Newbie with question re: plywood subfloor

[tilehound]

Hi all!
I've been reading this site for a little bit and you all have given me a little more confidence for a project I want to do. I've been planning on tiling my foyer, which currently has a vinyl floor on top of 1/4" plywood, which is itself on top of what looks like about a 3/4" plywood subfloor. I plan on removing the vinyl floor. But my question is this:

Do I need to add another layer of plywood on top of this? If so, how thick?

I tried to search the forums, but I'm a bit confused when everyone talks about needing at least 5/8" plywood and whether that is on top of a plywood subfloor. I am planning on installing porcelain tile on top of Ditra (you all have convinced me that's the way to go).

The Deflecto calculation says I meet the specs for the porcelain. The only thing that gives me pause is that in one section I hear a bit of a squeak, though the floor feels really sturdy. I would think screwing the existing plywood into the joists underneath would do the trick for that.

Thanks in advance for your help!

[Deckert]

Remove the vinyl and the 1/4" plywood. You don't need another layer for porcelain as long as the deflecto says you are good. I'd verify that you used the right numbers in the deflecto just to be sure.

[tilehound]

Thanks a bunch for answering so quickly. This site is awesome!

One more question: Is there an easy way to remove the 1/4" plywood? I'm not sure how it was fastened to the plywood subfloor. I'm assuming nails or screws. But I suspect someone used some mastic or something. What would happen if I didn't remove the plywood?

Thanks again in advance.

Steve

[cx]

Welcome, Steve. If you'll go to the UserCP above, find Edit Signature and enter that first name it'll appear automatically in each post for us to remember.

'Fraid I'd like to know more about your joist structure before I'd pronounce your subflooring adequate. A fella can meet the joist deflection requirements with 24"oc spacing, for example.

The problem with your underlayment for the vinyl is that it's probably Luan or similar faux mahogany plywood and not suitable for a tile installation. It's ranked with MDF and particle board in that respect. It's soft, not assembled with exterior glue, not stable when exposed to moisture, etc. Not what you want anywhere in your tile installation.

You'll just hafta try to get it off there any way you can. Large prybars and roofing shovels and such may help. If it's glued, your option is to remove the entire subfloor down to the joists and start over.

So try to take some of it up and see how it goes. Sometimes it's relatively easy, sometimes it's impossible without serious damage to the subflooring.

Then we'll talk more.

My opinion; worth price charged.

[tilehound]

Thanks, CX, for the quick reply. You guys never cease to amaze me.

The joists are 16" OC, 9.25" wide and 1.5" thick. Don't know what type of wood, but they're in good condition.

I'll take the plywood out as you suggested. I was just hoping screwing another sheet of plywood on top would be enough if I can't take it off. I take it that would not be a good idea.

If you have any other suggestions on how to proceed, I'm all ears. Thanks again.