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Old 06-05-2010, 02:24 PM   #1
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Subfloor thickness


I am having to replace my top subfloor because it was too much work to prep it for tile as the three layers of linoleum that the previous owners had left was just too much to handle. Anyway when I got to the first layer of subfloor it is 1/2". What would you recommend putting on top of that to make it nice an sturdy for tile? What would the professionals do to get it ready. I am going to be using 12x24" tiles with electric radiant heat below. Thanks.

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Old 06-05-2010, 05:52 PM   #2
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Subfloor thickness


the total of your subfloor for tile should be a minimum of 1 1/4" with 1 1/2" thickness being better (stiffer).
Are you using Kerdi under your tile?

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Old 06-05-2010, 06:23 PM   #3
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Subfloor thickness


So I should use either two more 1/2" or one 3/4" plywood. When you say kerdi are you referring to ditra? Should I? Will that make a difference with this thick of a floor?
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Old 06-05-2010, 08:05 PM   #4
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Subfloor thickness


I'm not sure what you meant when you said, "Will that make a difference with this thick of a floor?" I don't care if you have 3" of plywood, it's still gonna shrink & expand. So, it's never a good plan to install tile over plywood. You can and sometimes it works OK, but we don't recommend it. So yes, you need Ditra or a cement backer board so your tiles are bonded to a stable tile friendly substrate.

When your house was built 1/2" subfloor with a better quality 5/8" was the norm. the vinyl floor was then installed over the 5/8". The subfloor system was changed in the '70's to a single 3/4" with the flooring people installing the finishing 1/4" underlayment for the vinyl.

You need to install a min. 5/8" underlayment grade plywood or 3/4" if you have the room. Then I would install Ditra and you know the rest.

Those are large tiles, you better make sure the floor is flat and the joists are stiff enough for tiles. Have you run the number into a span chart? OH you haven't. Tell use the; size and type of joists. (species and grade would really help too), their spacing and the length of the unsupported span of the joists.

Since the tiles are 12x24 I reckon you're setting them in a half offset (brick) pattern. Have you checked to be sure the tile's warpage isn't going to give you fits.

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Old 06-05-2010, 08:38 PM   #5
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Subfloor thickness


I was planning on putting down 3/4 plywood. Is all 3/4" plywood the same strength?

The joists are 7.25", with an 11' span and 16"oc. The species of wood looks line pine? fir? looks like a good quality.

The pattern of tile is going to be a modern non brick pattern. They are a rectified tile.
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Old 06-06-2010, 08:04 AM   #6
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Subfloor thickness


How much better off would I be to take of the 1/2 inch also and then replace with 3/4? What would go on top of that for best results?
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Old 06-06-2010, 09:16 AM   #7
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Subfloor thickness


For tiles that big --two layers of CDX plywood --totaling 1 1/4 to 1 1/2 inch thick.

Then a tile backer--set in thinset--or Kerdi-- Your floor joists--2x8 seem a bit light for the tile'

Consider sistering in some new ones.--Someone will chime in with the deflection on 2x8's over 11 feet.

My gut tells me that is to springy for 12x24 tiles.---Mike---
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Old 06-06-2010, 09:41 AM   #8
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Subfloor thickness


Why wouldn't I use ADX plywood if I need more strength? ADX is stronger to some degree and gives even better support.

To sister in some would be very difficult because of wires that run through many of them. Would 2x4s give some help? What about 2x8s connecting the joists together?
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Old 06-06-2010, 01:22 PM   #9
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Subfloor thickness


"CDX" plywood is never never never ( did I mention NEVER) recommended for subfloor or underlayment when ceramic tile is involved. You won't find that suggestion in any of the books anywhere.

Exterior grade underlayment plywood (Exposure 1) is the current recommendation, never never never CDX. Never ever!
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Old 06-06-2010, 01:24 PM   #10
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Subfloor thickness


So is exterior the same as ADX?
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Old 06-06-2010, 01:57 PM   #11
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I've never heard of "ADX". But that's not to say it doesn't exist somewhere. Seems kind of counter-productive to me.

"X" simply means that "scrap wood" was used somewhere in the manufacture. In the case of an ADX panel that would mean the scrap wood was used in the core plys. How would that be a good thing?

"A" as we all know is the best-side of the panel.
"D" is going to be the back-side of the panel.
"X" is the worst of the worst that is allowed in the manufacture. In X voids and chips and knots can occur and nothing is done to repair them. A panel with an X core WOULD NOT be suitable for any ceramic or stone flooring installation. Never try to use "X" plywood for any flooring underlayments.

"CDX" is the worst of them all and never used in flooring underlayments.
"C" would offer a not so good surface layer but plugged and sanded.
"D" would be the worse than not-so-good back face.
"X" is as we now know even worse than C and D but hidden inside the laminates. "X" DOES NOT stand for EXTERIOR. "X" stands for: as bad as it gets. Are ya with me there Mike?

For ceramic and stone tile underlayments the best choice would be A-C Exposure 1. This means the face is the best you can get for the purpose and even C is suitable for the back-face and the panel is constructed with exterior glue.
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Last edited by Bud Cline; 06-06-2010 at 02:18 PM.
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Old 06-06-2010, 02:04 PM   #12
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Subfloor thickness


So I would look for AC exterior.
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Old 06-06-2010, 02:16 PM   #13
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Subfloor thickness


I doubt that 2 X 8 floor joists spanning eleven-feet is suitable for a tile installation. But I haven't looked it up. Seems that the maximum-span for 2 X 10's is about eleven feet.

KERDI is not typically used under a tile installation of this nature. That's not the correct product. The correct product is DITRA. Are ya with me there Mike?

I hate it when people start giving advice here and don't know what they are talking about.

How can anyone look at a piece of wood in a floor structure and declare "it looks like good quality"?

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So I would look for AC exterior.


For ceramic and stone tile underlayments the best choice would be A-C Exposure 1.

OK, the span tables for that floor say you are marginal in deflection at eleven feet of span. So marginal in fact that I personally wouldn't install ceramic tile without beefing something up somewhere. Especially tiles that are that large.

Either reduce the span or sister the joists.
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Old 06-06-2010, 02:45 PM   #14
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Subfloor thickness


Bud---X stands for exterior glue---CDX Grade Plywood–i.e. C-D Exposure 1 Plywood - See See Plywood

Learn a little,Bud.
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Old 06-06-2010, 03:30 PM   #15
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Subfloor thickness


How can I sister the joists? Are you meaning running the length of the span? If so, this will be almost impossible. If you mean sister by running perpendicular than that will be doable.

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Bud---X stands for exterior glue---CDX Grade Plywood–i.e. C-D Exposure 1 Plywood - See See Plywood

Learn a little,Bud.
So what would I ask for at the lumber yard for the strongest possible?

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