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Old 07-16-2015, 10:13 AM   #1
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No Membrane for Tile Subfloor


I just started a tile project in my kitchen and missed a step. First, let me tell you what I've done.
1.) ripped out old flooring to slat sub-floor (house built in 1958 so has diagonal slats as base sub-floor)
2.) Installed 19/32 plywood by screwing down in 6"x6" squared pattern.
3.) Stapled down wire mesh with 1/2 inch staples every 4"x4" squared pattern.

The guy at the tile store told me I had to lay down the wire mesh because mud/mortar won't stick to plywood. This makes sense so I did. But, after doing so I finally did some research and everyone is saying to use a membrane between the plywood and wire mesh. Why is this? Obviously for a moisture barrier but wouldn't the mud work do that? So my second guess as to why you need a membrane is so that the wood doesn't soak up the water too fast from the mud.

So, my questions are:
1.) Am I totally screwed? Will I have to rip up the wire mesh and lay down a membrane?
2.) If the membrane is just so the plywood doesn't soak up the water from the mortar too fast, can I just make the mud a bit wetter before using it?

One lesson I've learned is never base your project off one opinion. I hope this lessos won't be too costly, time wise and money wise..

Thanks in advance,
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Old 07-16-2015, 10:56 AM   #2
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Hi Shane,

OK, so you removed the old flooring down to the subfloor which is 1x what? You repaired and refastened the slats/planks and installed 19/32" (⅝" roughly), underlayment. Can you tell us what grade and type? Did you gap the sheets and perimeter? You then stapled wire lath in preparation to do what is know as a "Jersey Mud Job" aka "scratch-coat". Are you in Canada or the east coast?

That method of installation is not an approved method and has never passed when tested by any accredited testing association. It doesn't fail every time in real life, but has a high failure rate. You should consider a different method.

The reason you read to use a membrane is because you researched doing a "mud- job". You are NOT doing a mud job. Mud job for floors is made with sand and Portland and is placed about 1" thick or thicker. It creates the base for tiles, you're not doing that.

1. No, you're not screwed yet. Rip up the lath, no membrane. You should install a concrete backer board or an uncoupling mat such as Ditra.

2. See #1, you've been led down the wrong road.

Quote:
One lesson I've learned is never base your project off one opinion.
That's a good thing to keep in mind but even that isn't fool proof. It's possible that 3 or of 4 might give you bad info, all depends on who and where you ask.

Jaz
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Old 07-16-2015, 12:03 PM   #3
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Jaz, Thanks for the quick and thorough reply.

I'm in Northern Utah. It's not humid here at all and we don't get much precipitation. I can't remember the grade and type of plywood off the top of my head. The sheets are gaped at the perimeter and a little on the inside.

This is a major bummer. What a waste of money and time.

If I just mud over the wire mesh and lay the tile what kind of issues could I see?
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Old 07-16-2015, 02:58 PM   #4
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Years ago some of the younger guys entering the trade decided they wanted to call the thinset "mud" I think partially because it looked like drywall mud and at the same time they could make it sound like they were doing a real mud job. I know some were trying to deceive homeowners. So, thinset is not mud.

If you thinset over the lath you'l just be using a method that isn't the best way to set tiles. The world will not end right away. But it's no big deal to remove the lath and it didn't cost that much, did it? So you wasted a few hours of your time, better now than later. You just switch to the method you shoulda used in the first place.

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Old 07-16-2015, 02:59 PM   #5
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What Jaz is describing is how all the old 40's and 50's bathrooms I have gutted were originally done. Metal lath screwed to the subfloor with a about a 1 1/2 inch thick bed of mortar on top of it, and the tile on top of that. Those old floors were solid as a rock and last a lifetime but damn that is a lot of mortar and a lot of work.

If you continue going down that road be prepared to mix, pour, and level out however much mortar to get it 1"+ thick. Then you can start laying your tile but by now your floor is probably going to be raised up quite a bit. There is a reason no one does it this way any more

Follow Jaz's advice. Yeah it sucks to take a couple steps backwards but rip up that metal lath and put down the cement board. Less work, less material, less stuff that can go wrong.
Even with the new plywood you put down your floor might still be raised up pretty significantly. If your original subfloor is solid and in good shape you can install your cement board over that and be closer to being level with whatever other flooring butts up against it.

But, to make a long post short, Jaz is spot on and you should take his advice!
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Old 07-16-2015, 03:29 PM   #6
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Thanks Jaz and MrBryan. I guess I could use a big floor scraper to rip up the lathe quickly.
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Old 07-18-2015, 12:01 PM   #7
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Quote:
Originally Posted by shanefarr View Post
The guy at the tile store told me I had to lay down the wire mesh because mud/mortar won't stick to plywood.
Yikes. Well first of all, tile does stick to plywood. People tile right on top of plywood all the time. Many of those tile jobs last for decades. Some do not. The reason has to do with the expansion/contraction of plywood, not because tile won't stick to it. It's not recommended to tile on plywood directly, but again, it often works. Since there is also a failure rate for the method the guy gave you as well, I'd call it a waste of time.

Note that 95% of the time, people who choose to not tile directly over plywood choose a cementitious tile backer, such as Durock or Hardibacker. Tile certainly sticks to that. So your tile guy's advice is strange because that would be the #1 advised method.

However, I think it's easier to use Ditra anyway. Your tile guy's advice would probably be around #5 or worse on my list of options.

Note that Ditra is adhered to the plywood with thinset, and that sticks to the plywood. So, there ya go.
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Old 07-18-2015, 12:04 PM   #8
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Quote:
Originally Posted by JazMan View Post
That's a good thing to keep in mind but even that isn't fool proof. It's possible that 3 or of 4 might give you bad info, all depends on who and where you ask.
I'd guess about 3 out of 4 cement board installations don't have thinset under the board. (You've probably seen a lot more tile jobs than I, but I see a lot of this.) You can't even trust guys who've "been doing this my whole life". If you do the same thing wrong for 20 years, then that 20 years of experience isn't really worth all that much.
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