If you want tile, you need a good base.

I can't believe that there is ¼" ply directly over the joists. ½" maybe, but ¼"????

I could see the ¼" over the particle board, which is over ½"

Anyway, you need a stout base for tile & mortar. ¾" may not be enough beef. an additional ½", may need to go over the ¾".

Laminate would be your choice. I personally wouldn't have laminate in my house, as it cheapens the look, feel and value.

It really is 1/4 plywood on the joists. My house is like the "house that Jack built" (or the little pig that built the straw house) and my gut feeling is that while we have the subfloor exposed it seems like a good time to rectify the initial cheapskate building practises. What we were told about adding 3/4 plywood is to add the 1/4 hardiboard on top. Does that sound right to you if we go with the tile? Thanks for input!

Just got back from Lowes. The guy there said we didn't have to use plywood for a new subfloor but could use OSB. For some reason this seems like just a step above the particleboard in quality to me. He said that is what they are using for new home construction now exclusively and should be adequate. Opinions? Thanks!

Never heard of a 1/4" subfloor either, but I'm not doubting you.

Pull up the particle board and replace it with 3/4" BCX square edge underlayment, no T&G, nor OSB or AdvanTech. You are not supposed to have a 1/4" plywood anywhere under tile, but since it is the main subfloor I would just leave it.

Go over the 3/4" BCX with a 1/4" cement backer board laid in a thinset "setting bed" and install per the instructions on the label.

Lacahase, you should also be checking whats under the 1/4" subfloor. The joisting has to meet an L/360 deflection for porcelain tile. Measure the joist.... width, height, spacing, clear span ... and report back.

My subfloor sits above a basement so it is easy to get beneath the joists. The joists are 16"OC beam to beam, 9"H X 1 1/2"W and the clear span (if I'm understanding right, measuring from wall to the center of the room support beam) is 12' 1". My husband thinks the particleboard top piece is glued to the bottom plywood layer which I'm not sure how we will be able to take up one without taking up the other. Thanks for your help!

I would use T&G, just because you don't want any flex between panels between joists.

Thank you!

That is a very valid point. The problem is walls are now in your way trying to get it down. You could use T&G plywood and trim some off near the walls and put in some 2x blocking at the seams.

Your joist are plenty stout for porcelain.

Forget the OSB and any information you get from Lowes or Home Depot. You can check out John Bridges forum. He deals expressly with tile floors and tiling in general. jerry t's advice I agree with. You do not want particle board under a tile floor. If it's glued to the 1/4" plywood this will be a big problem. The whole situation is a problem because all your walls are (more then likely) sitting on this particle board. If that's the case I would seal the particle board and go over it with the plywood/cement backer board. Not the best way by any means.
Ron

I put in another post, but back to this one because it has all our job details. We were going to try to rip out the subfloor today and put in 3/4"T&G that my husband bought yesterday. After peeling back some adjacent carpeting and remeasuring, we decided that the base subfloor is actually 1/2" as someone initially suggested that surely it was. That is better, but the particleboard is going!
I was trying to get a reply whether we should take it down to the joists or now leave the 1/2" and top it off with the 3/4". Also, not sure about what to use to seal off the particleboard that must be left at the perimeter joists. Silicone or poly or what? Then, I have gotten this other idea which am not sure which is best. Was thinking that cement backer board comes in 1/2" and now wondering if it would be logical to just use the plywood and top it with 1/2" cement backer board. Would that be an adequate subfloor or should we top the base with another layer of plywood first? If so, better another 1/2" or the 3/4" which is going to raise the floor a bit by the time we are all done? Thanks!

Put the ¾ over the ½. Cement board adds no structural strength to the subfloor. It's purpose is to not let the movement of the wood structure transfer to the tiles.

As he said you still need additional plywood. When I tearout PB I replace it with 5/8" BC plywood and Ditra membrane instead of the 1/4" cement board to hold the height down.

We have almost gotten the particle board torn out. My husband has a work trip so we are taking a little break from it a few days. It looks so much better, just with the plywood surface, almost makes me want to forget tile and go with wood or laminate. But, since I probably will stay on course. Hey, we had decided to take the 3/4 back and use another 1/2" ply layer, just to keep the height lower. Do you think we shouldn't do that? I read on the John Bridge forum where I was trying to glean more info about the Ditra. Had never heard of it; do you really think that is superior to the cementboard? Is it a lot more expensive? We're trying to keep costs down as well. Oh, also, after raising the carpet next to the kitchen I can see the horrible particleboard lurking there as well. I cringed to see it, but really don't want to take out every floor at this time anyhow. Thanks for help!

You need at least an inch and an eigth of subfloor for tile, before the anti-fracture membrane, or tile board. And that is, if the joists are right for the span. You may need additional blocking between the joists, to beef up the substrate.

Remember, any movement at all, and the tile job is toast.