# Installing Durock correctly on a wood subfloor



## CWYfire (Nov 30, 2008)

This has probably been asked hundreds of times and I will be reading other threads in the mean time. But thought I would ask as I am about to undertake this in the next few days.  

Could someone describe how to install Durock correctly on a wood subfloor?  Does one need to pull a coat of something like thin coat prior to screwing down the Durock to the 3’4” plywood subfloor?  Currently we plan to cover the Durock with some sort of tile.  Any suggestions on that portion or what might be a better finished surface than tile would also be appreciated.
*Thanks for all of your incredible advice!*
Ed


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## webby3650 (Nov 30, 2008)

Some tile-setters lay the durarock in a bed of thinset before screwing the daylights out of it, this makes a rock solid base. Also, keep R-values in mind. Your stove may require more protection than others.


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## LLigetfa (Nov 30, 2008)

As mentioned, it depends on required R values and how stiff the tiles need it to be.  If you use really strong porcelain tile, it doesn't much matter how you lay the durock.

One way to get more R is to sandwich layers of metal diamond stretch mesh lath and durock.  Diamond lath is often used for stucco.  Two layers at right angles traps enough air to equal a couple of inches of masonry.


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## Elfin (Nov 30, 2008)

Webby is right, but another thing that should be added is a plywood support layer. Durock adds no structural support at all, and if used alone over subflooring you are likely to ultimately develop cracks in your tile and grout. 

But like webby said, the first and biggest consideration is the requirement for your particular stove... if only a non-combustible pad is required, plywood and durock covered with a layer of tile are adequate, with respect to all required clearances. 

So if that's your only pad requirement, the way I'd personally choose to layer this over subfloor is: 

1. 3/4" plywood, screwed into the subfloor but not into the joists
2. concrete board, set into modified thinset and also screwed into place
3. solid-bodied porcelain tile (much stronger than ceramic and unlikely to chip or show damage)


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## begreen (Nov 30, 2008)

Good advice. CWYfire, can you add your Century stove to your signature?


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## webby3650 (Nov 30, 2008)

Elfin said:
			
		

> Webby is right, but another thing that should be added is a plywood support layer. Durock adds no structural support at all, and if used alone over subfloor you are likely to ultimately develop cracks in your tile and grout.
> 
> But like webby said, the first and biggest consideration is the requirement for your particular stove... if only a non-combustible pad is required, plywood and durock covered with a layer of tile are adequate, with respect to all required clearances.
> 
> ...


I believe Durock installed correctly does provide plenty of support. I think the screws are supposed to be on a 4" grid. That is how I did mine, I didn't want a big elevation change since mine is in the middle of the room. Mine gets walked on occasionally, wood dropped on it and it has no signs of cracking or flexing. It's been about three years now.


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## LLigetfa (Nov 30, 2008)

When I built my house, I used 3/4" Strudifloor OSB which is much stronger than plywood.  I put down diamond lath in a 3/8" layer of modified thinset for the porcelain tile and have not had a single cracked tile or grout joint.  I realize a hearth requirement is different but this does make a point for comparison.

If you have access to the subfloor from underneath, you can double up the subfloor and/or install blocking on the underside to make it more rigid.


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## LLigetfa (Nov 30, 2008)

webby3650 said:
			
		

> I didn't want a big elevation change since mine is in the middle of room.


In my former home, I wanted the hearth to be flush with the floor because it was in a traffic area and a raised hearth would have been a trip hazard.  I actually cut out the original subfloor and a little off the tops of the joists and used blocking to support a sunken subfloor.  My stove required four inches of masonry for the hearth but through the magic of the layered diamond mesh, I got it down to an inch and a  half.  The tile never cracked from foot traffic and even dropping a few big splits didn't hurt it.


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## wellbuilt home (Nov 30, 2008)

I don't think most stoves need r value on the floor . My dog would dry out pretty bad under the stove and i would turn her over before i went to bed .I just lay a tar paper sheet under the wire lath and coat it with the cheep home depot thin set . Then i set the tile in a 2 ND layer of thin set . Ive done thousands  of sf of tile like this and never had a problem . Porcelain tile needs to be set in the modified thin set . I like to screw down the existing floor. If the height doesn't matter you could add a layer of ply wood


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## humpin iron (Nov 30, 2008)

Wellbuilt is incorrect, read your owners manual to find out what the R-value under your stove must be.  There are also areas in this web site that help you understand hearth building and R-values.
  If you do it wrong it"s like eating double cheese burgers everyday.....................eventually you meet a cardiologist.


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## LLigetfa (Nov 30, 2008)

wellbuilt home said:
			
		

> I don't think most stoves need r value on the floor...


My RSF is a zero clearance fireplace with glass door.  The four inch masonry requirement was for the hearth because of the radiant heat.  Most stoves are elevated enough not to require it.

BTW, the best way to apply blocking is to cut 2x4's at a very slight bevel so that they are a little longer than the distance between the floor joists.   You can then lather on some PL and hammer them into place and you only need to use one toe-nail per side.  Altenate their direction so you have kind of a herringbone pattern.


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## ansehnlich1 (Nov 30, 2008)

I got 4 layers of durock on 3/4 inch advantech subflooring with a couple LVL's underneath and crossbraced nice. The durock is layed in loose without screws to avoid heat transfer. You can see the brick edge, some mortar around the edges and seams, otherwise, no screws, no nothin'. Brick layed on top ain't budged.


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## fossil (Nov 30, 2008)

Depends entirely on the stove and how it's built and tested and certified.  Some stoves require nothing more than non-combustible ember protection for a combustible floor, while other stoves require considerable insulative (r-value) properties in the floor protection.  Not something that can be generalized, it must be verified for the particular appliance and then the hearth built to comply with the manufacturer's requirement.  Rick


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## webby3650 (Nov 30, 2008)

That's a great looking set-up there, I love my 500, bet you do too! That Advantech flooring is great stuff! A local lumber yard did a test the first year it was available, they put a 2'x4' piece in a 55 gallon drum of water, after 3 months it had no signs of swelling. That is tough stuff!


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## LLigetfa (Nov 30, 2008)

ansehnlich1 said:
			
		

> You can see...


What is it with workmen and baggy jeans?  Reminds me of babies with full diapers.  At least we were spared any serious cleavage.

I would think the weight would ensure some heat transfer.  The metal screws would add little more.


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## Elfin (Nov 30, 2008)

Wow... I've never seen a hearth-building thread go SPLAT in so many directions before! 

Webby, Durock and other concrete backerboards have no structural integrity and do not add to the structural strength of subflooring. They should also not be directly attached to any structural component such as joists. Don't take my word on it... google it, or ask the experts over at the http://www.johnbridge.com tile forum.

Durock is, however, one way to build up your R-value, as nicely demonstrated by ansehnlich1. Great photos of the process, and a nice-looking finished product too!


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## webby3650 (Nov 30, 2008)

Elfin said:
			
		

> Wow... I've never seen a hearth-building thread go SPLAT in so many directions before!
> 
> Webby, Durock and other concrete backerboards have no structural integrity and do not add to the structural strength of subflooring. They should also not be directly attached to any structural component such as joists. Don't take my word on it... google it, or ask the experts over at the http://www.johnbridge.com tile forum.
> 
> Durock is, however, one way to build up your R-value, as nicely demonstrated by ansehnlich1. Great photos of the process, and a nice-looking finished product too!


I am not talking about building a house out of Durock, I am simply saying, if you have only subfloor and it gives a little bit, after you install Durock over it with thinset and the amount of screws as recomended it will stiffen the area.


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## LLigetfa (Nov 30, 2008)

Elfin said:
			
		

> Durock and other concrete backerboards have no structural integrity and do not add to the structural strength of subflooring.


That's like saying the web of an I-beam doesn't either.  Tell it to an engineer.  Anything between a bottom cord and a top cord is a factor.


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## fossil (Nov 30, 2008)

If your wood subfloor is nice and tight and the underlying structure is adequate, then there's nothing in the world wrong with simply troweling a bed of thinset (using an appropriately serrated trowel) on the subfloor and screwing the Durock directly down onto that.  It's standard building practice for floor tile jobs of many descriptions.  If the single sheet of Durock and whatever you plan to finish it with on top provides all the floor protection required for your appliance (in terms of r-value), then there's nothing else you need do.  Rick


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## CWYfire (Nov 30, 2008)

*WOW!  I AM BLOWN AWAY!  Never expected to get so much help so quickly!  THANX EVERYONE!*

Fossil, I have posted a similar question and appreciate you looking at the specs to assure there is no elaborate process necessary for a hearth under my stove.  I am trying to be cautious rather than get over zealous and burn down my house.... LOL

I have built homes for years.  But this is my first wood stove install and it is on my own home where I keep all my goodies.... LOL  Rather be safe than sorry....

I do have a good solid 3/4" subfloor with plenty of support under it and doubt it will move.  It sounds like probably all I need to do is trowel on some thinset and screw down the Durock about every 8".   

*Is there a specific brand or type of thinset that I need to use and could someone elaborate more on the correct serrated trowel and manner of using it, etc.?*

Nice Pics of the ACTION shots ansehnlich1!  That hearth install is on a concrete slab rather than a wood floor correct?

*Thanks Everyone!*
Ed


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## Elfin (Nov 30, 2008)

Hey, it's your house... do it however you want. Many opt for the get-er-done-quick way, but that's certainly not the type of advice I'd feel comfortable giving.

No tile pro who is willing to guarantee their work is going to install durock directly on subfloor, especially when that install is followed by putting a heavy stove on top of it... repairs will be needed in short order. And technically, you also should be checking your joists for potential deflection of the substrate. Too much joist movement means some sorts of tile or stone will simply not be recommended unless the joists are reinforced. 

But again, when it's in your own house, and you are assuming responsibility for all repairs and installation failures, you can do whatever you wish.


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## fossil (Nov 30, 2008)

CWYfire said:
			
		

> ...Fossil, I have posted a similar question and appreciate you looking at the specs to assure there is no elaborate process necessary for a hearth under my stove...



That wasn't me in the previous thread who checked that out for you, I believe it was InTheRockies who pasted a page from the manual which said that a grouted ceramic floor tile surface, properly installed, is a suitable alternative to the 3/8" millboard.  Credit where credit's due.  On the floor, Durock and tile are sufficient, so long as it extends out the required distance all around the stove.  The wall clearances sound like the things you really need to be careful with in your installation.  Rick


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## ansehnlich1 (Nov 30, 2008)

CWYfire said:
			
		

> *
> 
> Nice Pics of the ACTION shots ansehnlich1!  That hearth install is on a concrete slab rather than a wood floor correct?
> 
> *


*

NO, that ain't concrete, that brick hearth is built directly on a 3/4 inch subfloor of stuff that's called Advantech. No screws, just laid in Durock as you see it. Same mortar used for brick was used for filling in around edges, and doing seams. 

No brick has moved, but I gotta tell ya, we have engineered floor trusses and planned ahead of time for the structural support of the hearth, including some heavy duty LVL's underneath. No movement in the floor whatsoever.

I wanted to build the hearth so that if I should ever get rid of the Oslo and buy something else, chances are I'll have the "r" value needed for several options of stoves and won't have to tear up the whole hearth to start over.*


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## LLigetfa (Nov 30, 2008)

Personally, I don't have faith in 3/4" plywood.  Are the floor joists cross-braced and strapped as well?  If it's a small area and you have access to the subfloor below, add additional bracing against the underside of the plywood.  I don't mean the cross bracing.  You should have that too.

You can buy modified thinset or you can add latex.  I'd pre-treat the wood with latex anyway so then I'd just add some latex to regular thinset rather than buy modified.  You can use a 1/4" square notched trowel.  I would back-butter the tiles.


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## fossil (Nov 30, 2008)

Elfin said:
			
		

> ...No tile pro who is willing to guarantee their work is going to install durock directly on subfloor...



Perhaps we're not speaking the same language here.  When I use the term "subfloor", I'm talking about tongue & groove plywood or OSB or whatever, typically 3/4" thick, specifically engineered to be applied directly to the floor joists/underlying structure.  It's the continuous surface that's underneath your carpet, or your hardwood, or your vinyl, or your whatever.  It's your _floor_, basically.  If you want tile on it, then the typical way that's applied is to affix a sheet of Durock or wonderboard, or other suitable tile substrate, either 1/4" or 1/2 " thick directly to that subfloor and then set the tile.  It almost sounds as if you think I'm saying it's OK to use Durock _as _subfloor...not!  Rick


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## Elfin (Nov 30, 2008)

fossil said:
			
		

> Elfin said:
> 
> 
> 
> ...



Nope, Rick, that's not it at all and I know I've been quite clear. I'm saying that there needs to be *another* layer of structural support between the subfloor (the definition of which is also quite clear) and the concrete board used as substrate. Subfloor alone is simply not enough support for a tile substrate such as Durock, which adds absolutely no structural support, and will allow for too much deflection, leading to cracking tile and grout failure.


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## webby3650 (Dec 1, 2008)

Well, Me and millions of other housholds have durock screwed to plywood with tile on top and have no failures, guess we all have a ticking time bomb! :ahhh:


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## Elfin (Dec 1, 2008)

Yes, I'm certain that you do. I know I do, each passing year bringing more failure to work done improperly by a previous homeowner. 

Crumbling grout, hairline cracks... very common in homes. All because of improper installs. But, on the plus side, frequent tile install failures do keep reputable tilesetters in high demand, with good job security.


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## webby3650 (Dec 1, 2008)

Elfin said:
			
		

> Yes, I'm certain that you do. I know I do, each passing year bringing more failure to work done improperly by a previous homeowner.
> 
> Crumbling grout, hairline cracks... very common in homes. All because of improper installs. But, on the plus side, frequent tile install failures do keep reputable tilesetters in high demand, with good job security.


Please try not to offend others with the I am always right no matter what attitude No one does that to you. Save room for people with practical suggestions.  Thank you


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## Mikeyvon (Dec 1, 2008)

Elfin is wrong. At least in modern construction, 3/4" T&G;ply subfloor, plus thinset, plus tile is all you will ever need.


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## Elfin (Dec 1, 2008)

You both can discredit me all you like; doesn't bother me a bit. All that matters is that some folks here will read what I wrote, follow the link I posted, and get solid advice on this topic before making a mistake that will haunt them for years to come.


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## LLigetfa (Dec 1, 2008)

Mikeyvon said:
			
		

> Elfin is wrong. At least in modern construction, 3/4" T&G;ply subfloor, plus thinset, plus tile is all you will ever need.


You give plywood and modern construction way too much credit.  I pity anyone that follows this advice.  I see a lot of modern construction that is done to minimum code.  My former home was that way and I could feel the floor shake when the cat ran across it!  That was why I chose strudifloor OSB and cross bracing when I built my current home.


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## fossil (Dec 1, 2008)

OK, I went to the website for which you provided the link, and this is what I found:

"_First of all, ceramic tiles (to include porcelain tiles, quarry tiles, various paver tiles and others) are hard, brittle and breakable. For them to be installed successfully on floors they must be well bonded and well supported underneath. If your floor is the least bit springy or “mushy” when you walk on it, it will be necessary to do some serious shoring up before tile setting begins. I cannot over-emphasize this. For our purposes, the tiles themselves have no structural value of their own. The use of cement backer boards or various membranes will not ameliorate an unsound floor structure.

Cement Backer Boards

Cement backer boards, such as Wonder Board http://www.custombuildingproducts.com , Durock  http://www.usg.com , Hardi-backer  http://www.jameshardie.com and others are used to “uncouple” a tile installation from the subfloor below. Before they are fastened, CBUs, as they are called, are bedded in thin set mortar, which is the usual adhesive used in setting floor tiles. The panels are then nailed or screwed to the subfloor following manufacturers’ specific directions. CBUs do NOT improve the stiffness or structural value of the floor."_

How is that different from anything any of us have been trying to say?  It's _exactly_ what I was trying to say.  Heck, if I had a house where the floor was "mushy", I'd do something about it even if tiling it was the_ last _thing from my mind.  But I'm certainly not going to argue with anyone about it...nor am I going to go back and rip out any of the myriad tile & marble  installations I've done on floors, countertops, walls, showers, etc., in 5 different houses over the last four decades.  Rick


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## Elfin (Dec 1, 2008)

Now Rick, we're talking about floors, which have nothing to do with countertops, walls or showers!!  Those are an entirely different topic, are installed with very different parameters in mind, and have no bearing on what's being discussed here. 

It's a small and old soundbite you have pulled from the site... partially true, needing updating. Keep looking. You'll find that uncoupling is achieved using Ditra, and that no matter how hard they try, CBUs *still* do not improve the stiffness or structual value of the floor


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## fossil (Dec 1, 2008)

Elfin said:
			
		

> ...CBUs *still* do not improve the stiffness or structual value of the floor



I agree totally, and I don't think I said anything to the contrary.  CBU's are _not_ structural materials, they add no stiffness nor rigidity, they just provide an appropriate substrate for proper adhesion of the tile.  If the underlying structure has a problem, then that most certainly must be addressed before the tile project begins.  But...not every light-duty residential application of tile on a floor requires some sort of extraordinary underpinning like floating a mortar bed.  (And I did say "floors", too...actually some really nice ones, if I may say so myself    ).  Rick


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## CWYfire (Dec 1, 2008)

Great discussion!

Actually thirty years ago when I built this home I poured a full footing under where I planned to build a rock fireplace.  Finally this Fall I decided to stick a brand new Zero Clearance in that I have had sitting in a storage building for almost that long....LOL  

Once starting the project I decided instead to go with a Wood Stove for the radiant heat and let someone else have the Zero Clearance rig.

Thus the support under my project is quite superior to what might normally be.  There is a concrete block stem wall around the outside wall of the bay alcove.  Then about three feet inward is the original outside stem wall that supports the joists like a fulcrum in the middle.  About 18” further inward the floor joists are headed off into the joists under the original house.  I had originally planned to build up with concrete blocks to support a stone hearth within this same area.  

On both ends of the alcove I doubled the joists that run from the girder to the mud sill which carry the weight of the three joists headed off in the middle. 

I nailed 2”x4” cross braces between the joists directly under where the footprint of the stove pedestal will distribute that weight.  Perpendicular to the joists there is dimensional lumber at a minimum of 16” on center so there is support running both directions like a checkerboard under the entire area that will be tiled.

I did cut out the old ½” subfloor and 5/8” flooring to replace it with ¾” tongue and groove nailed directly to the joists so the hearth would be below carpet level once tiled so I am not stumping my toes.... LOL   

Just wanted to clarify the substructure and assure that the discussion continues on focus with solution. 

*LLigetfa *you said < You can buy modified thinset or you can add latex.  I’d pre-treat the wood with latex anyway so then I’d just add some latex to regular thinset rather than buy modified. >

*Can you explain modified thinset and what you are calling latex?  Are you talking latex paint or some other material that needs to be mixed in with the thinset?  Are you saying to paint the subfloor before troweling on the thinset? * 

Thanks,
Ed


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## Elfin (Dec 1, 2008)

Deflection doesn't mean there is a problem with the underlying structure. There is going to be deflection of some variety, no matter how sound the structure is. The issue is the rigid fragility of tile and grout, and making the substrate appropriately sound for a successful installation. And one simple sheet of plywood can make all the difference. 

A side note: A lot of people are using natural stone these days, which is particularly fragile and has even higher requirements for a secure and sound substrate.


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## LLigetfa (Dec 1, 2008)

I'm talking about a latex additive made specifically for concrete.  It is a bonding agent used wherever new concrete meets old or where concrete needs to bond to dissimilar material.  It looks like white carpenter's glue and is often thinned down with water before applying.  A quick google search for *concrete latex additive* will give you many examples of use.


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## Elfin (Dec 1, 2008)

LLigetfa said:
			
		

> I'm talking about a latex additive made specifically for concrete.  It is a bonding agent used wherever new concrete meets old or where concrete needs to bond to dissimilar material.  It looks like white carpenter's glue and is often thinned down with water before applying.  A quick google search for *concrete latex additive* will give you many examples of use.



Just wanted to add another search term... "latex admix" is what it's commonly called as well. 

If you use a "modified" thinset, this means it is already "latex-modified" and no admix is needed... however, some thinsets are more "modified" than others (usually the cheaper they are, the less admix). 

We could get into the topic of whether expensive modified thinset is really necessary to use under your concrete board (it's not) but I think I've had about enough "debate" for one evening


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## CWYfire (Dec 1, 2008)

So your saying to put a coat of latex on the 3/4" plywood and then trowel on the thinset with a 1/4” square notched trowel, *CORRECT?* 
*
Do I trowel the latex on or use a brush?* 

*Do I need to allow either to set up any before applying the next and specifically before screwing the Durock to the plywood?* 

Will the screws suck the Durock down where it needs to be and I take it the heads be countersunk into the Durock *CORRECT?*


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## fossil (Dec 1, 2008)

You don't put down two different coats of anything.  You buy what's called latex-modified thinset mortar, either pre-mixed ready to spread, or as a powder you mix with water strictly in accordance with the instructions on the package.  Or, if you're really hard-core, you can buy the thinset and the latex modifier separate (looks like milk), and mix it yourself.  Whatever way you choose to acquire that, it's what you trowel down onto the area where you're going to lay the Durock, and then you set the sheet down and use Durock screws to fasten it to the subfloor.  The proprietary Durock screws will minimize damage to the surface of the Durock, and drive smoothly home, and countersink themselves.  They only have to be ever-so-slightly below the surface, so don't go into "over-drive" with them.  Snug 'em down pretty tight, but don't drive them through the Durock.  They're really just clamps, squishing out the thinset to a uniform layer, and holding everything together until the thinset cures.  Rick


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## CWYfire (Dec 1, 2008)

*Great job of explaining Rick!*  I appreciate all of the great advice on this thread!  Will be striking out into the next chapter of the adventure tomorrow. ;-)
*Thanks Again Everyone!*
Ed


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## LLigetfa (Dec 1, 2008)

Thin out the latex with water and whitewash the wood.  Add a bit of latex to the mortar mix.  By the time the mortar is ready, the latex application on the plywood would be set about right.  Apply mortar with a 1/4" square notched trowel.  Set the durock smooth side down immediately into the mortar and screw it down with bugle head screws.  The screws should be flush, not really countersunk.  Apply mortar to the butt joints in the durock and lay in fibreglass mesh tape.  If the mesh tape is self adhesive, you could apply it before the mortar.

If the tile you are laying is not too heavily textured on the back side or irregular, the same 1/4" square notched trowel should suffice.  To ensure a better bond, I like to back butter the tiles.  Depending on how thick you need the mortar to be you can butter it with the smooth or the notched side.  Apply mortar to the durock with the same 1/4" notched trowel.  Always set the tile tight to the next one and then slide it away from the adjacent tiles so that you don't push mortar up into the grout joint.  To prevent excessive drying, you can cover the tile with damp cardboard but not so wet that you dilute the mortar.  Wait a day or two before applying grout and use the same damp cardboard to prevent excessive drying.


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## LLigetfa (Dec 1, 2008)

The watered down latex pretreatment of the plywood also serves to moisten it so as to not draw too much moisture out of the mortar, drying it before it has time to set.  Mortar needs to set, not dry per se.


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## webby3650 (Dec 1, 2008)

LLigetfa said:
			
		

> The watered down latex pretreatment of the plywood also serves to moisten it so as to not draw too much moisture out of the mortar, drying it before it has time to set.  Mortar needs to set, not dry per se.


I like that idea, Is that a common practice?


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## LLigetfa (Dec 1, 2008)

webby3650 said:
			
		

> Is that a common practice?


I think it is but you would probably have to put it to a vote.

It's like common sense.  Perhaps not so common after all.


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## Gooserider (Dec 1, 2008)

First off, we need to be a bit nicer to each other in this thread...  I am seeing complaints about personal attacks, which is not cool...  I don't want to use my "Moderator Hammer", so let's be a bit nicer to each other...

Second, there is an article in the HearthWiki on building a hearth extension - I wrote a good bit of it myself as the result of research on what I needed to do to extend my own hearth...  

A few factors to consider - Hearths or extensions are generally SMALL - thus deflection is much less of an issue.  There is a deflection calculator on the Johnbridge tile site mentioned earlier in this thread, If I punch in my floor's specs for the entire room, there is NO WAY I could put down tiles without completely tearing the existing floor out, sistering all the joists, upgrading the subfloor etc.  But if I figured the 18" wide extension I was building, the calculator wouldn't even accept a number that small, the lowest value it would take (5' IIRC) my deflection was WAY under the requirements, no problem...

I also got a great deal of information from manufacturer spec sheets, and from the JB forums - BOTH the JB forum and the Durock tech sheet on making a hearth extension said that as long as the subfloor was flat, and the deflection was low enough, there would be NO PROBLEM with putting Durock directly on the plank subfloor... (Sorry Elfin!  You may be technically correct, but a plywood sheet isn't an absolute requirement)  What is needed is to put the Durock down on a layer of thinset, and screw it down - as the JB forum puts it, "the screws hold it down, the thinset holds it up..."  (There are specs for the thinset in the Durock application sheet, referenced in the Wiki article)

In the case of my extension, I cut out the carpet, and lifted the 1/2" particle board subfloor it was attached to, then put down a layer of thinset, a layer of roof flashing, another layer of thinset, and a layer of Durock, screwd down per reccomendations.  Over that I put a layer of random size slate tiles to match the entryway that the extension merges with on one end - I needed to stay flush with the existing carpeted floor, so I didn't have the room to put down more - 2 years, no problems, looks great...

There actually is some strength to Durock BTW - if you search, you will see many instances of people building raised hearths using metal 2x4 framing covered w/ 3 or 4 layers of Durock, bonded with thinset and screwed to the framing, and tiled, w/ no other support - holds up a stove and the people installing it just fine...  (Can't use plywood there due to R-value restrictions)

LLigetfa - can you provide a technical reference that describes this application for the diamond lath, and how to use it to get such a gap?  Not saying it isn't true, and it does sound like it would be a useful technique, but before I'd want to reccomend it to someone, I'd want to have a link to some reference that I could hand to a code official or insurance inspector to PROVE that it is a legitimate process...  (If the inspectors won't buy off on it, it doesn't work...)

Gooserider


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## humpin iron (Dec 1, 2008)

I think people here are not fully understanding there are two types of failure here.  There is the tile failure, cracks , etc.  Then there is the R-value failure.  Tile and thinset will not pass the exposure to heat over time.  To put it simply: take what you want to make your hearth out of and see if it passes the "O/S" test.  Hold it in your hand, heat the other side and see if you can figger out what "O/S" stands for............

To achive an R-value of 2.0 from common marble the hearth has to be in excess of 50 inches thick


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## begreen (Dec 1, 2008)

Few modern stoves radiate intense heat down at the hearth. There are some mfg. that have higher R value requirements, but often the tile just gets warm even after days of continuous running. 

FWIW, I can't remember a complaint of a cracked tile or hearth failure here on the forums. For all the angst, it seems the majority are just fine and some overkilled.


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## LLigetfa (Dec 1, 2008)

Gooserider said:
			
		

> LLigetfa - can you provide a technical reference that describes this application for the diamond lath, and how to use it to get such a gap?


The hearth pad thermal isolation technique I described was designed and certified by RSF engineers.  In my case I purchased the assembled product from RSF.  One could design/build their own but I doubt an inspector would sign off on it without an engineer's stamp.  One could however, deploy the technique to build above minimum spec for their own peace of mind.


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## Gooserider (Dec 1, 2008)

LLigetfa said:
			
		

> Gooserider said:
> 
> 
> 
> ...



Yes and no, based on my own experience, and remembering back to when Elk was posting a lot as an inspector...  There isn't a particular issue with home-built setups as long as you can show the inspector a manufacturer's technical data sheet or some other reference that says "This is a valid application" - at least to the effect that the material has a given R-value, and is appropriate for that application - (mineral wool has a high R-value, but doesn't work on a floor because it's compressible for instance)

If something you use doesn't have a technical backup, they might or might not buy it, and almost certainly won't give credit for the protection it might theoretically offer.  Given that metal lath is a non-combustible, I doubt that they'd reject something using it, but I don't think they'd credit you with any R-value for it - and in most installs the challenge tends to be getting to the minimum required R-value (which is a very conservative number in the first place) without getting to thick - there is probably little safety benefit in making a hearth pad higher in R-value, though I do see some theoretical advantage in making it bigger in area, especially in front of loading doors.

Gooserider


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