[andynick]

Hey all,
I'm down to the joists in my master bathroom, ready to install fresh 3/4" Plywood Subfloor.

When I ask friends their opinion on my flooring layers needed for tiling, I usually get one of the following (conflicting) answers:

1. Just put 1/2" Cement Board directly over your subfloor! That's enough structure to tile on for a bathroom floor.

2. You'll need an ADDITIONAL underlayment, ideally another 3/4" plywood/OSB layer. On top of that, you can use a thin 1/4" cement board or a membrane like Schluter Ditra.

I guess the confusion is whether subfloor IS underlayment.
Appreciate the advice in advance!

 

[chandler48]

You'll first need to know the size of your joists and the total unsupported span. Let us know that and we can go further. Usually 1 1/4" of subflooring (3/4" advantech and 1/2" plywood is sufficient, but knowing the span is important. You use 1/4" cbu on floors, and 1/2" on walls. CBU offers absolutely no structural strength.

 

[andynick]

Alright! My floor structure is unique due to the kitchen structure and soffits below, but each joist is 16” apart or less. I’m attaching some pictures to help illustrate.

I’ve fastened some extra braces where my shower curb will be, and will continue to sister 2x4’s to help get my subfloor as well supported as possible.

Appreciate the advice in advance!

Again, I'm currently planning on using this 3/4" tongue-in-groove plywood as subfloor, and questioning whether or not I need another layer of anything.

Pics Below!
Pic 1
Pic 2

 

[chandler48]

It appears your structure will support the flooring, so next determine the height at the doorway that you need to accomplish. Consider it with the height if the tile and thinset to try and match as close as possible. As I said, usually 1 1/4" of subflooring is normal. See if that will work.

 

[joecaption]

I have not used plywood for subfloor's since Advantech came out.
It's more stable and water resistant.
Use constrution adhesive on top of the joist.
For the second layer use AC plywood, never CDX!
Make sure any seams are offset with the layer below.
The second layer is fastened between the joist, not to them.
It mush be fastened every 4" on the edges and 6 to 8" in the field.
The cement board needs to be laid in a bed of thin set.

 

[HotRodx10]

At 16" spacing of joists (or less), 3/4" plywood should be stiff enough, but if you have the room to go thicker, and you're concerned, 1-1/8" plywood is more than 3 times stiffer than the 3/4" (and more than one and a half times stiffer than 2 layers of 3/4")

To give you a good bonding surface for the tile, cement board or tile backerboard over the ply, as joecaption said, bedded in thinset, and screwed down.

So, if thicker matches up with the adjacent floor better, then use the thickest plywood that works and the backerboard or cement board (or ditra? - I'm not familiar with how to use that). However, at the joist spacing you have, 3/4" Ply and a 1/4" underlayment, in thinset, should be sufficiently stiff.

 

[andynick]

This community never fails to amaze me — Appreciate all the advice!! I'll follow up with any more questions as I have them!

 

[huesmann]

I used Ditra on my floor, and it's way less messy than using cement board—I ended up using cement boards on the tub walls, and they're messy and a PITA to cut compared to Ditra.