Do I need to rip out the cement board and start over? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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01-01-2013, 08:49 PM
I think (?) Im probably just going to confirm what I already know.:confused:
Heres the scoop:
1940's house, we gutted the bathroom and intend to install 18x18 travertine tile on the floor and half way up the walls.
We ripped the floor down to the original plank floor. We did not install anything over that plank floor(no plywood) 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 hump is over a 3ft span and from high point to low point there is a 1/4" difference)
The more I read, the more confused I get....subfloor, double subfloor...self leveling head is spinning. And Im terrified that we should have used plywood over the original floor even though my neighbour did the exact same thing 2 years ago and his tile job still seems fine.

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? Or are we ok to use self leveling again for the hump issue?

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?

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01-01-2013, 10:10 PM
Welcome Julie. :)

For a natural stone installation, you'll need joists stiff enough and 2 layers of plywood or equal. With what you have now, you'll have a failure sooner rather than later if you continue with your existing plan.

Your best bet is to remove everything down to the planks, then evaluate what you have from there. First you'll need to check the joists with the Deflecto ( and tell us what you get. Also tell us the joist spacing.

Then take a look at your planks and tell us what you see. Are they in good shape or all cupped, cracked, big knots, etc...? How wide are they? How thick are they? Are they installed perpendicular to the joists or at an angle?

Lets get this much sorted out and we can go from there with your options. :)

Tiger Mountain Tile Inc
01-01-2013, 10:36 PM
At this point it's best to be safe rather than sorry. The materials are pretty inexpensive at this point to replace, but all the travertine won't be.

You don't have any photos of the plank floor do you?

Also, you'll want the self leveler to be over the cement board, not under.

01-02-2013, 12:54 AM
Sorry that you're not hearing what you had hoped to hear from us Julie but natural stone installations demand little to no movement whatsoever. With what you have now the hump in the floor should be the least of your concerns and can be dealt with after you get everything tore up and your joists evaluated.

In case you decide to ignore our advice and move forward with what you got, (your neighbor did the same thing 2 yrs ago and it still looks fine), let me point out that your layer of cement board adds little to no structural value to your floor. It is simply a tiling substrate. But hey, at least you know that it "might" last 2 yrs :D

01-02-2013, 06:48 AM
Well, I did kinda suspect we made a big boo-boo and were going to have to tear it up.:shake:
Safety glasses on, back to demolition.:crazy:
I do have pics, I'll share them when I get home from work today as well as all the measurments (joist spacing, planks etc)
You all have been very helpful and likely prevented us from making further mistakes that would have broke my heart.

01-02-2013, 08:55 PM
More info:
The plank flooring is in good shape, its just a hair shy of 1" in thickness, 5" wide plank tongue and groove laid on a diagonal.

The joists are 16" on center and also in good shape.

Should we be putting down 1/4" plywood?

01-02-2013, 09:02 PM
Min. 1/2" underlayment grade needed next. Did you run the numbers on the deflecto calculator?

01-02-2013, 09:09 PM
If the deflecto turns out good then screw down the planks first. Then install min 1/2" underlayment grade plywood screwed down just barely all the way through the planks. Leave 1/8" gap around all plywood sheets. Screw down perimeter of sheets every 4" and 6" through field. No drywall screws please. Caulk gaps, install ditra. Back burn all travertine and use min1/2"x1/2" trowel with premium unmodified thinset combed in one direction. You need to achieve min 95percent coverage. Seal stone before grouting.

01-03-2013, 06:46 AM
When I ran a deflecto calculator I got a number of 640 :uhh:

Joists, douglas fir in good condition.
Joist height 2x8
Joist width 2"
16" on center
10ft spacing

Oh, and a hearty congratulations that Im rated for ceramic tile. The silly calculator didn't take into account that Ive already bought my beautiful 18x18 travertine and it should have been offering me condolences.:uhh:

If I use Ditra, does that change anything?

01-03-2013, 06:42 PM
Ditra doesn't change it. You are looking to be at 720 min. You could laminate another joist to the side of the existing joists to beef up the floor or add another support wall below if possible.

Tiger Mountain Tile Inc
01-03-2013, 06:58 PM
I don't know what the mods will think of this, but I'll let them correct it if they choose.

I don't know about other thinset manufacturers but I know that Laticrete only requires L/480 for a natural stone installation. For example, here's the data sheet for 4xlt (, which is their thinset that would likely be used for a natural stone installation.


01-03-2013, 07:14 PM IS actually possible to add another support wall under the bathroom!! Its a laundry area, unfinished, so to add a wall would actually be a plus!

01-03-2013, 07:32 PM
I believe it is from the Marble institute of America MIA that the L720 come from. It may have changed though. Not at work to dig deeper.

Tiger Mountain Tile Inc
01-03-2013, 07:36 PM
I think you are right, Brad, and it hasn't changed.

I was just pointing out that Laticrete only requires L/480.

01-03-2013, 07:48 PM

To somewhat stiffen the joists you can "sister" another joist alongside. You can use the same size (2x8 in your case) or you can even add deeper (2x10) joists. When you put them together I would recommend using a wavy zigzag of liquid nails and then nail them together from both sides using 10 penny (10d) nails which are 3" long, nailing from BOTH sides. The nails will provide shear strength until the glue sets. I'd recommend using good kiln dried douglas fir. Sometimes Home Depot carries dougfir, but you may need to go to a good lumberyard if not.

You want to make the sistered joists as long as possible, ideally even sitting on the sill next to the original joist, but that's often not possible, other, but every little bit of length will help stiffness even if it doesn't extend the entire way. A side benefit is that it also gives you more screwing surface when putting down the plywood :)

To double check your deflection, put in 3" for the width instead of 1.5". Note, a single 2x10 is about the same stiffness since it's more a function of depth..