# proper subfloor



## AustinDB (Sep 11, 2006)

I recently read a thread in this forum that now has me confused. I had always thought that a 5/8" + additional 1/2" installed topped w/ Ditra or 1/4" Hardi was a very good practice. 

Current job has 3/4" ply on 12" joists-are there span charts to identify if the current plywood is sufficient? If the 3/4" is sufficient, is there any benefit of using 1/2" vs 1/4" Hardi? I like Ditra, but cost and ease of purchase play a factor given the height isn't an issue here.

when the TCA recommends 1 1/8", is that a minimum plywood recommendation or a 'best case' suggestion?


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## kevjob (Aug 14, 2006)

try this http://www.johnbridge.com/vbulletin/deflecto.pl

Basically you need a minimum L360 for porcelain tile and L720 for stone

No CBU(hardie, ditra etc..) add any structural stability to floor. You can sister joist, shorten the span with a wall, or add more plywood. 

What is the span and spacing (16" apart) of said joists? 

I use regular T&G real 3/4" plywood over joists with tile all the time. Most of the time when we do stone tiles we add a layer of 1/2 ply over the 3/4 then ditra.


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## Splinter (Apr 5, 2005)

No real benefit to 1/2" vs 1/4" cementboard... Im sure it does add some structural value, you're just not supposed to rely on that. The subfloor should be adequate by itself. 

Check the span of the joists and use the Deflectolator that Kevin posted above. 3/4" ply is usually acceptable over 16"oc framing, provided you're not installing natural stone.


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## AustinDB (Sep 11, 2006)

Thanks for the link Kevin-precisely what I needed. The joist spacing is 12" and the handy dandy calculator said the system netted L/420 which is sufficient for ceramic tile.

If the spacing were 16" OC and deflection were too much given a single layer of 3/4"-how could you determine if an additional layer of 3/8" would be enough? I've always slapped down a sheet of 1/2" 'just because', but there must be a more precise way, right?


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## angus242 (Oct 20, 2007)

72chevy4x4 said:


> when the TCA recommends 1 1/8", is that a minimum plywood recommendation or a 'best case' suggestion?


That's when you're tiling directing to the plywood.


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## JohnFRWhipple (Oct 20, 2009)

The American Wood Council has a better span calculator located here at this link.

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

You will notice that the span calculator's do not ask you for sub-floor thicknesses. Deflection ratings are for the floor joists and deflection needs to be measured in all direction and this includes in between the floor joists themselves. If you have 24" on center floor joists you can achieve a L/360 deflection rating with Spruce 2"x10" 's at 14' total span. Now if these floor joists are topped with a single layer of 5/8" plywood the chances are your plywood will deflect more in between the floor joists than they do over the span of the room. Here is where adding another layer of 1/2" plywood or 5/8" plywood helps out.

12" on center joists spans are ideal for tiling and nets a tank of a sub floor. As a tile contractor you should have a good understanding of framing requirements as outlined in this article http://www.ctioa.org/reports/fr81.html from the CERAMIC TILE INSTITUTE OF AMERICA, INC.

Here is more info on Plywood than you ever need know: http://www.fpl.fs.fed.us/documnts/fplrn/fplrn059a.pdf

If you take a common 16" on center floor joist make up you have about 14.5" in between each floor joist. If you take 14.5" and convert that to 32nds of an inch you have 464 32nds of an inch or 464/32". In a L/360 set up your plywood can not bend more than 1.3 32nds of an inch. For a L/720 rating the deflection is less than a 32nd of an inch and closer to one sixty fourth of an inch. You will not achieve this with wider joists spacing and single layers of plywood.

It's quite common online to read that cement board offers nothing to a floor assembly and this I find quite funny. A simple test at home to see for yourself the structural gain in the various subfloor options is to cut out some pieces of equal size.

Try standing on on a piece of 1/2" cement board thinset to 5/8" plywood on 16" inches.

Now try again with 1/4".

Now try again with Ditra, Spider Web

Try again with Noble Seal TS, Dal Seal TS

Once you have played around with this it's comical the posts stating that cement board adds nothing to a floor assembly. As the floor joists widen this stiffing effect of course lessens but anyone tiling a floor should be measuring this in the field. Bring a straight ruler with you to the job and pile your setting materials in one spot. You may need a simple off cut of plywood to concentrate the load in between the floor joists. Measure the sag. Remove your setting materials and recalculate.

I have done this in years past and found that unless your dealing with 3/4" plywood over 12" joists spacing the deflection is to much and a second layer of 1/2" or 5/8" plywood always is a good starting point. Then cover that with Ditra, Noble Seal TS, Wedi, or what ever you trust most.

JW


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## charimon (Nov 24, 2008)

OK I have an Issue with all the references to span tables with out actually checking to see if the floor you are going to set on meets spec. 
Span tables are for the framer/builder to choose the correct product. Our Requirements are deflection of 1/360 the length of span. VERIFY the ACTUAL floor you are about to tile meets Standard. It is simple. The deflection calculation is based on a concentrated 300# load placed center of span. 

So lets say you have a 20' span, that is 240" so to calculate deflection 240/360= .66 or between 5/8 and 11/16" that is your actual "on site" pass fail standard
How to check your floor
1, Mount your horizontal laser to a wall. 
2 Take a light weight tripod with an Inch scale mounted so the laser intersects it. Mark the placement of the legs so you can replace the tripod over the center of the span after you load it (see 4) 
3 Move to the edge of the room and take a digital picture of the beam where it hits the rule. 
4 load 300# of tile, grout,mortar in the center of the span replace tripod. 
5 go back to where you took your first pic and take another digital picture of the laser line and compare deflection. If the line has moved more than 11/16 you are out of Spec Less than 5/8 and you are good. Save the pics and add the relevant data in case you need to refer to it later. 
(remember the numbers i used are based on a 20' span yours will be differnet)

the same process can be to verify between joist deflection but you need to use a 4 wheeled cart the leingth of the joist span, first measurement is with wheels on joists second wheels center of span between 

The book may say it is ok but the editor didint take into acount the flood the house had 5 years ago or their neighbor giving them a "deal" on cleaning up the pipes hanging too low in the basement. As professionals our job is to verify the Actual Site conditions before we lay tile.


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## JohnFRWhipple (Oct 20, 2009)

charimon said:


> ...
> How to check your floor
> 1, Mount your horizontal laser to a wall.
> 2 Take a light weight tripod with an Inch scale mounted so the laser intersects it. Mark the placement of the legs so you can replace the tripod over the center of the span after you load it (see 4)
> ...


I like it. Great execution. :clap:

Perhaps even a digital camera with a timer. You can walk out with a few bags of setting materials on your shoulder (I'm a good 230) and take a picture. Then set the camera on a timer delay and walk out.

These new home builds are so bouncy we are pricing in a good half day of blocking on each new one we tackle.

JW


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## Splinter (Apr 5, 2005)

> Once you have played around with this it's comical the posts stating that cement board adds nothing to a floor assembly



I'll assume that's directed to my post 2 above yours... Do we really need this nonsense again John? Want to poke me with a stick too because I still use Kerdi?

Of course cement board will add some structural integrity to a floor, my post said it's better to have a subfloor that is already sufficient. You're not supposed to rely on the cement board to bring it up to spec.


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## angus242 (Oct 20, 2007)

Splinter said:


> Of course cement board will add some structural integrity to a floor, my post said it's better to have a subfloor that is already sufficient. _You're not supposed to rely on the cement board to bring it up to spec._


That goes for most underlayments with the exception of plywood when tiling directly over it.


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## JohnFRWhipple (Oct 20, 2009)

Splinter said:


> I'll assume that's directed to my post 2 above yours... Do we really need this nonsense again John? Want to poke me with a stick too because I still use Kerdi?...


You assume to much. The post is for anyone reading this thread so they understand that there is more to deflection of a subfloor than the size of the floor joist and it's span. I could care less that you use Kerdi.

Deflection is deflection. The joists span and distance between them is one part of the equation. Understanding that the plywood thickness and the deflection of that between floor joists should be checked is important.

As for your comment about using cement board to bring a floor's rating up to spec you really do need to get yourself a TCNA specification guide. Once again for those of you without a TCNA specification guide you can use;

1/4" thick cement backer board with 19/32" thick subfloor on 16" floor joists centers and get a "Residential Service Rating" TCNA Specification Detail F144-11

1/4" thick fiber cement backer board with 19/32" thick subfloor on 16" floor joists centers and get a "Light Commercial Service Rating" TCNA Specification Detail F144-11

7/16" thick cement backer board with 19/32" thick subfloor on 16" floor joists centers and get a "Light Commercial Service Rating" TCNA Specification Detail F144-11

1/4" thick cement backer board with 23/32" thick subfloor on 16" floor joists centers and get a "Light Commercial Service Rating" TCNA Specification Detail F144-11

This specification notes that if glass tile is used the service rating could be lower.

The TCNA Specification guide on page 30 specifies that subfloors both wood or concrete need to conform to the IRC for residential and IBC for commercial and that the deflection shall not exceed L/360. We can only assume that all homes are built to this minimum standard and like suggested above checking in the field is key to a successful install.

So again, why is that cement board can qualify for a tile ready surface and get a light commercial service rating if it does not add any structure to the floor? The double plywood service rating is TCNA Specification F150-11 and nets you a residential service rating with 15/32" plywood and light commercial with 19/32". This again is the second top layer of plywood.



Splinter said:


> ...You're not supposed to rely on the cement board to bring it up to spec.


You don't. Just adding the cement board does that for you. Look it up.

JW

Source of TCNA Specifications


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## The Coastal Craftsman (Jun 29, 2009)

Here we go again lol 


I spoke to my cousin more about this subject a week back. Told him about my test with the ditra and wedi board. He's a structural engineer/inspector for the government in the UK. He checks highways, bridges etc etc. I Explained this whole situation to him about materials adding extra strength to subfloors and he gave me all this technical jargan but Basicly said it comes down to compression. He said the better the compression ratings for said products the stiffer the sub floor would be. He gave me an example on a concrete slab secured to plywood subfloors then another example of plywood secured to concrete floor and asked me which is stronger under center load compression. I guessed subfloor on concrete because i didnt even think about it which he said was wrong. He said because concrete can take compression extremely well when you put the concrete ontop of the subfloor it's under compression which is adding stiffness to a subfloor. He then explained that it would be very weak under the subfloor and would fail much earlier. He said this the reason that re bar is placed lower down on a structural concrete beam because the concrete is then under compression and the rebar increase tension the more the concrtete flexys. He said this is why anything with any type of compression rateing will increase the floor stiffness. I showed him the pics of the test I done and he said without even seeing the specs of said products the wedi would add vastly more stiffness to a subfloor just because of its triple layer construction. He did say that this will only apply when the materials are properly bonded though. 


Hope this explains as to why sub floors feel so much stiffer with hardie, cbu and wedi compared to ditra or noble.


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## angus242 (Oct 20, 2007)




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## The Coastal Craftsman (Jun 29, 2009)

I'm surprised we didn't get this As its second time round.


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## charimon (Nov 24, 2008)

I guess the real question here, John, is there a "quantifiable" difference-- structurally between the 2 ratings. and if the difference between the 2 ratings is structural what is it. If the quantifiable differences are not structural than this point doesn't apply. But if they are than you have uncovered a Valid point... So what is the quantifiable differences between the 2 ratings


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## angus242 (Oct 20, 2007)

The real-world issue is all of these silly stats and "feelings" being attributed to what an acceptable floor is.

A light commercial rating because of CBU does NOT make a floor better.
Having a floor FEEL stiffer does not make a floor better.

An installer is doing an incredible disservice to a customer if they sell a job with the lie that a tile underlayment can make an unacceptable subfloor compliant simply by installing it.

Ratings for floor systems are for more than just deflection. 

Flexural and compressive strength differ from material to material. Those are also stats that determine rating. Compressive strength has nothing to do with deflection. 

If you want to drive a forklift on your floor, you probably would want 1/2" CBU instead of Ditra. If you want your kitchen tile installation to last, make sure the framed floor substructure is more than sufficient BEFORE you specify a tile underlayment.


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## The Coastal Craftsman (Jun 29, 2009)

angus242 said:


> Compressive strength has nothing to do with deflection.
> 
> 
> > This is where you are wrong. Just IM'd Me cuz and he said it has everything to do with Deflection. He said the Deflection is based on a materials compressive and Tension properties. He said this is why a 20MPa concrete is stiffer than a 5MPa concrete and why some bridges have to have a much higher Compressive strength concrete to stop deflection. He said he could get into allowable flex but its not something that has anything to do with what we talking about.
> ...


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## The Coastal Craftsman (Jun 29, 2009)

angus242 said:


> Compressive strength has nothing to do with deflection.


This is where you are wrong. Just IM'd Me cuz and he said it has everything to do with Deflection. He said the Deflection is based on a materials compressive and Tension properties. He said this is why a 20MPa concrete is stiffer than a 5MPa concrete and why some bridges have to have a much higher Compressive strength concrete to stop deflection. He said he could get into allowable flex but its not something that has anything to do with what we talking about. 

He said this is something you learn in your first 6 months of uni and that my test with the ditra and wedi shows full well why a floor would be stiffer with wedi/cbu than ditra. But hey this guy designs and checks bridges for a living so i guess he know a thing or 2 about deflection. 
:laughing:


He just sent me a pic of why compressive strength has everything to do with deflection. 

The lower cables would be what you call a "subfloor" and this would be under tension. The more layers you add the more the stiffness/deflection lowers but compresion is added more to the upper layers this would mean you would then need a material that handles compression better. Which would be your "CBU/Wedi" which in turn gives less deflection the more layers you add. If you add too many layers the layers that were under compression would then be under tension.


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## Tinstaafl (Jan 6, 2008)

How many years has your cousin been tiling floors?


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## The Coastal Craftsman (Jun 29, 2009)

Tinstaafl said:


> How many years has your cousin been tiling floors?


We are talking about structure. deflection rates aint just for tile!


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## angus242 (Oct 20, 2007)

Deflection of the framing is all you should be worried about, not the "added" benefit of an underlayment. You're either within the TCNA specs or you're not. 

Underlayment does not change that.


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## world llc (Dec 9, 2008)

here we go again....


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## Tinstaafl (Jan 6, 2008)

BCConstruction said:


> We are talking about structure. deflection rates aint just for tile!


Engineers are great at filling blackboards and sheets of paper with squiggly lines, and on many occasions theory does cross disciplines. But in my experience, those same engineers often suck at practical solutions to real-world situations.

TCNA specs are based not only upon theory, but empirical evidence as well. Play nerd all you want, but bottom line, building to "industry standards" and/or warranty requirements does not allow you to change the recipe according to what you _think_ is acceptable.


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## JohnFRWhipple (Oct 20, 2009)

A floor built to a L/360 Deflection rating is required in any home that is built here in Vancouver. Few are built better than this.

In Vancouver most framing I see is 16" on center floor joists with 5/8" sheathing. That's it. Some glue, some diagonal blocking (maybe), a few nails from the framing gun - tap, tap, tap. And you have the "Vancouver Special" subfloor. This floor meets the requirements from the TCNA for deflection and the local building code. Is it good for tile? No, not if you want to follow a TCNA specification guideline.

Now you can add some more plywood. An extra sheet of 5/8" plywood and your good to go.

You can add some Wedi - good to go.

You can add some Ditra - good to go.

You can add some Hardi Backer - good to go.

You can add some cement board - good to go.

The original post talked about a Proper Subfloor. All of the above are proper subfloors for tile. All of the above are listed in the TCNA guidelines plus many more options. Regardless of what you add on top your adding an underlayment of some kind to get the Sub Floor Tile ready and making it proper.

Now if you don't entertain, weigh 120 pounds and don't mind minimum code you can build it to a "residential service rating" or you can built it better. Just ensure you install each floor as per the manufacture's guidelines and TCNA specs.

There is a huge number of ways to build a proper subfloor for tile. Choose the one that works best for you. Just don't tile over the basic "Vancouver Special".

JW


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## world llc (Dec 9, 2008)

i really hate to be involved in this thread but i cannot find the definition to res. service rating and comm. service rating.

where is it, or just copy paste it so i know


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## Splinter (Apr 5, 2005)

JW said:


> > Originally Posted by Splinter
> > ...You're not supposed to rely on the cement board to bring it up to spec.
> 
> 
> You don't. Just adding the cement board does that for you.



So you agree with me.. Thanks.. That's what I wanted to hear.


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## The Coastal Craftsman (Jun 29, 2009)

Tinstaafl said:


> Engineers are great at filling blackboards and sheets of paper with squiggly lines, and on many occasions theory does cross disciplines. But in my experience, those same engineers often suck at practical solutions to real-world situations.
> 
> TCNA specs are based not only upon theory, but empirical evidence as well. Play nerd all you want, but bottom line, building to "industry standards" and/or warranty requirements does not allow you to change the recipe according to what you _think_ is acceptable.


Like I have said before I build way above what the stupid TCNA book says. I have never once read a book on tiling yet no failed installs in 15+ years. Just because TCNA may say adding wedi does not make floor stiffer and deflect less does that mean it don't! Is my experience not correct and have I been "lucky" on my last 200+ installs? I will always build what I think is acceptable if it's minimum or not but trying to tell someone there method is wrong because a book don't say it's right is crazy talk. But like my stance has been on this whole thing yes wedi will increase a floors stiffness more so than ditra.


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## JohnFRWhipple (Oct 20, 2009)

BCConstruction said:


> Like I have said before I build way above what the stupid TCNA book says. ...


As do I in most renovations but the TCNA is not stupid. It's your best guide for multiple install scenerio's. Wedi is a member of the TCNA. So is Schluter. Noble. Hardi Board. Custom.

These products tested and specified in line drawings grouped by common areas of tiling, ie floors, walls, showers etc.

The original post was on a proper sublfoor. The Specification Guide has dozens and dozens of ways to do this. You will find online people asking this question time and time again and a common response people inquiring about cement board is it does nothing to increase the deflection of a subfloor. These people might be talking about the span over the floor joists but not the span between the floor joists. And with 16" centers cement board can be used to make a proper subfloor like we have mentioned above.

The testing procedures for residential, light commercial and heavy duty are not posted online to my knowledge and you would most likely not find them unless you joined the TCNA as a manufacturer. Try emailing Eric from Noble Company. He might know the procedure.

Eric Edelmayer <[email protected]>

JW


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## world llc (Dec 9, 2008)

do we have a definition yet?


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## JohnFRWhipple (Oct 20, 2009)

world llc said:


> do we have a definition yet?


I thought you where looking it up. Where's Eric when we need him.

I shoot him a link to this thread - lets see what the top tech at Noble has to say about service ratings.

JW


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## ee3 (Feb 10, 2006)

WOW--After reading all that I need a nap.
The service ratings are based on ASTM C627
(the robinson test) It is based on both loads and wear.You load it with up to 300# on different wheels ,soft rubber,hard rubber,steel, that go around and around..Cracked Tile or Grout is recorded until failure based on a % ...


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## world llc (Dec 9, 2008)

JohnFRWhipple said:


> I thought you where looking it up. Where's Eric when we need him.
> 
> I shoot him a link to this thread - lets see what the top tech at Noble has to say about service ratings.
> 
> JW


Lol I thought you were going to look it up...

I did look through tcna but could not find info

Is it a tcna rating? Or ansi? Or that other I think iso?


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## world llc (Dec 9, 2008)

ee3 said:


> WOW--After reading all that I need a nap.
> The service ratings are based on ASTM C627
> (the robinson test) It is based on both loads and wear.You load it with up to 300# on different wheels ,soft rubber,hard rubber,steel, that go around and around..Cracked Tile or Grout is recorded until failure based on a % ...


So does light com. mean it made it to the steal wheels?


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## JohnFRWhipple (Oct 20, 2009)

ee3 said:


> WOW--After reading all that I need a nap.
> The service ratings are based on ASTM C627
> (the robinson test) It is based on both loads and wear.You load it with up to 300# on different wheels ,soft rubber,hard rubber,steel, that go around and around..Cracked Tile or Grout is recorded until failure based on a % ...


Thanks Eric. Then someone weighing 320 can't use their new bathroom?

JW


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## The Coastal Craftsman (Jun 29, 2009)

JohnFRWhipple said:


> Thanks Eric. Then someone weighing 320 can't use their new bathroom?
> 
> JW


If they walk around with high heels on then prob not. I wonder what surface area of the wheel is in contact with the floor when being tested. There's a massive difference between a load being spread out across a 5sq in area compared to a 1sqin area.


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## JohnFRWhipple (Oct 20, 2009)

BCConstruction said:


> If they walk around with high heels on then prob not. I wonder what surface area of the wheel is in contact with the floor when being tested. There's a massive difference between a load being spread out across a 5sq in area compared to a 1sqin area.


I spend a couple weeks in Disney Land just recently and as a general snap shot of America both Canadians and Americans alike I would think a large amount of our population is 250 plus, and I'm sure they walk like me heel to toe, one foot after the other.

What is even sadder than this is how many kids are so huge. Like massive huge. 

Is the framing requirements for our floors taking this into account? Is your subfloor OK to handle your fat ###? :no: :blink: A touchy subject but man if your building your home and ever plan on entertaining you better check that your floors can take the weight!

JW


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