[soberjulie]

I think (?) Im probably just going to confirm what I already know.
Heres the scoop:
1940's house, we gutted the bathroom and intend to install 18x18 travertine tile.
We ripped the floor down to the original plank floor. We did not install anything over that plank floor before we used some self leveling compound, then thin set, then the cement backer board.
We thought we did a lovely job.
And then....we noticed the floor is not completely level. There is a slight hump in the center. Ugggggg
The more I read, the more confused I get....subfloor, double subfloor...self leveling compound...my head is spinning.

Do we need to rip out the cement board and begin again by installing plywood subfloor, and take care of the hump along the way?

Another thought: Can we just count the existing cement board as our subfloor, and begin again on top of that, using self leveling compound and another layer of cement board?

[mterry]

How much of a hump? Anyway, you can use self leveling compound over the cement board if needed

[soberjulie]

Not a huge hump, but if I were to dry lay the tiles there would definitely be some rocking. If I lay my level down over the hump theres about a 1/4" space between the highest spot and the lowest spot spanning over a 3ft area

[rusty baker]

You really need plywood over the plank floor.

[soberjulie]

So then can the exisiting cement board act as a subfloor if we lay another level of cement board over top of it?
What could/will happen if we don't rip it up and install subfloor?
My neighbour did the exact tile in his bathroom 2 years ago, also didn't use subfloor and his tile job still looks ok. (mind you, I want more than a few years out of this tile!)

[rusty baker]

CBU gives no support and tile is never supposed to be installed over plank flooring without plywood.

[soberjulie]

Well...I suppose if I wanna be an optimist, I should be happy I learned this now and not after we laid the tile. Safety glasses out, more demo to be done.

Lesson learned for when we move on to the kitchen when this is done.

[oh'mike]

Help Laying Marble Floor Tile - Flooring - DIY Chatroom - DIY Home Improvement Forum

This thread is similar to yours----Rusty is right---remove that work---start over---

Read that link first and then come back with questions.

[jeffnc]

The problem here is the wood in the subfloor - there isn't enough of it.

Cement backerboard is not a structural element. It's not there to make your floor stronger. Rip it out, add a layer of plywood on top of the original plank (get the total thickness up to about 1 1/4"). Then leveling compound on top of that. At that point you have a better floor without cement board than you did with it before. You could tile directly on top of that, although I wouldn't. I'd use Ditra, or if not that then Hardibacker.

[oh'mike]

Well meaning thread above --but confusing----leveling compound is applied UNDER Ditra---But on top of Cement backer board--which must have thinset under it---

There--not isn't that just as clear as mud!!!!!

[soberjulie]

Thankyou all.
Back to demo. Ugggggg. Hubby is ripping it out today.
Mind you, I feel more knowledgable and this is only the beginning of our home improvement project.
Very glad to have found this site!

[soberjulie]

Thankyou all.
Back to demo. Ugggggg. Hubby is ripping it out today.
Mind you, I feel more knowledgable and this is only the beginning of our home improvement project.
Very glad to have found this site!

[soberjulie]

Thankyou all.
Back to demo. Ugggggg. Hubby is ripping it out today. Then we'll double check the thickness of the plank floor before deciding how much plywood to use.
I feel more knowledgable and this is only the beginning of our home improvement project....so while backtracking sucks, better to do it right and educate ourselves to boot.
Very glad to have found this site!

[oh'mike]

Our site has the hiccups today-----

[soberjulie]

Ok, the plank flooring is just shy of 1" thick, tongue and groove laid on a diagonal. The joists are 16" on center.
So that means just 1/4" plywood yes?

When do I lay the electric in floor heating pad?