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Old 09-22-2014, 07:57 PM   #1
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Questions on subfloor and leveling.


Just removed old vinyl flooring (2 layers) and got down to the plank subfloor. Spoke with someone at a local hardware store and they advised to put down 1/4 plywood then hardy board on top of that followed by my 12x24 porcelain tile. I began this process and had a couple questions.

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1. Can the plywood sheets be laid butted directly to each other or does there need to be some sort of a gap?
2. The plywood made the floor a lot more uniform but isn't %100 flat. Is this an issue? Will the hardy board laid on top of the plywood correct some of this issue or should I be using some self leveling cement?

Depending on what you guys say I may have a few more questions but I appreciate you taking a look.
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Old 09-22-2014, 08:26 PM   #2
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Even the plywood is just going to follow the contour the floor underneath. It may look like it helped flatten the floor but it didn't and neither will the backer. How bad is it out of level? Some of it can be fixed when you lay the tile, but from your pics it looks like you need some SLC to help level it out a bit.

But the seams of the plywood and leave a 1/8" space in your backer. Make sure to use thinset and screw every 6" or as directed by the manufacturers instructions.
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Old 09-22-2014, 09:03 PM   #3
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Useless info from that hardware guy.
1/4" is never ever used under tile !!!
Since you have nothing but old slat subflooring flooring you need at least 3/4" underlayment rated T & G sub flooring. Floor lever, then the tile.
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Old 09-22-2014, 09:17 PM   #4
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Porcelain takes a very stiff floor. And don't forget the thinset under the CBU, after you get the floor stiff enough. Oh yeah, we need size, spacing and grade of the floor joists.
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Old 09-22-2014, 09:30 PM   #5
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So do I need to pull up what I have/take back the plywood and get something like this?

Or is there someway to salvage the way I am currently doing it?

At what point is the self leveling concrete applied? After the wooden subfloor or after the hardy board?
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Old 09-22-2014, 09:30 PM   #6
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What Rusty said...you need to calculate deflection.

1/2" is all that is recommended over floor boards, but most professionals would up it to 3/4". I also agree with Rusty that you need a stiff floor for your installation. From the pics it looks like they are on 16's but it's hard to tell.
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Old 09-22-2014, 09:48 PM   #7
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Measuring between the nailing lines they are between 13 in and 18 inches apart with the most common being in the 16 inch range. Thanks for the good input so far i'm just freaking out thinking that i have to pull everything back up and argue with people at lowes to refund the money for the used boards.
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Old 09-23-2014, 08:18 PM   #8
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Any other input on this?
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Old 09-23-2014, 08:44 PM   #9
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You have gotten some good pro advice that I agree with.I doubt that your joists are between 13 and 18" spacing.even in the old days they could read a tape .For ceramic you need to have a stiff floor as Rusty has said.It does not have to be perfectly level but flat and stiff.
One point you may want to ponder is.If the guys at the box store know all that much why are they working there for $10 or so an hour when they could be out there doing it for $25 or so an hour?
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Old 09-23-2014, 08:46 PM   #10
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You can base the stiffness calculations on 16 inch on center joists. In order to compute the stiffness of the floor, you need to know the depth of the joists. This needs to be the actual depth, not the nominal depth, measure it with a tape measure. You are supposed to know the species and grade of the joists, but unless the joists are stamped, it is hard to know exactly, so in most cases you can assume you have SPF (spruce pine fir) construction grade.

You also need to know the actual span of the joists, this is the distance from one support to the other. Once you know all this, go the John Bridge website, where there is a handy deflection calculator. For 12x24 inch porcelain tile, you are going to want no less than 480 deflection ratio. Less than that, you risk cracking the tile. Hardi board does nothing to improve the stiffness of the floor, plywood helps a little. See this link http://www.johnbridge.com/vbulletin/deflecto.pl
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Old 09-24-2014, 11:14 AM   #11
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Quote:
Originally Posted by Daniel Holzman View Post
You can base the stiffness calculations on 16 inch on center joists. In order to compute the stiffness of the floor, you need to know the depth of the joists. This needs to be the actual depth, not the nominal depth, measure it with a tape measure. You are supposed to know the species and grade of the joists, but unless the joists are stamped, it is hard to know exactly, so in most cases you can assume you have SPF (spruce pine fir) construction grade.

You also need to know the actual span of the joists, this is the distance from one support to the other. Once you know all this, go the John Bridge website, where there is a handy deflection calculator. For 12x24 inch porcelain tile, you are going to want no less than 480 deflection ratio. Less than that, you risk cracking the tile. Hardi board does nothing to improve the stiffness of the floor, plywood helps a little. See this link http://www.johnbridge.com/vbulletin/deflecto.pl
IMO, 480 isn't enough. The TCNA documents that they have seen failures at 600. Shoot for 720 and you won't have to really ever worry.
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Old 09-24-2014, 11:44 AM   #12
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Quote:
Originally Posted by ChiTownPro View Post

IMO, 480 isn't enough. The TCNA documents that they have seen failures at 600. Shoot for 720 and you won't have to really ever worry.
How do I go about stiffening my floor for porcelain?
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Old 09-24-2014, 05:07 PM   #13
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The most practical way to stiffen a floor that does not meet deflection criteria is to sister joists to the existing joists. This topic has been discussed many times on this forum over the past few years, do a search and you will see many threads that discuss in detail how to do it, length of sister, fastening techniques, potential issues, value of blocking, and other relevant issues. An alternative to stiffening the floor, which can be a major project if access is difficult, is to use a floor material that requires a lower stiffness rating.

Smaller porcelain tile are much less susceptible to cracking than large tile. Vinyl is even less susceptible to cracking due to deflection. Natural stone is the most susceptible.
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Old 09-24-2014, 09:00 PM   #14
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So I decided to really map out my Joists to figure out what I need to do because my floor is not extremely standard. There are a few joists that are already doubled up and there is not a super standard spacing for all of the joists.



The numbers between the joists show the spacing. The numbers at the top of the joists shows the span between support and also show which joists are doubled up. There are also X joist braces where the span is longer. These joist braces are about 5'6" from the bottom of the image (about half way in between supports). The x braces are shown with red x's. The x-Joist braces are 1x3s and are pictured below.



I'm wondering if the doubled up joists as well as the small spaces between joists in some spots will give me enough leeway to go forward with the 12x24 porcelain tiles. Also I know that the deflection calculator gives the deflection length wise down the joist. I am planning on laying the 12x24 tiles perpendicular to the joists so i didn't know if that would work in my favor as well. Thanks again for any input.
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Old 09-24-2014, 09:07 PM   #15
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Also forgot to add that the joists are 7.5"x1.5".
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