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Unread 10-05-2022, 12:34 PM   #1
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Tile on wood subfloor

redoing two small bathrooms
(60x76 inches and 58x54 inches not including the shower surface areas)

after gutting the bathroom we found the old plywood i believe it is 1/2" or 5/8"
my contractor added one more layer of plywood. he wants to "float" a mud floor of 1 1/2" to level the bathroom floors.

For the showers we are using ready-tile shower bases which will be set in mortar.

my concerns/questions are:

1.) shouldn't we use self leveling cement instead? it won't lift the floor height as much and 1 1/2" for this floating mud floor is more than max recommend thickness for mortar?

2.) do we need to treat the plywood under the showerbases where we use the mortar? tile-ready instructions says the plywood can soak up the liquid in the mortar. however from watching installation videos online, everyone seems to just throw the mortar straight on the plywood

3.) found a nice mosaic marble for the floor. do we need to beef up the floor more or can 1"-1 1/4" plywood handle. the tile is 5.25lb per square foot.

4.) water proofing. do we need to apply a liquid water membrane to the plywood before we start, i dont see many people doing this most are using the membranes like ditra heat which we are considering since we are installing heated floors. does this provide the same waterproofing as the liquid membrane? the membrane will have gaps do you need to tape over these ?

thanks for all your help/advice

greatly appreciated
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Unread 10-05-2022, 03:27 PM   #2
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Welcome, Martin.

It's important to know whether your original subfloor was 1/2" or 5/8ths" plywood. The 5/8ths may have proper T&G edges, while the 1/2" does not, nor is the 1/2" suitable even to be a first layer of structural subflooring. The orientation of the second layer is also important.

You are using a tiile-redi shower receptor, rather than a "ready-tile" product?

1. Depends upon the application. First, your floor does not need to be level for a ceramic tile installation, but it needs to be very flat. The use of an unbonded, reinforced mortar bed (minimum 1 1/4" thick with welded wire mesh in the vertical center) is a very good way to achieve both if you have the vertical space and floor structure to accommodate it. There is also an industry accepted method using a minimum of 3/4" of mortar over a cleavage membrane and expended metal mesh that might work in your application. Or, you could use a SLC if your application permits. Most of those also require a reinforcing mesh of some sort and have a minimum thickness requirement. You could also consider just using a Cementitious Backing Unit (CBU) or other tililing substrate.

2. Be careful whose YouTube videos you watch. The manufacturer of your shower receptor (If I guessed correctly) requires a cleavage membrane of some sort over the plywood subfloor and under the mortar bed for his receptor installation. I recommend you read and follow the manufacturer's instructions for his product if you want to use it. Creating your own shower receptor can be much, much less expensive is easier to fit your exact shower footprint and drain location. You would still want a cleavage membrane under the mortar used for the pre-slope or single mortar bed in that case, but the requirement for the floor to be dead level doesn't apply.

3. First, I recommend you not use a marble on your shower floor. Type marble shower into our Advanced Search feature and read a little for information on that.

As for the floor structure, we'd need to know a good deal more to about it to judge. You can enter your information into the Deflectometer in the dark blue bar near the top of the page for an initial go/no-go reading on that.

4. The bathroom floor is not usually considered a wet area, but if you want to consider yours a wet area, yes, you'd need to consider some sort of waterproofing. It's more complicated than just painting a liquid membrane on the floor, though, and may be more than you actually need.

As for the use of other types of membranes, such as those designed for radiant heating systems, you'll again want to read the product manufacturer's instructions.

My opinion; worth price charged.

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Unread 10-06-2022, 12:49 PM   #3
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Hi Martin,

You can post pictures here, and I recommend it.
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