Replace it with ply and then cement board. You can't install cement board directly over dimensional lumber.

As for taping, I would tape any and all seams regardless of what the manufacturer says.

Thanks so much Bill, I am almost done removing the pressed wood. Is it OK if I use 1/2 inch ply and 1/4 inch cementboard? That way I will be at the same starting height as before.
Thanks again,
Marisa

1/2" ply over the plank decking is fine.

When you install the backer board over the ply, use a 1/4" square notched trowel and unmodified thinset to bed the backer board on the ply, then use screws to secure it. Read the wonderboard installation manual so you know the fastening schedule, and be careful when you get to the edges that you don't fracture the cement board when nailing or screwing close to the edges.

You can tape (mesh tape) and thinset the seams right after installing the cement board or do it when you tile.

For first-time DIY it's sometimes easier to do the seams and let the thinset cure, then tile the next day. Letting the seams cure gives you a nice surface to work on without having to worry about the seams while you tile.

When you thinset the seams, keep the thinset level with the top of the backer board, try not to end up with a hump of thinset on each seam.

Here's a tip to make tiling even easier: use Ditra instead of cement board as an underlayment. My first two tiled rooms I used cement board (messy with the dust when you cut it, heavy to carry). Then I discovered Ditra (can carry a 300-square foot roll, cut with a utility knife) and I'll never go back. It costs about twice what cement board does but saves gazillions of hours and is actually better technology.

And BTW, on your "can I DIY it" question, absolutely yes.

Mongo, thanks so much, that walks me through a lot of my questions. Do I use modified thinset for the seams? Also, when putting down the plywood, do I just screw it into the planks or do I need to go through the planks and into the joists?

Home Depot does not carry unmodified thinset. Is it a problem to use the modified on plywood?

Also, what is your opinion of using polished honey onyx mosaic tiles for a bathroom floor?
Thanks again,
Marisa

Weedyacres, thanks for the encouragement and advice about the ditra. I will look into that. I did buy the cement board but haven't put it down yet. Thanks again!

You can use unmodified or modified for the seams.

When putting down the plywood, screw it to just the planks, not the joists. And prior to that, make sure your plank decking is secure, no loose nails, etc.

Home Depot carries Custom thinsets, look for {{gwi:1467434}} CustomBlend which is unmodified or also referred to as a "dryset". The gray is a couple dollars cheaper than the white thinset and is perfect for that application.

Their modifieds are {{gwi:1467435}} VersaBond, which is lightly modified, and {{gwi:1467436}} FlexBond, which is highly modified.

Or you can add a liquid latex admix to CustomBlend in place of some of the water to modify it yourself.

For setting the tile I'd recommend using the FlexBond and if using a light grout then use the white FlexBond. If a dark grout, you can use the gray FlexBond.

If you can't find unmodified anywhere fior between the ply and the cement board, no worries, you can use a modified thinset. It's just that all you're using it for is a void-filler and not for bonding strength, that's why we use the unmodified, to save money.

Mongo, thanks again. I did buy the flexbond yesterday in Gray, but not sure of my grout color yet, so may just use it for the cementboard and will look for it in white for the grout if I go lighter. Thanks again, I really appreciate your help.

Mongo/Bill, how important is it to thinset the cementboard to the plywood? The reason being is that I have to really slide/shimmy these pieces of cementboard to get them around some protruding pipes and the baseboard heater. I am afraid if I mastic the floor, I will be scraping most of it off when I slide the cementboard in.

Thanks again

It's important to use thinset between the ply and the cement board. Not using it can lead to flexure cracks in the grout and/or tiles popping off the cement board.

Another option would be to use a membrane like Ditra. It'll serve as an uncoupling membrane, and it's flexible and might be easier to place for your application. There are two thicknesses of Ditra to help with floor thickness compatibility.

Going back to the cement board. While it depends on access, you could cut the cement board to size, thinset the area where it will be set, then place 4 or 5 long dowels on the floor right on the thinset. Slide the cement board in place on top of the dowels. The slide the dowels out, dropping the cement board into the thinset.

Easy peasy.

Thanks Mongo. I hope it is easy peasy! I have done more acrobatics trying to get this stuff down on the floor. My shoe got caught under the cement board doing the dry fit and I had to retrieve it with the crowbar.
For the ditra, I think I would need a 3/4 inch ply under it, which currently I have 1/2 inch. I may try the dowel thing. Sounds like a good idea. I may only need one or two. What are dowels, just small wooden poles?

Just a quick update. I got the cementboard down with the thinset underneath. Wasn't too bad. It is screwed in and the seams are taped with thinset. Thanks so much for walking me through that process. I felt much more confident knowing I had good advice.

Now I have to search for the right tile for the bathroom. I really wanted onyx mosaic, but the tile people say that it scratches easily and also is a softer stone which could chip. Also needs to be cleaned with only a stone cleaner. Not sure my two kids would treat it too kindly. She said glass tiles also would scratch. I had my heart set on something really beautiful for the floor and since most natural stone and also glass tiles may be too "high maintenance" I may need to search for something else.

Coming late, but I'm really glad to see you're not using onyx on the floor! It's *so* not right for that.
Save the extravagant high-maintenance stuff for the Master Bath and get a cool porcelain for this room. Go to a high-end stone showroom and look at their imported rectified porcelains. You'll find something cool *and* kid-friendly :)

Hi Flyleft, thanks for the advice. I will take a look at the rectified porcelains.

I am thinking about tiling the wall up to about chair rail height. Does anyone know if I need to put any special underlayment on the wall or if I can go right over the current wall. (there is no tile on the wall now)

Thanks

If it's not in the shower area, you can go right over the drywall.