Super Lumen - thanks for your input. From what I have read on these 1/8" - 1/4" tiles, they can be put over existing flooring. I see I will need a flooring workman to come over and take a look. I appreciate your advice.

Its not a matter of going over existing flooring. It's a matter of deflection. If your fir had another 3/4" of subfloor under it, then you could *probably* put a lot of nails into the fir + a crack prevention membrane like Ditra and get away with it. The fir planks aren't rigid enough because they are individual instead of a sheet. And yes, a flooring guy will be able to give you some advice.

I knew this wasn't going to be easy! No, the fir flooring has four inches of open space underneath it. I understand what you're saying. Time to search for a flooring person. Thanks.

I was trying to avoid having the kitchen floor built up so much that I'd have to jump into the room from the dining room. LOL

Well anything is possible if you want to spend some $$. The fir can be taken up, and then your joists sistered down 3/4" on each side with rails, so 3/4" of subfloor can be set in between the joists flush. Then another 3/4" on top where your fir current is, making for a 1.5" thick base. Then DITRA + your tile probably adds another 1/2" so you are 1/2" above where you are now.

All of this of course would be some serious demolition. If you want something very thin and easy, I'd look into the very high end luxury vinyl. The expensive stuff looks a lot like real tile, is only about 1/8" thick at most, and can be "grouted" with an epoxy. It can go right down over your fir no problem and even the most expensive vinyl tiles will be a lot cheaper than cutting up your floor.

Yes, that sounds like a lot of serious work and $$. Actually, the thin porcelain tile I was inquiring about is only 1/8" to 1/4" thick but now I know it would require too much prep. I'll be looking into the luxury vinyl which is much more realistic. Thanks, again.

To install vinyl (sheet or otherwise) you will need a flat sheet of "something" on top of your planked subfloor. Vinyl is quite soft and will snuggle down into ANY gap. If you lay vinyl over top of your planks, you are going to see a "washer board" pattern within a few days. And it will only get worse.

Vinyl often allows luan board which comes in very thin formats (1/4" is possible). The board would be put down to create "flat and continuous" and then you can apply any vinyl product of your choice.

Or you can work with RIGID vinyl like CoreTec. These types of vinyl plank are rigid enough to span the gaps of the planks. The only thing needed is they must be "flat" which might require some sanding....but that's about it.

I realize I will have to have some kind of underlayment under the vinyl tile. I just didn't want anything too thick to raise the height of the floor too much. The luan board sounds like a good idea and I thank you for that suggestion and your input.

Not that I would use it but this may be an option for your situation.

http://www.ardexamericas.com/sp/Documents/TechData_En_ARDEXUI720.pdf

Thanks. I will check it out.