# New subfloor and SLC in bathroom.



## TNTRenovate (Aug 19, 2010)

Tom M said:


> What is your 1/4" on the ply? the ditra? You dont wait a day before topping it? I knew what method you were going to put out. At the end of the day I just still prefer the mud.


Could be Ditra or backer. Why would I need to wait a day before topping? Nothing in any of the products requires it.

I have no issue with what you prefer. What I had issue with its saying that mud is the best and flatest when neither is true. The are other methods that are just as flat and in some aspects better.


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## TNTRenovate (Aug 19, 2010)

carp.addict said:


> I was taught to never tile a floor with less than an 1" under it, other wise there is to much room for deflection. I'd be putting a layer of 1/4" ply over the floor boards, glued and nailed every 4" with ring shank nails, locate the low spots, fill with thinset, use a large notched trowel to butter the ply, lay the Backer board and screw it off to spec.


I've heard a few different measurements pertaining to for thickness. But the thickness of the floor doesn't effect deflection the host deflection and typically 3/4" spacing 16 o.c. meets deflection ratings between the joists. It's a false sense of security to think adding plywood will decrease joist deflection.


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## TNTRenovate (Aug 19, 2010)

Tom M said:


> Another horrible idea in my eyes is gluing the plywood. You give the owner a major problem down the road.
> 
> Go ahead now bash that... IDC


No that's actually sound advice for any method.


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## carp.addict (Dec 26, 2015)

TNTSERVICES said:


> I've heard a few different measurements pertaining to for thickness. But the thickness of the floor doesn't effect deflection the host deflection and typically 3/4" spacing 16 o.c. meets deflection ratings between the joists. It's a false sense of security to think adding plywood will decrease joist deflection.


If what you say is true then no one would bother to put any type of underlay under their tile. It may meet code standards, but adding another layer of plywood definetly increases the strength of the floor, if you think otherwise, I suspect you've never jumped up and down on a floor before and after installation.


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## TNTRenovate (Aug 19, 2010)

carp.addict said:


> If what you say is true then no one would bother to put any type of underlay under their tile. It may meet code standards, but adding another layer of plywood definetly increases the strength of the floor, if you think otherwise, I suspect you've never jumped up and down on a floor before and after installation.


I am not sure how you come to that conclusion by what I said. I am well aware of the "jump test", but we need not throw out the science.

What's the formula for calculating floor joist deflection? Once you answer that you'll know why adding plywood doesn't really reduce joist deflection. I have yet to see a calculation or calculator that asks for the thickness of the sub-floor.

Where it does help is in between the joist deflection.


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## heavy_d (Dec 4, 2012)

I bet 100 layers of plywood would reduce joist deflection.


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## TNTRenovate (Aug 19, 2010)

heavy_d said:


> I bet 100 layers of plywood would reduce joist deflection.


I'd collapse well before you got to 100 sheets. But that illustrates another issue adding plywood to stiffen the floor, it adds weight to the system with no real return on reducing the joist deflection.


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## GO Remodeling (Apr 5, 2005)

Interesting about floor thickness. 1 1/4" was standard when I started in construction. But "big" floor tile was 6x6 and soft body. 

Now, floor thickness has been reduced due to new products that accommodate movement. Schulter over 3/4". Permat replacing much thicker backers. Mudbeds are for NJ and Cali. 

There is deflection between joists as well. An added layer of the right plywood does help. Schluter calls for an extra layer of plywood for stone installs. That's above the framing deflection requirement.

What I don't trust are thin membranes over 3/4".


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## carp.addict (Dec 26, 2015)

olzo55 said:


> Interesting about floor thickness. 1 1/4" was standard when I started in construction. But "big" floor tile was 6x6 and soft body.
> 
> Now, floor thickness has been reduced due to new products that accommodate movement. Schulter over 3/4". Permat replacing much thicker backers. Mudbeds are for NJ and Cali.
> 
> ...


I've heard 1"1/4 as well, although around here the standard in new construction is a layer of 1/2" over 5/8' t&g, and that's only 1 1/18" thick.


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## Tom M (Jan 3, 2007)

Thicker plywoods absolutely helps in deflection. Stand on a bare joist, then stand between 2 over a piece of plywood. Its not supposed to be counted as an assembly but it factors in. Many new home builder use 1" plywood in living rooms to stiffen these spaces.

And...because of height some guys just mud over dimensional planks and skip a layer of anything. Too me I dont trust the deflection of diagonal planks.


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## carp.addict (Dec 26, 2015)

Tom M said:


> Thicker plywoods absolutely helps in deflection. Stand on a bare joist, then stand between 2 over a piece of plywood. Its not supposed to be counted as an assembly but it factors in. Many new home builder use 1" plywood in living rooms to stiffen these spaces.
> 
> And...because of height some guys just mud over dimensional planks and skip a layer of anything. Too me I dont trust the deflection of diagonal planks.


The planks are always loose and have checked in my experience, that's why I like to glue a layer of ply on top of it after adding some nails.


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## EthanB (Sep 28, 2011)

More plywood does reduce deflection. Check TCNA and the Deflectolator. 

I don't see the purpose of putting cbu on top of slc, as someone mentioned.


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## Tom M (Jan 3, 2007)

I have the TCNA. They are not real clear or at least dont thoroughly address wood planks. 

In the substrate section for stone tiles they say you must have 2 layers of wood structural panels (plywood )if using a backer board, but they allow a single wood layer subfloor if you are using an unbonded mortar bed.

Then in the setting material section, (Neat cement/mud can also be used a setting material) they say "rough wood floors", "plywood floors",foam insulation board etc... are suitable backing.

What we dont know is the deflection of the planks on a diagonal, the deflection of the mudbed and the total when the 2 are together. 

But..... it must be okay. IDK I have ripped out many old baths with no subflooring. Mortar beds were used as setting beds and poured in between old floor joists on cribbing. Often small framing members cut to hell thanks to the plumbers.


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## TNTRenovate (Aug 19, 2010)

Problem with saying plywood helps deflection is its misleading.

There are many factors in construction that effect a tile installation. 

There is joist deflection, which is what is commonly referred to a deflection. So to say plywood helps deflection is misleading. The deflectolator doesn't ask for subfloor thickness. Why? Because it's calculating joist deflection.

Then the it's deflection between the joists. The TCNA and many other sources absolutely agree that additional layers of plywood decrease this deflection, and I agree. 

Then there is compression of materials. Which is a while other discussion.

To say a subfloor thickness of 1" or 1 1/4" is sufficient enough isn't accurate. To say adding a layer of plywood reduces deflection is misleading and also not quite accurate.


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## Tom M (Jan 3, 2007)

And the deflectolator is misleading as well. Because an assembly does have value.

I just picked up the new IRC and they have again reduced span ratings. I would say the older growth framing members had more tile friendly tolerances specially as part of an assembly.


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## EthanB (Sep 28, 2011)

TNTSERVICES said:


> Problem with saying plywood helps deflection is its misleading.
> 
> There are many factors in construction that effect a tile installation.
> 
> ...


Oh lovely. Your semantics always help so much with the conversation.

With VERY FEW exceptions(maybe one or two in my life) adding ply will reduce deflection above, and in between, joists because of load spread. I don't really care whether you call it "joist" deflection or "mayonnaise" deflection. We all know exactly what we're talking about here. Feel free to win this argument by dissecting the etymology of the term back to the 1600's.

If the joists are undersized then you're going to have too much deflection no matter how stiff the subfloor.


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## TNTRenovate (Aug 19, 2010)

EthanB said:


> Oh lovely. Your semantics always help so much with the conversation.
> 
> With VERY FEW exceptions(maybe one or two in my life) adding ply will reduce deflection above, and in between, joists because of load spread. I don't really care whether you call it "joist" deflection or "mayonnaise" deflection. We all know exactly what we're talking about here. Feel free to win this argument by dissecting the etymology of the term back to the 1600's.
> 
> If the joists are undersized then you're going to have too much deflection no matter how stiff the subfloor.


You are confused my friend, sorely confused. Just like everyone who refers to deflection as a general term. It shows ones ignorance on the subject. Even the TCNA explains the confusion and difference.

https://www.tcnatile.com/faqs/30-deflection.html

_What is the acceptable deflection for a floor that will be tiled?_

_Traditionally, the accepted minimum requirement for floor rigidity is L/360 - before the tile underlayment is installed. The L/360 standard means that the floor should not deflect more than the "span" divided by 360. If the span of the joists is 10 feet (between supports), then the deflection should not be more than 1/3" between the center and the end. Frequently, there is misunderstanding regarding deflection between joists. For example, while joist manufacturers regularly meet the standard L/360 criteria for code construction with 24" o.c. (on center) systems, these floors often have deflection between the joists exceeding L/360._

You also need to go back and read what I have been writing. I have said numerous times that you have to first determine the existing deflection rating of your floor assembly and determine if you need to address undersized joists, before you consider adding plywood, as adding plywood will not reduce joist deflection, or also refereed to as deflection of the joists. However, it will reduce the deflection of the plywood between the joists. That isn't semantics, it's using proper terms to explain two very different factors.

And it's not just me, there are some other guys on here that I know you actually respect who discuss this exact subject a few years ago. Notice they mention the difference between joist deflection, deflection between the joists as well as compression of the materials. 

http://www.contractortalk.com/f73/proper-subfloor-117829/

Now back pedal or try and bring this back to me and semantics.


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## madmax718 (Dec 7, 2012)

carp.addict said:


> Why does gluing the ply give them problems? If anything they would be thanking you for reducing their squeak's and giving them a solid floor.


You got squeaks under tiles you definitively have some issues.


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## madmax718 (Dec 7, 2012)

Not to get into the middle of this discussion, but removing a full mud bed + old (possibly cast iron) probably reduced at least 500-1000 lbs of weight over those joists. Full mud beds also provided their own support (cement + wire lathe). 

We also have to keep in mind that you no longer have to build bathrooms to last 50 years. Nobody keeps bathrooms for 50 years anymore. People will update way sooner than that. 

Mud beds are not flatter than SLC

3/4" T&G, using the proper sub floor screws is just fine. If your floor is so far out of level, the joists have probably been over loaded- one of the common effects of mud beds + tiles+ cast iron. (excessive weight). Again, lots of options here, sister the joists, level the tops, and the power planer does a great job leveling out any high spots of the ply.


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## heavy_d (Dec 4, 2012)

TNTSERVICES said:


> You are confused my friend, sorely confused. Just like everyone who refers to deflection as a general term. It shows ones ignorance on the subject. Even the TCNA explains the confusion and difference.
> 
> https://www.tcnatile.com/faqs/30-deflection.html
> 
> ...


For fun, imagine you have an empty room. Joists run one end to the other, so this room is the entire joist span. If you add 3-4 layers of 5/8 ply (for fun), you don't believe that will reduce joist deflection? I believe it would. I believe all those layers screwed/glued together and to the joists would assist greatly, despite the added weight.


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## Tinstaafl (Jan 6, 2008)

If you add a couple layers of plywood on top of a joist, you are effectively increasing the height of that joist. Obviously the weight of the ply and the fastener schedule would play into the total effect, but overall I believe the floor would be a bit stiffer. :thumbsup:


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## TNTRenovate (Aug 19, 2010)

heavy_d said:


> For fun, imagine you have an empty room. Joists run one end to the other, so this room is the entire joist span. If you add 3-4 layers of 5/8 ply (for fun), you don't believe that will reduce joist deflection? I believe it would. I believe all those layers screwed/glued together and to the joists would assist greatly, despite the added weight.


Weird though that no joist deflection calculation factors in subfloor thickness.


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## TNTRenovate (Aug 19, 2010)

Tinstaafl said:


> If you add a couple layers of plywood on top of a joist, you are effectively increasing the height of that joist. Obviously the weight of the ply and the fastener schedule would play into the total effect, but overall I believe the floor would be a bit stiffer. :thumbsup:


Stiffer for sure, but why? It would decrease the deflecting between the joists more than decrease the joist deflection. The floor would in fact feel stiffer. It would also add weight causing the joists to deflect from their original position. But that's not the same as addressing the issue of undersized joists. Nor is it a substitute fot proper structure. With a little extra framing you can increase the deflection by over 50%.

Adding plywood isn't a solution to undersized joists.


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## Tinstaafl (Jan 6, 2008)

TNTSERVICES said:


> ... It would decrease the deflecting between the joists more than decrease the joist deflection. ... Nor is it a substitute fot proper structure.


Not arguing either point. Just clarifying that it's tough to make an absolute statement that adding plywood does nothing at all to lower joist deflection. :thumbsup:


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## EthanB (Sep 28, 2011)

Put a 500lb point load on one joist. Then put a steel I beam across 5 identical joists and put the 500 lbs on the beam above the center joist. There will be less deflection because the load is spread to the other joists. That's how a subfloor assembly reduces joist deflection.


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## TNTRenovate (Aug 19, 2010)

Tinstaafl said:


> Not arguing either point. Just clarifying that it's tough to make an absolute statement that adding plywood does nothing at all to lower joist deflection. :thumbsup:


Its pretty easy to say it isn't significant enough to be a factor since it's not factored in any joist deflection calculation.


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## TNTRenovate (Aug 19, 2010)

EthanB said:


> Put a 500lb point load on one joist. Then put a steel I beam across 5 identical joists and put the 500 lbs on the beam above the center joist. There will be less deflection because the load is spread to the other joists. That's how a subfloor assembly reduces joist deflection.


Plywood does not carry load like a beam carries a load. Put that same 500lb weight in the middle of an unsupported piece of plywood. Now put that same 500lb weight on the beam.


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## TNTRenovate (Aug 19, 2010)

So what's the factor then fellas. When calculating deflection what do I add to the equation to factor in sun for thickness. 

How do you figure your for rating for tile installation? Hey close enough and then throw some plywood on top and hope you have enough?


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## carp.addict (Dec 26, 2015)

madmax718 said:


> Not to get into the middle of this discussion, but removing a full mud bed + old (possibly cast iron) probably reduced at least 500-1000 lbs of weight over those joists. Full mud beds also provided their own support (cement + wire lathe).
> 
> We also have to keep in mind that you no longer have to build bathrooms to last 50 years. Nobody keeps bathrooms for 50 years anymore. People will update way sooner than that.
> 
> ...


You should never tile straight to subfloor, it's fastened directly to structure and will move with it, by adding underlayment or "Backer board" your adding a slip sheet between your tile and the movement of the structure itself. 

I always try not to hit joists when screwing/nailing the sheets for that reason. That's how I was taught anyway.


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## TNTRenovate (Aug 19, 2010)

carp.addict said:


> You should never tile straight to subfloor, it's fastened directly to structure and will move with it, by adding underlayment or "Backer board" your adding a slip sheet between your tile and the movement of the structure itself.
> 
> I always try not to hit joists when screwing/nailing the sheets for that reason. That's how I was taught anyway.


If I had to install direct to subfloor I would apply a roll on membrane, one that specifically was a crack isolation membrane such as AquaDefense.


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## GO Remodeling (Apr 5, 2005)

I never trust a subfloor only with roll on or Tavy Skin or Green Skin. Just me; don't believe in that thin layer providing enough protection. I prefer a sheet of something over the subfloor then membrane. The exceptions to that are Ditra and Permat


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## Tom M (Jan 3, 2007)

heavy_d said:


> For fun, imagine you have an empty room. Joists run one end to the other, so this room is the entire joist span. If you add 3-4 layers of 5/8 ply (for fun), you don't believe that will reduce joist deflection? I believe it would. I believe all those layers screwed/glued together and to the joists would assist greatly, despite the added weight.


Take it a step further and screw a 2x across the joists. There has also been a debate that joist blocking adds floor strength and I think the notion it doesnt has been debunked. Anything that join members, to act unity reduces deflection. With in limits of course.


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## GO Remodeling (Apr 5, 2005)

we use to stiffen subfloors from below by dropping the solid bridging 1 1/2" and strapping a 2x4 or 2x6 next to the bridging and attached to the joists. Everything was tied together and worked as a unit.


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## carp.addict (Dec 26, 2015)

olzo55 said:


> we use to stiffen subfloors from below by dropping the solid bridging 1 1/2" and strapping a 2x4 or 2x6 next to the bridging and attached to the joists. Everything was tied together and worked as a unit.


If your installing blocks, why not make them full width?


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## mike gunderson (May 23, 2010)

I have wondered why the deflector chart doesn't figure bridging as well. I'm doing a floor that has 2x8 doug fir, spanning 12 feet. it has 1x3 cross bridging. That floor does not meet the deflector requirements but the testing i've done it shows hardly any deflection at all. Plus the "bounce test" doesn't show any bounce at all.


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## TNTRenovate (Aug 19, 2010)

mike gunderson said:


> I have wondered why the deflector chart doesn't figure bridging as well. I'm doing a floor that has 2x8 doug fir, spanning 12 feet. it has 1x3 cross bridging. That floor does not meet the deflector requirements but the testing i've done it shows hardly any deflection at all. Plus the "bounce test" doesn't show any bounce at all.


Measure the deflection yourself. Laser the center, measure, add 300lbs to the center of the room, measure. What's your deflection? If you meet your desired deflection have fun.

Here's the problem that I have with all of these guesses and "feelings" from jump or bounce tests, they are subjective. We need to be objective in these situations. I am not saying throw the subjective out, just don't rely on it on it's own. It would suck to be called back on a failing installation that one felt was sufficient.


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## GO Remodeling (Apr 5, 2005)

carp.addict said:


> If your installing blocks, why not make them full width?


Perhaps I wasn't clear enough.

Solid bridging is full height of joist space (usually a 2x12). But dropped down 1 1/2" to meet a flat 2x4 laid perpendicular across the joist. The flat 2x4 is nailed into the joist. The bridging is nailed to the joist and into the flat 2x4. Understand?


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## GO Remodeling (Apr 5, 2005)

I had to call a TGI manufacturer on a tile failure job was consulting on. The manufacturer"s rep said that having drywall attached to the bottom of the joists added "x" % more strength to the floor.

Why don't the calculator"s have the plywood figured in? Maybe the there were too many variables or the calculators were developed years ago and no one has bothered to update them. Just because the plywood isn't included doesn't mean it is insignificant. It might just be excluded for lack of interest


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## 91782 (Sep 6, 2012)

Learned long ago: Glued and screwed ply on both the top and bottom chord will increase span rating(live/dead load).

This I have had done (calculations & approval) by an architect in the past.

I know that strapping bridging blocking can reduce bounce, but never have I heard of it changing span ratings.

And I have asked and been schooled on this subject.


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## TNTRenovate (Aug 19, 2010)

olzo55 said:


> I had to call a TGI manufacturer on a tile failure job was consulting on. The manufacturer"s rep said that having drywall attached to the bottom of the joists added "x" % more strength to the floor.
> 
> Why don't the calculator"s have the plywood figured in? Maybe the there were too many variables or the calculators were developed years ago and no one has bothered to update them. Just because the plywood isn't included doesn't mean it is insignificant. It might just be excluded for lack of interest


Until they include them, to me, it isn't significant. For one reason, what is the factor?

Weyerhaeuser has a good article that quotes Art Lewis an engineer for Weyerhaeuser. He does say that sub-floor thickness can help, but again, there is no data. However, he does say that extra framing material can improve deflection by 50%.

http://www.woodbywy.com/trus-joist/taking-the-bounce-out-of-floors/


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## carp.addict (Dec 26, 2015)

SmallTownGuy said:


> Learned long ago: Glued and screwed ply on both the top and bottom chord will increase span rating(live/dead load).
> 
> This I have had done (calculations & approval) by an architect in the past.
> 
> ...


Span books and charts use bridging and strapping in the equation to produce total span limits. It's actually integrated into the charts themselves.


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