# durock multiple layers



## rob bennett (Dec 7, 2009)

Howdy-
    I just realized that I am going to need probably 5 layers of durock (old style) for my hearth.  I can't find Mircore locally.  If you have done this, did you screw layers progressively to each other with mortar in between?   Or should I just screw them together and call it good? Or just mortar them together?  I need a K value of .84 I believe.  Its for a keystone I'm putting in.

Thanks


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## jefferson1 (Dec 7, 2009)

i needed 1.5 for mine so i used 6 sheets of it. we just screwed it together- no need to cement them.


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## jefferson1 (Dec 7, 2009)

oh and each sheet is .26 so you'd need a bit more than 3 - call it 4 and you would be well over .84 at 1.04.


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## Highbeam (Dec 7, 2009)

Last I saw durock was 0.20 per 1/2" thickness. The woodstock keystone needs 0.84 for hearth R-value? I thought they only needed ember protection.

I also could not get micore in WA so I used "dead air space" per the hearthstone manual.


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## begreen (Dec 7, 2009)

4 layers should do it if the goal is to achieve that high rating. Is this required? Cement board's R value  .52/1" or .26/ 1/2" according to USG and Craig's chart. But for some reason thechimneysweep has it as .20. I would go by the mfg rating.

Here is the spec:

"Space-saving Durock® brand cement board is listed by Underwriters Laboratories, Inc., for use with UL-listed
solid-fuel room heaters and fireplace stoves. Used as a wall shield, Durock cement board reduces by two-thirds
the manufacturer-specified clearance (minimum 12") between the room heater or stove and a combustible wall
surface. Durock cement board may also be used as a floor protector in place of one layer of 3/8" thick millboard.
(Having a thermal conductivity of k ≤ 0.84 Btu in./(ft. ² h °F) in the minimum dimensions specified by the room
heater/stove manufacturer.)"
http://www.usg.com/USG_Marketing_Co...d_Submittal_Sheets/CB198_Durock_Cement_Bd.pdf

and specs here on page 2:
http://www.usg.com/USG_Marketing_Co...ts/DrckCement_Board-Submittal_Sheet_CB399.pdf
1/2" Durock R= .26, K value = 1.92


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## begreen (Dec 7, 2009)

Highbeam you are exactly correct. For Woodstock stoves only 1/2" sheet of cement board is required per their installation instructions.


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## Highbeam (Dec 7, 2009)

The instructions appear to require that sheet of steel as well. Ugh, why can't they just give us an R-value requirement and let us be creative?


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## rob bennett (Dec 8, 2009)

The back plate says the floor protection "must me be a minimum of 3/8 of an inch thick with an K value of .84"

So we are not talking about R value.  k= 1/R according to an article on this site.   So I am looking at 2.3 inches of durock to equal that.  

If that is the case, that diagram in the instructions and above is really misleading.  1/2 inch cement board only works if that cement is micore 300.

I'll call the friendly folks at woodstock tomorrow.  I'd be pretty frustrated to finish my pad only to find out it didn't meet inspection.


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## begreen (Dec 8, 2009)

Please let us know what they say. I am confused.  We have USG saying:

"Durock cement board may also be used as a floor protector in place of one layer of 3/8” thick millboard.
(Having a thermal conductivity of k ≤ 0.84 Btu in./(ft. ² h °F) in the minimum dimensions specified by the room
heater/stove manufacturer.)”

In the Keystone manual we have Woodstock saying:

"1) a 1/2 inch layer of an approved non-combustible insulation board (such as
DUROCK cement board or WonderBoard Backer Board), then apply
2) 1/4” or more of a decorative, non-combustible material such as tile, slate,
stone or brick."

Yet the Hearth.com chart is calling for 2.3" of cement board to achieve k.84.

I could be completely befuddled, it sure wouldn't be the first time, but if k is the reciprocal, then doubling the thickness would half the k value, no? If 1/2" = k 1.92 then 1" = k .96.  Somethin's goosey lucy.


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## rob bennett (Dec 8, 2009)

Well if I have my math figured out, and the calculation right

1/2 inch durock has an r of .26

so 5 layers of durock     .26+.26+.26+.26+.26 = 1.3 total R

1/1.3 = .769  (lower k = lower heat conductivity)

Either way I am confused


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## scotsman (Dec 8, 2009)

Okay, so if I build what the page in BeGreen's message says, I'm good to go--no chance of issues? If this is the case, why do some hearth pads measure 1.25" in thickness? I saw some this past Saturday at a stove dealer in Clovis for $495.00. They were very nice, but for $495? Hody Cow! If the plywood, durock, tile will do just as well, why $495?? BTW, is this durock the same as concrete Hardi-Board or not. If not, can it be gotten at Home Depot or Lowes? Thanks--


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## begreen (Dec 8, 2009)

This is not the same as HardiBoard. Ask your HD where they keep the pure cement board. Durock or Wonderboard, and not the lightweight next generation stuff. 

A good hearth pad can cost $500 retail. You can do the same for about $80 material, your time and labor, depending on the tile chosen.


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## Wet1 (Dec 8, 2009)

I bonded my layers of Durock together with thinset and then screwed them as well.  It came out very rigid.


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## ChipTam (Dec 8, 2009)

I agree with BeGreen.  This is all very confusing business.  I went through this about a year ago when installing my Jotul 3TD.  I had to convert k-value to R-value and then use the Hearth wiki chart to determine the R-value of 1/2" durock and 1/4" tile.  I used two layers of durock (bonded with thinset and screws) followed by a layer of tile.  That put me slightly under the required R-value for my stove.  However, in practice, I can place and keep my hand on the tile under the stove with a roaring fire going and the tile is not really even warm to the touch.  I'm all in favor of following manufacturer's recommendations but sometimes I think the lawyers rather than the engineers come up with the requirements.
ChipTam


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## emurphy@eclumber.com (Dec 8, 2009)

You may have already taken care of this, but hardiebacker has a rating of  .13 per 1/4" or .26 for 1/2" http://www.jameshardie.com/homeowner/products_backerboard_halfInch.shtml

so that would work for you as well.

If it help I also used thinset between the mess of stuff I built it worked out just fine.


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## scotsman (Dec 8, 2009)

murph said:
			
		

> You may have already taken care of this, but hardiebacker has a rating of  .13 per 1/4" or .26 for 1/2" http://www.jameshardie.com/homeowner/products_backerboard_halfInch.shtml
> 
> so that would work for you as well.
> 
> If it help I also used thinset between the mess of stuff I built it worked out just fine.



So, the 1/2" HardiBoard IS okay? I've been mulling this over a lot and was wondering exactly what it is we're trying to protect the floor and sub-floor from. Does anyone have a handle on this and exactly how much of a danger we're facing?

My floor is 1/2" tile 16" square laid over a plywood subfloor. For the pad maybe 1/2" plywood, then a layer of small-cell expanded metal, then 1/2" Hardi-Board, screw that together and then set more of the floor tile on top of that with the thin-set. The thinking is that the expanded metal layer would be a barrier to and/or disperse the heat. In addition, I thought I might use the leftover 1/2" HardiBoard pieces to raise the whole shebang off the floor creating an airspace for more heat dissapation. What is the braintrust's thinking on this arrangement? Is this overkill? If so, would it be worth it? I don't mind overkill if it's not a killing cost.

Then how can we know we've accomplished the objective?


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## scotsman (Dec 8, 2009)

ChipTam said:
			
		

> I agree with BeGreen.  This is all very confusing business.  I went through this about a year ago when installing my Jotul 3TD.  I had to convert k-value to R-value and then use the Hearth wiki chart to determine the R-value of 1/2" durock and 1/4" tile.  I used two layers of durock (bonded with thinset and screws) followed by a layer of tile.  That put me slightly under the required R-value for my stove.  However, in practice, I can place and keep my hand on the tile under the stove with a roaring fire going and the tile is not really even warm to the touch.  I'm all in favor of following manufacturer's recommendations but sometimes I think the lawyers rather than the engineers come up with the requirements.
> ChipTam



So, is that roughly how we can tell if there is a potential problem? If one can place one's hand underneath the stove, as in this case, does that mean that the underlying surfaces are safe? If one created an airspace underneath the protection pad, would that help or is it radiational heating we're dealing with? Just trying to figure this all out! Thanks--


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## Highbeam (Dec 8, 2009)

What does woodstock say? Being so high on customer service I suspect they can send you a memo or TSB to show the inspector. I could swear that they also required a sheet of steel in there which I always thought was overkill.


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## emurphy@eclumber.com (Dec 8, 2009)

Here in NH The only place we could find Micore, was a company that specializes in acoustic celiings. Do you have that type of company in your area? This all gets a lot easier if you can find Micore.  - I just went through this scenario myself this year, with the help from this forum.


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## blel (Dec 8, 2009)

I don't know if these are near you but they are micore distributors I got from Micore/USG website.


BUILDING SPECIALTIES
14980 NE 90TH STREET
P O BOX 350
REDMOND, Washington     
Phone:      425-882-3116
Fax:      425-881-5830

# Micore
BUILDING SPECIALTIES, INC.
EAST 6819 MISSION
SPOKANE, Washington     
Phone:      800-909-2526
Fax:      509-893-0866


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## begreen (Dec 8, 2009)

Highbeam said:
			
		

> What does woodstock say? Being so high on customer service I suspect they can send you a memo or TSB to show the inspector. I could swear that they also required a sheet of steel in there which I always thought was overkill.



I was just perusing Quad docs and think I might have seen that for the Quad Isle Royale?


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## Highbeam (Dec 8, 2009)

It's ugly formatting but this was cut from the fireview manual. They still require the sheet metal. Oddly omitted from the other stoves it seems.



Your Woodstock Soapstone stove must stand on either:
1) a hearth of solid masonry (brick, stone, or tile supported by concrete),
2) a prefabricated hearth pad listed to UL standards. These pads can be placed on
top of a wood or carpeted floor. (Woodstock Soapstone Company carries a
selection of these pads.)
3) a custom designed pad made up of approved non-combustible materials which
will protect the floor from sparks, hot coals and ashes; and prevents heat from
being radiated onto the floor underneath.
If you decide to build your own hearth to go over a combustible floor, start with a
plywood base. Over this apply:
1) a layer of 24 gauge galvanized sheetmetal,
2) a 1/2 inch layer of an approved non-combustible insulation board (such as
DUROCK cement board or WonderBoard Backer Board),
3) decorative non-combustible material such as tile, slate, stone or brick.


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## begreen (Dec 8, 2009)

Weird. The screenshot posted above is straight from the Keystone online manual. No mention of the metal for that stove.


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## emurphy@eclumber.com (Dec 8, 2009)

Highbeam said:
			
		

> It's ugly formatting but this was cut from the fireview manual. They still require the sheet metal. Oddly omitted from the other stoves it seems.
> 
> 
> 
> ...



If you use the metal you can get away with one  1/2" layer of cement board? I'm suprised they don't say anything about a need for air space. This sounds like the way to go.


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## Highbeam (Dec 8, 2009)

I don't think an air gap is ever required.


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## rob bennett (Dec 8, 2009)

I just got off the phone.  
I was told to follow the picture.  One sheet of durock and tile will get it done.

The metal that highbeam is talking about is only if you decide to not use grout between the tiles.

I'm still confused about the .84 k value.  I wonder if I have the wrong numbers or equation.  I'll have to look it up.


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## emurphy@eclumber.com (Dec 8, 2009)

I just took this from the Englander website -While my simple mind can't comprehend all the numbers, you might.

EXAMPLE:
The specified floor protector should be ¾” thick material with a k-factor of 0.84. The proposed
alternate is 4” brick with a C-factor of 1.25 over 1/8” mineral board with a k-factor of 0.29.
Step (a): Use formula above to convert specification to R-value.
              R = 1/k x T = 1/0.84 x .75 = 0.893
Step (b): Calculate R of proposed system.
              4” brick of C = 1.25, therefore R brick = 1/C =1/1.25 = 0.80
             1/8” mineral board of k = 0.29, therefore Rmin.bd. = 1/0.29 x 0.125 = 0.431
             Total R = Rbrick + Rmineral board = 0.8 + 0.431 = 1.231
Step (c): Compare proposed system of R of 1.231 to specified R of 0.893. Since proposed
              system R is greater than required, the system is acceptable.
*Definitions:*
Thermal conductance = C = _____Btu____ = ____W____
(hr)(ft2)(deg F) (m2)(deg K)
Thermal conductivity = k = __(Btu)(inch)__ = ___W____ = ____Btu____
(hr)(ft2)(deg F) (m)(deg K) (hr)(ft)(deg F)
Thermal resistance = R = (ft2)(hr)(deg F) = (m2)(deg K)
Btu W


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## rob bennett (Dec 9, 2009)

Well I think it all boils down to this statement in the durock pdf mentioned a few posts back.


Durock cement board may also be used as a floor protector in place of one layer of 3/8” thick millboard.
(Having a thermal conductivity of k ≤ 0.84 Btu in./(ft. ² h °F) in the minimum dimensions specified by the room
heater/stove manufacturer.)”

The tag on the back of the keystone says floor protection should be a minimum of 3/8 inch and k value of .84

I think my problem was that the 1/r =k is an over simplistic equation.  

I'm going to stick with what Woodstock told me today.  And put the slate on this weekend


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## akennyd (Dec 9, 2009)

Highbeam,

It is my understanding, by the woodstock information, that you only need to use sheet metal as a ember/spark barrier if you have open gaps in your stone work or tiling (i.e. no grouting between them).

I sure hope the drawing shown in an earlier post is still correct because that is what I have spent SEVERAL hours building for my fireview.  1/2" cement board (wonderboard) and then 1/4" porcelain bonded with thinset and with grout between the tiling so no need for a sheet metal plate between or underneath.

Also, my understanding is that hardie board is NOT a viable option for a hearth anymore.  I still have 2 unused sheets at home that I need to return to HD, I purchased them first and then decided not to use them after discussion with the helpful group here on this forum.

Hope some of that helps...

Kenny
John 3:16


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## Highbeam (Dec 9, 2009)

Trouble is that the picture doesn't say IF no grout then use the sheet metal. My inspection went very well but I would hate to have a picky inspector looking for that sheet stock. 

I don't think hardi ever was a viable hearth product since it was always combustible.


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## emurphy@eclumber.com (Dec 10, 2009)

Looks like this thread is all set. But I don't want anyone to get the wrong Idea. The below is cut and paste from the Hardi website. I had to check again because I used Hardi based on this info. ( On top of micore, and unfortunatetly below -oops! ) 


Non-Combustibility
HardieBacker® cement board is recognized for use in non-combustible construction in NER-405. 


Surface Burning Characteristics
When tested in accordance with ASTM test method E-84: 

Flame Spread 0
Fuel Contributed 0
Smoke Developed  5

Thermal Resistance
(Approximate value) 1/4'' thick: R:0.13 

http://www.jameshardie.com/homeowner/products_backerboard_halfInch.shtml


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## akennyd (Dec 10, 2009)

HB,

I know what you are saying about inspectors but I'm not to worried because it is in black and white just below the picture that was previously posted.  My inspector told me they would be using the manufacturers info as their guide.

From Woodstock:

"Brick, flagstone, or stone should be mortared at all joints to prevent sparks and embers from falling through cracks."

"If the finishing materials are simply fitted together “dry”, with no mortar or grout, the NFPA requires that a piece of at least 24 gauge sheet metal be placed under the cement board in order to prevent embers from making their way down to the plywood base."


murph,

I think it's more of an emissions thing than a burn/catch fire thing.

Look here:  https://www.hearth.com/econtent/index.php/forums/viewthread/45188/

Kenny
John 3:16


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## emurphy@eclumber.com (Dec 10, 2009)

Interesting - clearly wonderboard - durock seem to be better choices - or atleast I see nothing to say otherwise. For my part thought, I think I'll take my chances with off gasing, only becasue it's already done,  And I can't see anything in the hardi information that scares me.  But If I were to build another hearth I would use the older "cement board".


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## firefighterjake (Dec 11, 2009)

Our Inspector specifies the 1/2 inch listed Durock . . . the sheets that actually have a sticker on them saying they are approved for hearths. That said, we don't make the home owner show us the actual label or dig up the tile or stone to verify it's Durock and not Hardiboard or another cement board underneath.


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