# Subfloor thickness



## ingeborgdot (Mar 30, 2008)

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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## jlhaslip (Dec 31, 2009)

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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## ingeborgdot (Mar 30, 2008)

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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## JazMan (Feb 17, 2007)

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.

Jaz


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## ingeborgdot (Mar 30, 2008)

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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## ingeborgdot (Mar 30, 2008)

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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## oh'mike (Sep 18, 2009)

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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## ingeborgdot (Mar 30, 2008)

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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## Bud Cline (Mar 12, 2006)

"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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## ingeborgdot (Mar 30, 2008)

So is exterior the same as ADX?


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## Bud Cline (Mar 12, 2006)

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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## ingeborgdot (Mar 30, 2008)

So I would look for AC exterior.


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## Bud Cline (Mar 12, 2006)

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"?:laughing:



> 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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## oh'mike (Sep 18, 2009)

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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## ingeborgdot (Mar 30, 2008)

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.



oh'mike said:


> 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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## oh'mike (Sep 18, 2009)

Inge--

3/4 cdx tongue and groove plywood has been the standard for the last 35 years(in the u.s. any way.)

Two layers of that will do the job. Top that with a tile backer like Durrock set in thinset and you are set.

I may have used the wrong name for that Schluter product--They make fine products but I don't use them my self. sorry.

Your floor Joists are to light for safe tile installation. I think that you should add more joists of the same size. I know that is a lot of work.--If you don't beef up the joists you take the risk of tile failure.

--Good luck--Mike--


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## Bud Cline (Mar 12, 2006)

CDX (t&g) is OK for a SUBFLOOR. CDX IS NOT OK for an UNDERLAYMENT over a subfloor and under tile. A second layer of plywood would be an improvement but........

A second layer of subfloor plywood WILL NOT necessarily improve the deflection of the floor structure.


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## ingeborgdot (Mar 30, 2008)

Here is something new. I just remembered there is a false wall down below this that comes out 3' on the joists. It is not a great support but does add some support. I forgot about this as I don't think it is a great support but like I said it does add some. 
The other issue is I wonder if you guys think my tile job will be big. If you call a 4' strip between the cupboards big then yes. Most of the tile will be about a 4' strip. Other parts will be a 6x4 section and another 6x4 section. It is not a big kitchen at all. 
My kitchen is only 8' wide. The rest of the span goes into another room on the other side of a wall. 
Could I post a picture to show you my kitchen?


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## ingeborgdot (Mar 30, 2008)

I ended up taking out the crappy 1/2" plywood and have replace it with some nice 3/4"stuff. It has nine layers and both sides are usable. I told them I wanted the strongest stuff they had and they sold it to me. What next? Radiant, thinset, ditra, thinset, and then tile? Or 1/2" ply and then radiant, thinset, ditra, and then thinset and tile? Anyone?
I also sistered the joists and most of them I got about 70% of the joist sistered. Some were more than 80% and some were less than70%.


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## lazzlazz (Mar 29, 2010)

This thread has info re: subfloor/underlayment; floor strength; tiling
http://www.diychatroom.com/f5/considering-replacing-playwood-subfloor-many-questions-68639/
There are many threads on here discussing the relevant factors; it's well worth digging through them. johnbridge.com also has useful information pertaining to tiling, ditra, etc.


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## flooringgirl (May 29, 2010)

My best tile installer would say to use 1/2" exterior grade plywood, radiant heat, then tile. He hasn't had a failure in 40 years, so take it or leave it!

Tia


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## ingeborgdot (Mar 30, 2008)

1/2 on top of 3/4 right?


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## flooringgirl (May 29, 2010)

ingeborgdot said:


> 1/2 on top of 3/4 right?


Yes, to equal the 1-1/4" standard. Both mortar and screw the plywood to the base floor. As far as the issues with your joists are concerned, you probably don't have as much of an issue for such a small area as there would be for a large expanse, so long as it is solid. Running the length or perependicular doesn't really apply for tile, since it doesn't expand and contract like solid hardwood, for example.

Tia


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## ingeborgdot (Mar 30, 2008)

Would I not use ditra?


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## flooringgirl (May 29, 2010)

ingeborgdot said:


> Would I not use ditra?


Not necessary, Ditra is for situations where you don't have room to build up. It's a stabilizer when there is no option to increase floor height.

Tia


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## ingeborgdot (Mar 30, 2008)

So you are saying with my joists, that I now sistered I could have 3/4, 1/2 and then just put my wire over that with thinset? 
Would cement board be good to use under the wire?


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## flooringgirl (May 29, 2010)

ingeborgdot said:


> So you are saying with my joists, that I now sistered I could have 3/4, 1/2 and then just put my wire over that with thinset?
> Would cement board be good to use under the wire?


Cement board, exterior-grade plywood, both fine. Then you can go over with the radiant and tile. I don't understand this "sistered" thing.

Tia


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## ingeborgdot (Mar 30, 2008)

It is where you glue and screw another joist to the original joist as long as you can make it to help give it more support. 
Would I put the cement board on top of the 1/2 or leave off the 1/2 and put on the cement board? Sorry for all the questions but just picking the pro's brains to get as much info as possible. Thanks for all the help though.


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## JazMan (Feb 17, 2007)

The Flooringgirl says..... *"My best tile installer would say to use 1/2" exterior grade plywood, radiant heat, then tile. He hasn't had a failure in 40 years, so take it or leave it!"*

And...... *"Both mortar and screw the plywood to the base floor."*

Oh boy.......:no:

Jaz


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## ingeborgdot (Mar 30, 2008)

Any your point is?


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## lazzlazz (Mar 29, 2010)

ingeborgdot said:


> Any your point is?


Ingeborgdot,
If you put the time into reading through many threads on this site (and on johnbridge.com), you'll learn a lot of valuable information, some of which does not directly apply to your immediate situation but which may alert you to relevant things down the road. Another thing you will realize is that Jazman and Bud Cline know their stuff where tiling is concerned (spending time reading many threads will help you figure out who really knows their stuff vs. who is somewhat knowledgeable, but isn't quite at expert level). While there are places where experts may do things differently, they still worry about fundamentals.

You might get away with certain things, but that may depend on whether you did things like orient the subfloor and underlayment plywood to get the highest strength, built up the strength of your joists, etc. It's up to you to decide whether you want to risk ripping up a floor with popped-up tiles in a few years.

Your underlayment should not be screwed to the joists - fasten it only to the subfloor (I used ring shank nails long enough to just barely go through both layers).

When you get to the electric radiant heat, research that process well and follow the manufacturer's instruction and others' advice carefully. It seems (based on what I read) quite easy to accidentally cut a wire in the process of installing it - in the end, I decided it was more of a headache than I wanted to deal with.


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## JazMan (Feb 17, 2007)

Tiles direct over plywood is doable in a light duty, residential dry area, but it is not a very good installation method. High failure rate, and I don't mean a total failure necessarily, but grout cracks are likely. 

Thin set under plywood underlayment is almost insane. You will not find that recommendation anywhere that I know of. 

Without re-reading this thread again, I recommend tiling over either a cement backer properly installed into thin set per specific directions, or my favorite which is installing Ditra. 

Jaz


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## ingeborgdot (Mar 30, 2008)

So what are you trying to tell me? That is what I would like to know????
Are you saying that I should go with 3/4, 1/2, wire, ditra and then tile? Would you please explain to me what you would recommend as Tia did? Please don't leave anything out. Thanks.


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## flooringgirl (May 29, 2010)

JazMan said:


> The Flooringgirl says..... *"My best tile installer would say to use 1/2" exterior grade plywood, radiant heat, then tile. He hasn't had a failure in 40 years, so take it or leave it!"*
> 
> And...... *"Both mortar and screw the plywood to the base floor."*
> 
> ...


Apparently, my expertise in the floor covering field is not appreciated here. Darn, since I was turned on to this site by someone from another flooring forum who seemed to think it was a good place for me. Tell me to bow out of your circle, that's fine. I probably do have enough on my plate already anyway.

The sad thing is that you haven't even considered the value of my opinions. My family is 32 years in floor covering retail and installation, my grandma was an international architect, and my mom is a color expert. Don't you think that makes me well-qualified to talk about floor covering?

What is wrong with mortar and screw? Or with exterior grade plywood and no failures?

Tia


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## JazMan (Feb 17, 2007)

OK, re-read the thread again. Sounds like you did a good job with the joists.

So, you installed the 3/4" ply subfloor over the joists right? You removed the 1/2" right? OK good. A single 3/4" meets and exceeds minimum specs for a subfloor in a basic ceramic or porcelain installation using either a cement backer or Ditra. It's always good to add a layer of plywood underlayment if you want to and have the room to do so. Plywood adds strength between the joists, cement backers add little or nothing, they just give you a tile friendly surface.

So, decide if you're gonna add some more plywood, then install the warming wires or mat per directions, then Ditra etc. If you decide to install a cement backer instead of Ditra, install the heat over the board.

I'm gonna assume you know how to install the plywood underlayment, Ditra etc. If not sure please ax. :thumbsup:

Jaz


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## JazMan (Feb 17, 2007)

Tia,

The professional tile setters here try to give info that follows proven methods as outlined in The Tile Council of North America Handbook. We also recommend always following the instructions supplied by the manufacturer of each product. 

I have a disagreement with a few things you have suggested. And if my memory serves me correctly there are other suggestions from older posts I take issue with. It's just too much work to respond to every post. 

I hate to "correct" everything I disagree with, since while I believe I am right, it's possible to do things wrong or to minimum standards and still have a successful installation. We just need to recommend methods that have been proven to work by many tile setters and over a long period of time. 

You will not find the recommendation to install plywood underlayment into fresh thin set mortar anywhere. Heck, plywood underlayment shouldn't even be glued to the subfloor just fastened to the subfloor. 

I hate to hurt your feelings but, what your family knows has nothing to do with what you can do. Anyway, most architects do not have a clue about the proper installation of tiles, that is why they hire consultants as needed. They also hire consultants in HVAC, plumbing, electrical and several other specialties. 

As for retail flooring stores, there are very few that install much ceramic tiles since they can not be competitive with independent tile setters. And let me add that I don't know of a professional tile setter that believes setting direct over plywood is the right way to do things. Those people are usually referred to as hacks.

Jaz


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## Bud Cline (Mar 12, 2006)

Just curious as to why (every time) someone is called-out on some advice they have given that isn't industry-approved the next thing they do is post up their resume? What's that all about? That's happened three times here in just the past ten days.



> *My family* is 32 years in floor covering retail and installation, *my grandma* was an international architect, and *my mom* is a color expert. *Don't you think that makes me well-qualified* to talk about floor covering?


No, apparently not so much. Sorry.


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## ingeborgdot (Mar 30, 2008)

Ok, the one thing I did wrong was glue the plywood to the joists. I thought that was a good thing. 
Yes, I will put 1/2 on top. I wish 3/8 would be acceptable but I think 1/2 should still give me enough room. 
From the wire on up, how much height should that take up? Any idea? I am planning on using ditra which is what you recommend right?


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## JazMan (Feb 17, 2007)

No no, you did it right. You're supposed to glue the subfloor to the joists, it's any underlayment that isn't glue to the subfloor, but just fastened to the subfloor. :thumbsup: Make sure to use screws that are just a bit longer than the total of the sub & new underlayment being installed. In other words, do not fasten the underlayment into the joists. 

3/8" is acceptable, but 1/2" adds much more strength and you'll save 1/8" by using Ditra.

The wire should add about 1/8". ( I don't know what brand you've got ). Ditra adds 1/8" total. Then your tiles should be 3/8-1/2". So about 7/8 - 1" total.

Jaz


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## ingeborgdot (Mar 30, 2008)

Ok, great. 
The wire is from
http://www.warmzone.com/
I am using the 1/8" wire from them.


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