due to a "twist" as noted above.

thanks already.......

If the subfloor seam is in the doorway, your flooring will run in lengths across that seam and into the other room. With the boards runing lengthwise through the doorway, it has strength with the staggered flooring.

Don't put a seam at that doorway in the underlayment, if the flooring runs across the doorway, instead of through it.