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Unread 04-19-2014, 10:13 AM   #1
ericthibeault
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Ditra or 2nd ply w/heated floor

Hello,

I'm about to install ceramic tile in a new bathroom in my basement but I have some questions before installation that I hope somebody will be kind enough to answer.

I remodelled all the entire basement and my contractor installed a single tongue and groove 5/8 ply on 16" o.c. on 1"x3" over the basement concrete slab. All is screwed down the concrete slab

Now I have to put 12"x24" ceramic tiles in the bathroom section (about 32 square feet, 6'x7' minus the 32"x48" shower base). The trick is that I have a heating floor to install (flextherm.com/en/products/cable-system/green-surface-cable-2w-3w) and since we had to go down to the concrete slab to redo something, we can consider that the bathroom part of the 5/8 plywood is no more tongue and groove since we've cut it with a sawzall. But it has been put back with 1"x3" on the perimeter.

My questions, Is it best to put in this order:
- 5/8 > heating wire -> ditra (not xl) ceramic tile
- 5/8 -> 3/8 ply -> heating wire -> ceramic tile
- Other order options

I'm curious about the fact that the 5/8 could... maybe... move a bit vertically near the cut we made (even if screwed back). That fact makes me think it would be best to put a 3/8 ply on top of those instead of the other option. It would "smooth out" that joint.

On a side note, since my shower base is already installed (over the 5/8 ply), do I need to remove it and installed it back on the other ply or I can install the other layer (whatever layer it is) aside of the shower base?

Last question: I'm not sure to understand if I need polymere modified or not for:
- tile layer
- ditra layer (if I use this option)
- self levelling of the heating wire)

Thanks for helping me out guys.
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Unread 04-19-2014, 10:23 AM   #2
cx
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Welcome, Eric.

We'll skip my opinion on putting plywood over sleepers on a concrete basement floor installation at all, eh?

For your patched 5/8ths" subfloor I'd add nothing less than nominal 1/2" exterior glue plywood as a second subflooring layer.

I can't see your layout from here so can't tell if there would be any advantage to removing the "shower base" to install the plywood under it.

For the required thinset mortar types to use with the Schluter products I very strongly recommend you visit their website, download the Installation Handbook and make yourself very familiar with the installation requirements before you go any further in your planning. You cannot bond anything to plywood with any thinset mortar that does not meet ANSI A118.11 requirements.

My opinion; worth price charged.
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Unread 04-19-2014, 10:47 AM   #3
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Since I'm a french speaking person, I'm not sure to understand two of your sentences meaning:

- We'll skip my opinion on putting plywood over sleepers on a concrete basement floor installation at all, eh (note that the sleepers are on vapor barrier poly and little foam)
- My opinion; worth price charged. (kind of FWIW, IMHO?)

I know that the best is to put another 5/8 perpendicular on top of the first one but I want as small a transition as possible between my other floor (12mm laminate on 3mm backing).

For the shower base, is it common to put a layer of ply + thinset + heating wire + ceramic aside of it instead of put the shower base on top of all that?

As for the ditra, wrong idea to thinkg that it would be enough over a 5/8 to support 12"x24" tiles, even if it's a small bathroom and the ply is not on "springy" joists but on sleepers directly on concrete?

Thanks
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Unread 04-19-2014, 11:26 AM   #4
Steve in Denver
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Quote:
Originally Posted by CX
We'll skip my opinion on putting plywood over sleepers on a concrete basement floor installation at all, eh?
CX - Why ought we skip your opinion? Inquiring minds want to know...
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Unread 04-19-2014, 11:33 AM   #5
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Forgive my imperfect use of the Queen's English, Eric. I frequently type in Texan rather than even the American version of English.

Yes, my disclaimer means FWIW.

Another 5/8ths" plywood layer would be fine, of course, but it, or any other layer of subflooring must be installed with the strength axis perpendicular to the joist structure, never perpendicular to the first layer.

But a nominal 1/2" layer would be fine in your application. I do not like 3/8ths" plywood, even of the correct type, due to it being so very thin and usually not at all flat. You can end up doing more harm than good by adding that product.

There are two areas of deflection that are important to a tile installation. One is joist deflection, which is not a problem for you, and the other is between-joist deflection, which is a serious problem for you.

Your 5/8ths" plywood is the absolute minimum required by any ceramic tile substrate manufacturer and that is based upon testing using material in perfect condition, perfectly installed over joists with zero deflection. And it needs to pass only once.

With your cut-up floor, you almost certainly do not have the required three joist spans and fastening that would be necessary for a good subfloor installation. That's why I would recommend a second subfloor layer.

My opinion; worth price charged.
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Unread 04-19-2014, 11:58 AM   #6
jadnashua
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FWIW, if you can return the heating materials, I'd seriously consider using Ditra Heat - it will be much faster and easier to install. With Ditra Heat, you get a version of Ditra with a special shape that holds their wire in place. YOu install the Ditra on the floor, snap in the wire, then add your tile. No hassle with trying to level things after installing the wire on the floor in prep for installing the uncoupling mat (Ditra). In essence, you get all of that in one layer, minimizing buildup, and making your life much easier. http://www.schluter.com/media/brochu...v=201404032010
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Unread 04-21-2014, 10:32 PM   #7
ericthibeault
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Thanks for your answers.

So 2nd ply layer in the same axis than the 1st one? How far apart (joints)?

Also, do I only screw the 2nd layer in the first, screw it through the 1x3, screw it through the concrete slab or a mix of those?

Thanks
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Unread 04-21-2014, 11:28 PM   #8
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I would recommend using the information in this article for installing another layer of plywood.
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Unread 04-29-2014, 08:04 AM   #9
ericthibeault
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SLC hair thick cracks

Hi,

Last friday I've installed a heating floor (Flextherm Surface green cable) over two 5/8 plywoods in a 6'x7' bathroom. After the cable installation I've poored about 7/16 of self-levelling compound (Mapei Ultraplan 1 plus). Yesterday (monday) I've noticed hair thick cracks from one side to the other on the 6' axis at about 2'-3' intervals. I also found cracks in the other axis from the corner of my shower base to one other axis crack and a crack from my toilet drain to the wall behind.

I didn't put the primer prior of the installation (the guys from where I bought the mapei didn't told me so) but when I tested the bound with a hammer (lightly tap at different places) it seems to be a hard bound sound.

My questions:
- Are those cracks related to the fact that I didn't put a primer, or it's a "normal-ish" result since the SLC shrinks after installation? Note that I've followed the mix proportion to the milliliter.
- Will those cracks create a problem afterward when I'll start heating it? Note that I know I'll have to wait about a month for the SLC to cure, before starting the heating wire.
- Will the fact that I'll use a good polymer modified mortar for my 12"x24" tiles (Mapei kerabond/keralastic, 1/2" notches) will arrange all that and I can forget those hair thick cracks for good, or I need to scrap ALL off and restart with a primer (scrap the SLC, salvage my heating wire if possible, clean the top layer ply...)?

Finally, note that I've put that SLC in another place in my basement, on the same type of ply but in only a 2'x4' span and it didn't crack (about same depth, even a bit more).

Thanks for your time on this.
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Unread 04-29-2014, 08:13 AM   #10
ericthibeault
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Two things I forgot:
- I installed SLC in my main bathroom 5 years ago (without primer) but installed the ceramic tiles after 16 hours so maybe those cracks are "normal" but I didn't saw them in my first installation.
- Today, the SLC layer was kind of cold (basement, urethane isolated, the basement is heated but not in this room). We feel a 20-21 celcius in the room but the SLC floor is below that for sure.
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Unread 04-29-2014, 08:16 AM   #11
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Show me your crack!

Some hairline shrinkage cracking can be normal--I had some in mine show up about 72-96 hours after the pour. Can you take some pics of your cracks?

Failure to follow manufacturer’s instruction is the number one reason for SLU/SLC failure. Typically it’s the lack of primer, lack of mesh/lath (if required), and lack of gaps around the edges. The gaps around the edge are typically accomplished by wrapping the room in sill seal or edge strips. It forms your barrier and provides for movement. Did you provide a gap around the edge or use mesh/lath?

We have a great Mapei Tech that frequents the forums. I’m sure he’ll chime in.

My opinion is that if the manufacturer’s recommendations weren’t followed to the T you’re better off ripping up and replacing now. Sucks to do it later when tile is installed if it fails. Heated floor will promote more movement and decoupling of the floor, IMO.
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Unread 04-29-2014, 08:20 AM   #12
ericthibeault
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I'll try to take picture later and post them.

What can fail afterward (after installing the good poly-modified mortar and tiles?

Thanks
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Unread 04-29-2014, 08:34 AM   #13
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Quote:
Originally Posted by ericthibeault View Post
What can fail afterward (after installing the good poly-modified mortar and tiles?
The self leveling compound/underlayment itself can fail. If it begins to break up under the tile you're going to lose the bond of the tile. Would be like expecting your tile to stay stuck to the beach sand.
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Unread 04-29-2014, 09:12 AM   #14
jadnashua
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I haven't read the instructions on that specific SLC, but over wood, many of them require not only the primer, but lath AND a minimum depth (often 1/2" above the highest surface). So, reread the instructions to see what that one requires. You may get away with it, or it may become one very expensive mistake. Cracking sometimes is an indication of too much water, or inadequate mixing.
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Unread 04-29-2014, 12:33 PM   #15
ericthibeault
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Hi guys,

I managed to take some pictures earlier.

crack-overlay.jpg:
- I outlined the position of all the cracks
crack1.jpg:
- the first when we enter the bathroom that goes from side to side
crack2.jpg:
- between the toilet drain and the wall behind it
crack3.jpg:
- the crack from the shower base to the first/long crack (crack1)

The photos are resized so if not able to see well, I'll make other ones.

Note that I called Mapei's technical departement. I don't remember the technical term for this but the guy told me that those cracks can happen. The important things are:
- to let the SLC cure for a month
- to assure that the SLC is bounded to the plywood (by gently tapping on it with a hammer).

The thing is that I didn't mention that I didn't used the primer and I'm afraid that if told him, the answer would've changed.

I tested with a hammer and:
- 90% of the places sound hard and stiff, the other 10% I'm not sure, it doesn't sound hollow but the sound is slightly different.
- those "maybe hollow places" aren't where the cracks are
- gently hitting directly on the cracks doesn't change the surronding of the cracks.

Edit: I find those images to small, here are the links:
http://www.eric-thibeault.com/Storag...ks-overlay.jpg
http://www.eric-thibeault.com/Storag...cks/crack1.jpg
http://www.eric-thibeault.com/Storag...cks/crack2.jpg
http://www.eric-thibeault.com/Storag...cks/crack3.jpg
Attached Images
    
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Last edited by ericthibeault; 04-29-2014 at 12:35 PM. Reason: add links to better image resolution
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