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Unread 08-14-2006, 06:01 PM   #1
candlman
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Bathroom Remodel Subfloor Q's

I am getting ready to install backer board for an 13 X 7 bathroom. The floor joists are are5"w x 6"d and approx 24" on center. This leaves approx. 19" space between joists. Half of the room has 3/4" pine boards ranging from 4" to 7" and the other half has 3/4 plywood. I screwed the plywood down with 2" deckmate screws and though the boards were nailed down I used the same screws to
secure them as well. I put 1/2" plywood over the entire floor, seems staggered, and screwed it down with the 2" screws on the joists and 1 1/4 scews every 6" in the fields.
The floor is fairly level (within 1/4"). My friend advised me to use 3/8" PTS screwed down every 6" and thinset the ceramic tile to the PTS. I was thinking about using 1/4" backer board or Durock. What do you think about the subfloor
with the joist spacing and what about PTS instead of backer board? Assuming
you advise the backer board, what do you recomend for thinset an should I use a water mix or acrylic? I read through several of your threads but the 5 x 6 floor joists are pretty uncommon. Thanks for the help
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Unread 08-14-2006, 06:06 PM   #2
jadnashua
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Not sure what PTS is!?

The normal recommendation is cbu or a membrane, your choice. Don't use mastic. You don't need a modified thinset under the cbu, but it is okay - just costs more, doesn't gain you anything. The screws or nails hold the cbu in place, the thinset holds it up, so if your subfloor is good, then any thinset should be okay.
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Unread 08-14-2006, 06:12 PM   #3
candlman
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Pts

PTS, according to my friend, is plugged and sanded plywood. He says it's all his buddy who installs tile for a living, uses these days. What is cbu? Thanks for the help.
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Unread 08-14-2006, 06:19 PM   #4
jadnashua
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Setting tile on plywood can be done, but is not often recommended for non-pros, and even they don't do it often. It requires two layers of plywood, installed properly (joints offest specified amount, second layer not screwed into the joists) with NO mistakes or shortcuts AND a quite expensive thinset. CBU = cementaious backer unit (cement board).

All plywood used under tile should be no less than a C face, exterior glue, and plugged. Sanded isn't a big deal, but doesn't hurt.

You could use a membrane on the plywood in place of the cbu, if you wish. Various makers and styles: Ditra, Nobel, and others.
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Unread 08-14-2006, 06:28 PM   #5
candlman
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What do you think about thickness of the subfloor with the joist spacing? Also, do the 3/4" pine boards under the 1/2" plywood sound sufficient?
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Unread 08-15-2006, 07:20 AM   #6
candlman
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Redgard crack prevention

My post below, titled Bathroom remodel subfloor Q's, details my subfloor concerns. I have decided to use wonderboard or hardibacker board. What is your opinion of the extra step of using Redguard WP/Crack Prevention Membrain on top of the backerboard? Is this just overkill or is it worth the added waterproofing/crack prevention? The Redguard instructions say to use a custom polymer-modified morter for setting the tiles. Is this standard morter mixed with acrylic instead of water or is it a different type of morter? Thanks.
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Unread 08-15-2006, 08:39 AM   #7
bbcamp
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Candlman, I merged your threads so we have all your project information in one place. It really helps us if you keep everything here, rather than starting a new thread with each question.

I assume that the 5x6 joists are supported every 6 feet or so? If longer, let me know. Also, I assume you are installing a ceramic tile, not stone.

A also assume that your pine boards are in good condition, no rot, no big knotoles, etc. If so, the 1/2" plywood is fine, provided it is rated "Exterior" or "Exposure I."

I don't think you get much from waterproofing the floor, since water will simply flow through the doorway or under the walls. If you install the backerboard properly, you won't need the Redgard's crack suppression properties, and if you don't install the board correctly, the Redgard won't help. So save your money.

FYI, modified thinset can be found in 2 forms: admix already mixed in the powder, or admix you add to the powder instead of water. Both products meet the minimum requirements for a modified thinset, so pick the one that fits your work habits.
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Unread 08-15-2006, 09:42 AM   #8
candlman
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Sorry for the confusion. Yes the joists are supported at about 7 feet and the boards are in good shape. The tile is ceramic. I'm not sure if the plywood is exterior grade. I will check into it. If not, any good alternatives to ripping it back up? If I am not using the water seal membrane, should I still use the modified thinset? One last question, Wonderboard instructions say taping the seams is not needed, do you find this to be true? Thanks for the help.
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Unread 08-15-2006, 09:52 AM   #9
bbcamp
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Yes, use modified thinset to set the tiles.

Wonderboard's fuzzy edges eliminate the need to use tape, but you still have to work some thinset in there. Also, if you cut the board and joint 2 cuts together, you must tape and mud the joint.
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Unread 08-15-2006, 12:02 PM   #10
candlman
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Ditra

After reading more of these threads I am convinced that Ditra may be the best option for me. The install instructons make it sound pretty easy. Any tips or things to be cautious for? Planning to special order through Home Dump. What would you recomend for a modified thinset?
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Unread 08-15-2006, 12:13 PM   #11
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Versabond will do nicely to bond the Ditra to the subfloor. Get the gray, since you will be able to see any air bubbles. It will also dry lighter, so you will see that too. Mix the thinset a bit looser than usual, almost runny (Schluter uses "fluid" to describe the consistancy). Back roll the Ditra the day or 2 before you plan to work, this helps the Ditra stay flat. Finally, DO NOT TUG AT THE CORNERS TO TEST THE BOND. It will peel up easily like that, but since tile rarely pulls on the corners in that fashion, you shouldn't either.

Consider ordering your Ditra from our friends at www.tile-experts.com. You'll get it faster, and enjoy the experience, too.
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Unread 08-15-2006, 12:16 PM   #12
Dan Clark
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tile-experts.com

Try Tile-Experts. Prices are competitive and they have the Schluter expertise.

I ordered Kerdi and Ditra from them. I actually ordered LESS than I had originally planned because David pointed out that I didn't need some things. Good people.

Dan.
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Unread 08-15-2006, 07:30 PM   #13
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Using a 7' board I checked the floor for humps/dips. The floor looks pretty good except for one hump created by what must be a floor joist. The hump falls in the center of the entry way in the direction of the entry way the entire depth of the 6 1/2' room. (if that makes sense) Setting the board to the left of the hump shows the floor to only have 1/8" or less dips. To the right (about 5' to the wall) shows to have a 3/16" dip. If you hold the board tight to the left of the hump with the end up to the wall to the right of the hump it shows the floor to be 1/2" low at the wall. Does a hump in the floor result in structual problems where tiles may pop or is it a problem strictly for asthetics? I could use SLC to the right of the hump but the toilet (which the flange is already set and plumbd to allow for 3/4" for Ditra, thinset and tile) is next to the wall where it would have to be filled 1/2" to completely hide the hump 5' away. Any ideas? Thanks.
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Unread 08-16-2006, 09:07 AM   #14
candlman
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Slc

Any advice in this area would be greatly appreciated. If I use a SLC, what would you recomend for under Ditra? Thanks again for all of your help on this board.
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Unread 08-16-2006, 09:44 AM   #15
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The joist will not cause pile failure, but will make your life difficult for setting the tile. You could remove the subfloor, plane the joist down, then reinstall the subfloor.

You could use SLC, even with the toilet flange already set. Add a flange extender to raise the flange to the new height, or have the toilet flange re-set.

Since you will have 2 layers of subfloor, you can prime the plywood and pour the SLC before installing the Ditra. Do not do it the other way around! Dam all areas where the SLC may flow, including under walls, around pipes and through cracks in teh subfloor. Tape and caulk all these openings. Use 1/4" foam rubber product called Sil-seal around the perimeter walls. This will be an expansion joint for the SLC.
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