In process of bathroom renovation and have gutted it down to the 2x10 floor joists.

Supply and drain plumbing is in and I'm ready to cover it up.

It's a 42 year old house and previous subfloor was 1/2 inch plywood, which is actually still in fairly good condition, except for being cut up like a puzzle to access the plumbing.

Just how thick is "thick enough" for new plywood subfloor? We are planning on using CBU or some other sort of backer board. (It used to be mud below the old tile.)

I'm afraid I don't know what the code is for this area of Maryland. I assume that thicker plywood is better but it also costs more and isn't as easy to deal with as far as getting it home, in the house and up into the upstairs bathroom.

I was just going to use 1/2 inch.

There's some details that need to be addressed first.
What type of tile are you going to use, IE: porcelain or natural stone?

What is the span of the existing floor joists and their spacing?

The answers to those questions will determine your sub floor requirements.

First off 1/2" is not enough for any tile flooring. The absolute minimum would be 5/8" exterior grade plywood for a ceramic or porcelain tile floor. This also depends on the joist spans and spacing.

Cement board has no structural characteristics. It is only a backer for applying tile to rather than tiling directly to plywood.

joists are on 16" centers which run lengthwise down 14ft room.

Not really sure the type of tile. Prob stone but should prob use something that be used for any tile if we go that route.

Thanks for the help!

If in fact you have a 14' span your existing joists will just barely support a ceramic / porcelain type floor. There's just too much deflection in what you have now. And that does not take into account for the type and thickness of subflooring.

If you are going to build for the worst case scenario, then you need to construct your floor system to natural stone specifications.

For stone, your floor joist either need to be doubled up or the span reduced significantly.
For a stone install you will also need to have a double layer of plywood. IE: two layers of 5/8" or 3/4" plus a 1/2" layer, plus a tile underlayment such as 1/4" cemetboard or something like Schluter Ditra.

Uh... I guess this is gonna be one of those "I'm sorry I asked.. but actually far more glad that I asked" issues.

Kinda like the new window that we "almost" installed in the bathroom. The exterior wall over-hangs the lower level wall by a couple of feet. There are actually short 4 ft long pieces of 2x10 joists attached to the main joists for the rest of the room, which are 11 ft long. They overlap by 3 feet and the supporting wall is right in the middle of the overlap. The tub will be along this wall and centered over that wall which we should probably reinforce.

There is noticeable deflection already after 40 years of settling with the overhang. I would estimate no more than an inch total. Because of the overhang there is no way to reinforce the floor and install the window (which is 6 ft wide.)

Even if we go with ceramic would two layers of 5/8 or 3/4 be needed? Height really isn't a problem as there was mud below the tile before and was fairly thick.

Thank you for your insight.

I would recommend you remove the existing 1/2 inch plywood and replace it with two layers of 5/8" plywood. The first layer gets screwed into to joists, the second layer gets screwed to the first layer only on 8" grid. Second best would be to go with 3/4" plywood on top of your existing subfloor.

Most of the 1/2 inch was removed to access plumbing. However, walls were built on top of the subfloor, so around the perimeter of room there is about a one inch "lip" of 1/2 plywood as my circular saw could only get so close.

I could remove that lip with Sawzall, but thought it might make a good attachment point for new subfloor. OR, if putting in new 1/2, was gonna put some blocking below that lip to attach new 1/2 to. Then put a layer of 3/4 on top of the 1/2.

That would give same thickness as 2 layers of 5/8".

This probably all sounds just ridiculously stupid but I don't know what the standard procedure would be to replace the subfloor around the edges. I can't lift up the walls.

You can screw and glue a strip of 2x4 or a 4 inch wide strip of 3/4 inch plywood to the bottom side of your existing subfloor. 1/2 of it showind. this will act as backing for splicing in the new plywood. screw and glue the first layer, screw only for the second layer.

I was thinking something like that would work.

One last stupid question (in this post, that is!)... would it be acceptable to put down new 1/2 and then 3/4 on top of that?

Thanks for your patience!

No, the 3/4" must go down first. It is glued and screwed to the floor joists. The second 1/2" layer is only screwed to the first layer of plywood with 1-1/4" screws.
Here's a link to the installation of the second layer.
http://www.schluter.com/media/DitraHandbook.pdf Go to page 23 of the Handbook and follow the directions under Wood Underlayment.

Cool. I'll check that out.

Thanks. I appreciate it. You guys are awesome.

My response would be this, since everyone is suggesting 2 layers of proper plywood than 1/2 on top of that, why don't you just hardpack the entire floor area with 1 1/4" mortar over the top of the 5/8" plywood same thickness. Than it does't matter what the finished flooring is ?

If height is not a problem and the floor joists are 2x10 why can't you drop the 5/8" to 3/4" plywood subfloor and install an 1" 1/4" hard pack over than the tile or stone ?. That is what we used to do in the older days.