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Unread 09-24-2009, 10:24 PM   #1
Mr. Riley
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Riley's First Tile Job

Hello, I'm new to the site and new some advice. I'm getting ready to tile my small bathroom floor (4'x6') and need opinions on whether I need to put down ply under the hardie backer. The subfloor is 2x6 TG doug fir and it is SOLID and level as i just finished replacing more than half the floor (see photo). I was planning to just go with 1/2" hardie right on top of the subfloor, with 3/8" ceramic tile on top of that. This will make for a perfect transition into the next room, which is about an inch higher than the subfloor in the bath.

I know the hardie website says to install it on 5/8" ply or 3/4" OSB, but my floor is 1.5 inches thick and should be a good base, right? Not sure if this makes a difference, but the floor joists are 4"x8"s and about 3' on center. Please let me know what you all think.

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Unread 09-24-2009, 10:34 PM   #2
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You really don't need to use 1/2" CBU on the floor. You might get by with 3/8" ply over the top, then Ditra instead of CBU. This would put you at the same height as 1/2" Hardi.

It would be better to use 1/2" ply, and this would put you at only 1/8" or so above the adjacent floor. Having the installation done correctly is the most important. Having the transition at the same height is just a bonus.
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Unread 09-24-2009, 10:44 PM   #3
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Welcome, Riley.

What Kevin said, 'cept maybe for some of his math.

You really need half-inch plywood over any board floor, even in California. Fella could cheat and use the 3/8ths" but it's tougher to get that to lay flat.

I suspect your joist spacing is closer to 48" on center, but we know that to be suitable for a ceramic tile (but not natural stone) installation with the addition of the plywood.

You could use a thinner CBU over that, or the suggested Ditra, or, for an even thinner substrate, a sheet membrane such as one from the Noble Company.

My opinion; worth price charged.
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Unread 09-24-2009, 10:53 PM   #4
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Thanks for the responses. why is 3/8" tough to lay flat?
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Unread 09-24-2009, 11:13 PM   #5
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3/8" ply is kinna warp-happy.
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Unread 09-24-2009, 11:15 PM   #6
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What Kevin said, 'cept maybe for some of his math.
So you're saying my Chinese-made calculator is wrong?
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Unread 09-24-2009, 11:32 PM   #7
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OK, so is the plywood for support or to provide a smooth surface to apply the mortar and affix the hardie? What are the potential consequences of putting the hardi directly over my 1.5" subfloor?

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Unread 09-24-2009, 11:38 PM   #8
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Solid wood expands and contracts far too much and that movement translates into cracking tiles. The hardi needs the stability of plywood under it.
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Unread 09-25-2009, 10:53 PM   #9
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OK, I actually have about 1 and 1/2 inches to work with before I have to cut the bottom of the door off, so I'm going with the 1/2" plywood route followed by 1/4" hardie and then my 3/8" tile. What type of plywood do I look for -- I know i need exterior grade but all I see is stuff like cdx, ac, tongue and groove, etc. Also, will 1 and 5/8" drywall screws be OK to attach the plywood to the subfloor? And what kind of screw spacing do I use?

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Unread 09-26-2009, 11:24 AM   #10
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You want exterior glue plywood, Riley, with no face grade below C. AC or BC are commonly available grades. Should say EXT or Exposure 1 on the grade stamp (the Grade is the designation of the facial plies).

Do not use sheetrock screws, use some kind of coated deck screws. For half-inch plywood I'd recommend a schedule of 6" on the edges and 8" in the field.

My opinion; worth price charged.
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Unread 10-13-2009, 10:33 PM   #11
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Do I keep the plywood about an 1/8" from the walls, and the same for the hardi? When I tape up the seams between hardiboards, I've heard that I should do that while I'm laying tile. If I do it the the day before, I'm bound to get a build up of mortor along the seams that will create high spots. Any thoughts?
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Unread 10-14-2009, 05:01 AM   #12
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Check the wall spacing again, Riley. I remember it being 1/4" against walls and other fixed obstructions, and 1/8" between the sheets.

You are correct about taping and mudding the Hardi joints as you tile. Do not get wrapped up in the tile setting and forget the tape, though.
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Unread 10-24-2009, 12:52 PM   #13
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Cool Plywood Help

OK, let me preface this by saying that I'm probably overthinking this, but this is my first tile job and I want to do it right the first time.

My bathroom floor measures 52"X71". The long side is from the shower (from where the photo below is taken) to the toilet stub. The subfloor is 2"x6" T&G doug fir. I'm putting in 1/2" plywood and will need two pieces cut to size to cover the floor. I'll use 1/4" hardi on top of that and 3/8" 13"x13" porcelain tiles.

My question is what the orientation of the two pieces of plywood should be. I assume I wouldn't want to have one large piece, say 48"x71", and a small strip to fill in the rest. Assuming I should use two nearly equal size pieces, should the seam run perpendicular to the long side of the room (i.e., from left to right in the photo) or parallel to it (front to back in the photo)? There's one joist in the center of the floor. If the plywood seam should be perpendicular to long side of the room, should one piece of ply extend across the joist location? If so, which side of the floor should have the larger piece of ply -- the one closest to the toilet or the one closest to the shower/sink.

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Unread 10-24-2009, 01:01 PM   #14
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Riley, it'll help if you'll bookmark this thread and use it for all your project questions so folks can see what you're working on and what's been previously asked and answered. We can change the title to something more generic if you'd like.

On subflooring, you want all plywood panels oriented with the face grain perpendicular to the joist structure for maximum effectiveness.

In your situation, you can avoid having a very narrow strip by simply ripping some off the first sheet such that you have two wider, but not full width sheets for your installation. Make your rip such that the plywood joint falls near the center of one of your floor boards.

[Edit]

Now that I look again, it appears most, or all of the left side inna picher will be covered with cabinet work or/and be in a non-traffic area. Think I'd just use most of one full sheet to cover most of the floor and a narrow strip along that wall.

My opinion; worth price charged.
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Unread 10-24-2009, 01:15 PM   #15
Mr. Riley
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Riley's First Tile Job

CX, thanks for the advice. Actually, I haven't sheetrocked the left side yet because we changed our mind from a vanity to a pedistal sink and need the plumber to come back out and adjust the plumbing. But you're right, that left side will have little to no traffic. If I use a thin strip, won't the small strip tend to sink below the large piece of ply a litte when I screw it down?
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