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Unread 06-24-2019, 10:09 AM   #1
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New Bathroom floor and tub surround project advice

Hi All,

I've had such good results from projects that I did with the help of the forum, I'm back for my next one.

I have two tile jobs for this project:
1. Floor of a second floor bathroom. Floor Joists are 2x12 @16" OC with 5/8 plywood subfloor. I plan on using ditra and 6x24 porcelain tiles. Do I need to do any further work on the subfloor? What size trowel for the 6x24 tiles, 1/2"?

2. For the tub surround, the tile is 6x12 porcelain. is a 1/4 trowel ok or should I go with the 1/2"?

Thanks in advance for the advice!

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Unread 06-24-2019, 12:02 PM   #2
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Need to know the unsupported span of those joists. Nit room size but support walls in floor below.
#2. 1/2 is good but they always say use as big as it takes for full coverage when you pull a tile to check. Flatness of wall and tile come into play. 1/4" trowel leaves 1/8" if ran at 90 degrees which often times scrapes off more thinset than stays on the wall. Your tiles probably have a 1/16" bow. That's not going to work to well is it?
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Unread 06-24-2019, 05:28 PM   #3
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Thanks for the response Teddy,

1/2 seems right to me. As to your question (which I should have answered in the original post), is that the 2x12s under the bathroom are unsupported for up to 15 feet.


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Unread 06-24-2019, 05:29 PM   #4
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Welcome back, Adam.

Once you determine if your joist structure is suitable for ceramic tile, I think you should reconsider your subflooring. While Schluter, and other manufacturers of tile substrate products, say that nominal 5/8ths" plywood is sufficient over 16" joist centers, there is no way you could ever make me tile over such a subfloor. Not for my own house, nor for a customer's house.

The absolute minimum I'd ever start with for a subfloor for any application would be nominal 3/4" plywood or OSB. For ceramic tile I'd want a second layer of nominal 1/2" plywood, but that's entirely up to you. Lots of people tile over single layer nominal 3/4" subfloors over 16" centers and get by with it.

Flatness is gonna be your second issue. It's unlikely your subfloor will meet the industry standard requirement for flatness for those large format tiles without some help. That's no deviation from intended plane of more than 1/8th" in ten feet nor 1/16th" in two feet. Especially true using that thin plywood you've got. You'll wanna check that.

My opinion; worth price charged.

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