Self leveling compound, cement board or Ditra?

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Old 01-23-15, 08:57 AM
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Self leveling compound, cement board or Ditra?

Okay, please be kind and show me some patience. I am a woman who is tackling a project that I need help with.

I have small bathroom subfloor that I am trying to prepare for tile. The subfloor was plywood, but one corner of the room was visibly lower (by at least 1/4 -1/2 inch). Additionally, two boards were not flush, not bad, but not flush. I decided I was going to pour some self leveling compound to level up the floor before tiling.

I purchased the Level Quik and the primer. I cleaned the floor of all debris, applied the primer and allowed to dry per specifications. I watched countless videos on applying the self leveling compound, followed the manufacturer's instructions and poured and spread the floor. I allowed it to dry and upon inspecting it, noticed that although it helped to fill in the low spot, the surface all across the floor has these subtle "ripples". Subtle, but when I dry set a couple tiles on it to check the surface, the tiles didn't lay flat in a couple spots (sort of a mild teeter-tottering). Enough that I know cracked tiles or grout would surely occur. Argh.

I tried getting out a sanding block and sanding down some of the higher spots, but it has become apparent that I wasn't going to be able to get that perfectly flat surface I am striving for.

So. My question is this; Do I re-prime and pour a second layer of the self leveling compound? I have to wonder if it was too thick the first time? The reason that I say this, is that if you pour water into a glass, the surface levels, if you pour pudding into a glass, you may still have that uneven surface. Again, the original mixture was exactly the consistency that the videos and manufacturer specified, but I cannot conclude what else would have caused it not to level and leave these high/low spots?

As an alternative, someone suggested just putting cement board over it. Okay, but if I do that, won't I crack (or affect the integrity of) the layer that I just poured? I would hate for the layer of self leveling compound to deteriorate underneath my cement board.

Another person suggested the newer Ditra product. I am getting mixed opinions on this stuff. Old schoolers seem to dislike it, but others seem to swear by it. I am wondering if the Ditra will lie into any lower spots and make it more forgiving with the tile. I looked at it at the tile store, and it didn't appear to be very thick. I am not sure that it's application is meant to even out any uneven spots on the floor.

Or third option, should I try a second pour and THEN the Ditra?

Appreciate any suggestions! Thanks all!
 
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Old 01-23-15, 09:05 AM
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Welcome to the forums.

Let's start basic: What is the subfloor made of and what is the size, spacing and unsupported span of the floor joists?
 
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Old 01-23-15, 01:37 PM
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You can not pour SLC directly on plywood without the addition of a wire lath nailed to the floor to give the leveling compound something to grab onto. Correct procedures to tile and level a floor are as follows:

-Determine proper stiffness of the floor (see Mitch's note above).
-I shoot for a minimum 1 1/2" thick subfloor (1/2" + 3/4" plywood).
-Trowel on Thinset mortar and set 1/4" cement backer board into it and secure to the subfloor.
-Thinset and mesh tape all seams between pieces of cement backer underlayment (CBU)
-Now you can level the subfloor by applying primer and the SLC.
-Once leveled, you can use thinset and apply your tile.
 
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Old 01-23-15, 01:50 PM
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Thanks for your reply. I needed to move some things and crawl into the crawl space to be sure.

It would appear that it is 2x8's, 16" on center. Looks like two separate sheets of plywood on top of this, each 1" thick. This is an old house, pieced together by many owners over the years.

Sadly, I watched numerous videos where they poured the SLC directly over the plywood. I simply assumed it would be okay. The primer for the SLC also said that it could be applied directly to plywood (with the use of the primer), as did the salesperson. Yikes. I would sure hate to have to try to remove it now.
 

Last edited by womanstouch; 01-23-15 at 01:51 PM. Reason: needed to add more info
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Old 01-23-15, 02:21 PM
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Here is a direct lift from the installation instruction for level quick self leveling underlayment (emphasis added).

Bonding to Plywood Surfaces
Plywood floors, including those under resilient flooring, must
be structurally sound and must meet all industry guidelines. A 2.5
lb/yd˛ metal lath must be fastened every 6" * 8" (15 * 20 cm) with
fasteners that have a galvanized or corrosion-resistant coating over
primed surface.
A minimum of a 1/2" pour is required for single layer
plywood; for multi-layered plywood, the lath must be fully
encapsulated. For questions about proper subfloor installation, call
Custom Technical Services.


The pour you made should release fairly easily with the assistance of a stiff putty knife and a hammer. Plan on removing the existing, adding the cement backer underlayment in a trowel bed and then your SLC.
 
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Old 01-23-15, 07:45 PM
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Womastouch,

You left out the span of the joists, the distance between the two supports. Any chance the joists have stampings telling you their species and grade? How old is your home, you said old. You think there's two layers of plywood above the joists. Can you confirm and also tell us the thickness of each? You said each is 1", are you sure? Doesn't sound right to me.

You asked the clerk? Big mistake.

The primer for the SLC also said that it could be applied directly to plywood (with the use of the primer), as did the salesperson.
Well, sure. The primer goes on the plywood before you install the lath, then the SLC.. This method is for the base for the tiles direct, or Ditra. If you go with CBU, do the SLC over the boards. Also note the min. thickness for various installations.

What went wrong to cause the ripples? Could be several things. Worked too slow. Not enough SLC to flow flat. You poured the ridges. etc. Next time you may need to help the SLC find its level. A steel garden rake might be handy.

Jaz
 
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