Building a Hearth Pad

dont know about the steel studs we used 2x4 under ours and I just went through all 3 or 4 sheets of cement board then thr tiles when you put down your tiles dont use thinset like they say use mortar the thinflex flex's with the heat and will pop the tiles loose at least that is what happened to me I had to retile the outer row on my pad this last summer

I used steel studs, then a layer of sheet metal, then screwed 1/2" durock through sheet metal into studs, then put a second layer of 1/2" durock perpendicular to the first and bonded it only with latex modified thinset (Flexbond) - then I added a third layer of durock - perpendicular to the second layer, again, only bonded with thinset. The thinset does a few things for you in this situation: 1. by not using screws between layers, heat can not be transfered to lower layers through the screw creating hot spots. 2. the thickness of the thinset gives you "some" addition R value 3. it's a better bond than a hundred screws would be anyway, keeping all 3 layers as 1 mass and more rigid than if screwed.

offroadaudio said:

I used steel studs, then a layer of sheet metal, then screwed 1/2" durock through sheet metal into studs, then put a second layer of 1/2" durock perpendicular to the first and bonded it only with latex modified thinset (Flexbond) - then I added a third layer of durock - perpendicular to the second layer, again, only bonded with thinset. The thinset does a few things for you in this situation: 1. by not using screws between layers, heat can not be transfered to lower layers through the screw creating hot spots. 2. the thickness of the thinset gives you "some" addition R value 3. it's a better bond than a hundred screws would be anyway, keeping all 3 layers as 1 mass and more rigid than if screwed.

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A friend of mine built his hearth pad with a 2x4 frame, 3/4" Plywood, applied Thinset and then Screwed the Durock to the Plywood. Did he do this wrong? If he did, I want to tell him before he lays the tile this weekend.

Well, there is no real way to know with out knowing the specs for his stove.
If hearth requirements are a simple non-combustible floor covering, then he is ok. If he needs an R value of 1 or more, than this will not meet the requirement.
That 3/4" plywood is the nearest combustible under the stove, he must have the required R value between the stove and the plywood.

offroadaudio said:

Well, there is no real way to know with out knowing the specs for his stove.
If hearth requirements are a simple non-combustible floor covering, then he is ok. If he needs an R value of 1 or more, than this will not meet the requirement.
That 3/4" plywood is the nearest combustible under the stove, he must have the required R value between the stove and the plywood.

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His stove requirments are 3/4" plywood, Thinset, 1/2 Durock, 1/4" Tile. My question was since he screwed the Durock to the Plywood with Thinset in-between, will this effect the R Value? Your earlier post said not to screw the Durock to the Plywood.

He is laying more Thinset and installing the required 1/4" tile on top of the Durock.

The use of latex modified thinset is required for the installation of porcelean tiles. It IMPROVES the bonding strength of the thinset which is required for porcelean because it is so dense. Standard clay based tiles actually absorb some of the setting material into the bisque (body of the tile) to make a strong bond. This does not happen with porcelean because it does not absorb the setting material (due to density of the tile), so latex modified thinset is required. It also does provide some minute flexability to the setting bed (natural movement from hot and cold cycles) so once again, latex modified thinset is preffered.

Stay away from the cellulose based backer board. Durock and wonder board are the products to use. Keep looking for a cement based product. It may be a hassle to locate in your area, but you need to do this the right way.

HearthKB, I would seriously question your freinds stove requirements. I don't think your friend has enough r value ( could be wrong) and yes, those screws can conduct the heat. If it's not too late, the easiest thing to do would be to add another layer of durock on top, but just thinset it in place (using latex modified thinset), then tile it. This will also add to the r value and reduce the heat conduction of the screws.

BeGreen said:

I would visit a tiling website as a double check, but from what I have read Durock needs to be screwed down frequently in order to provide a stable backer surface for tile.

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There was an earlier post from a carpenter that said the same thing (can't find the post). He said to use a lot of screws when applying the Durock to the Plywood. He didn't mention anything about the heat being transferred through the screws.

Thinset is a mortar. Often latex modified to improve flex. From a tile install perspective the cement board should be "glued and screwed" - i.e. thinset and screws to minimize deflection. Can't comment on the heat transfer through screws affecting R value (seems like a Fossil question). I would think any transfer would be dissapate if the screws were not contigous through the layers. Hearthstone recognizes an 1/8 inch of still air as R value, which should achieve by using the steel studs. I agree with BG above - I would place them 12 OC to improve the strength of the cement board.

EDIT: I take back the comment regarding still air. In this design the studs will penetrate the still air and I think your R starts from the top of the studs.

Just to keep the debate going. Durock (or similiar) does need to be screwed and glued. IF your tiling say your kitchen floor, then you lay down a layer of latex modified thinset, then put your durock down, then screw it down using durock screws every 4 to 6 inches along the outside of the sheet, and every 6 to 8 inches on the inside of the panels. You leave gaps betwwen the panels, and "mud" them in using thinset and durock brand fiberglass tape to make all of the individual panels one continuous sheet or substrate. Let that set up and then tile your floor using the thinset recommended for your tile.

For a hearth, we are not walking on it. We put a stove on it at most. Usually we use multiple layers of durock, so the first layer can get glued and screwed. Then the next layers get thinsetted to each other. Thats it. Think about it. The thinset is good enough to hold the tile down, then it is good enough to hold the durock down.

You do not want to use mortar to bond tile. It does not have any bond strength. Don't take my word for it. Go look on some of the tile setting websites. The main purpose of latex modified thinset is not to provide flexability (even though it does do a small degree). It's main purpose is to increase bond strength.

While I don't claim to be a "hearth expert", I have been in the ceramic tile industry for around 25 years now. Follow the manufacturer's recommendations and you will be safe. Go do your homework on one of the tile setting forums to get informed on the installation process. KD

KD,
I for one am completely unqualified to debate ;-) - only passing along what I have read and done; mostly in examples like your kitchen floor - where I have done exactly as you state. My thinking was if it is good enough for your Kitchen floor why not your hearth. What you say makes good sense with respect to the subsequent layers of cement board, and would minimize (laregly) the concerns regarding heat transfer through the screws. The bag of thinset I recently used (TEC3N) is labled "performance mortar" Ive also seen these called polymer cement mortars. It seems the use of this term "mortar" is potentially confusing as it captures a whole range of products (sand, binder (often cement) and water mixed as a paste that hardens). If additives are in there, or other binders are used, then it would seem there would be different properties. For this application I think we are in complete agreement that Thinset is the correct mortar. If I am out to lunch I would like to draw on your apparent knowledge base to correct any misunderstanding.

buono fuoco said:

KD,
I for one am completely unqualified to debate ;-) - only passing along what I have read and done; mostly in examples like your kitchen floor - where I have done exactly as you state. My thinking was if it is good enough for your Kitchen floor why not your hearth. What you say makes good sense with respect to the subsequent layers of cement board, and would minimize (laregly) the concerns regarding heat transfer through the screws. The bag of thinset I recently used (TEC3N) is labled "performance mortar" Ive also seen these called polymer cement mortars. It seems the use of this term "mortar" is potentially confusing as it captures a whole range of products (sand, binder (often cement) and water mixed as a paste that hardens). If additives are in there, or other binders are used, then it would seem there would be different properties. For this application I think we are in complete agreement that Thinset is the correct mortar. If I am out to lunch I would like to draw on your apparent knowledge base to correct any misunderstanding.

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I'm with you on this one. Right on the money. How much of a factor the screws are-I dunno, but still conductive of heat. I feel if the bottom layer of durock is screwed and glued (glue meaning latex/polymer modified thinset) and another layer of durock is thinsetted only on that, then the conduction is greatly minimized (IMO). Especially if you use 3 or 4 layers of durock to get the desired r (k) value. The only layer that has screws in it is the bottom one, all the others are just thinsetted on top of each other.

My point regarding the comparison of the kitchen floor (or a foyer for that matter), vs a hearth is, I feel a kitchen floor or a foyer will receive more abuse (use) and the installation should be more "by the book" as compared to a hearth. While a heavy object may rest on the hearth (like a stove), it is a constant. Not the flexing and regular use like a kitchen or a foyer. So the placement of screws for a hearth could probably be minimized some.

Finally to qualify, the term "latex modified thinset" is kind of incorrect (as you pointed out). Latex was one of the first additives the industry used to "modify" standard thinsets. In the old days, you actually poured liquid latex additive into a standard bag of thinset and it was now "modified" (you used this liquid instead of water). We called it "milk" as it looked just like whole milk. So I guess thats how the term "latex modified" got started.

Nowadays, the additives are in the thinset already in powder form, so you just add water, and the newer additives are more than just latex. The term "polymer modified" would be more apropriate. Good stuff nowadays, even when you compare it to products just 10 years ago. So again, this works to our advantage in increasing bond strength and providing some "flex" in the install to compensate for less than ideal flexion and or expansion conditions in the substrate. I feel the newer products let us get away with less than ideal conditions that 10 years ago may have caused tiles to pop, crack, work loose, or whatever.

It's not a super dooper install tile on 1/2 inch particle board on 24 inch on center joists and life is all good product yet, but it's getting there. Regards, KD

It's tough to clear this up and find the true story about Hardibacker, because the James Hardie website has been down for some time (not sure what that's all about). However, I did find an online Hardibacker brochure that should clear up the questions with correct information. Thermal information is in the column to the far right. http://www.loweslink.com/pubdocuments/Hardibacker Brochure.pdf

Maybe the moral of the story is that it's a whole lot easier to build a pad that only needs to be non-combustible! But when I called PE to discuss the pad requirements for their stoves, they told me that R value requirements are coming, and soon... for all stoves.

well, the link doesn't want to paste in. There should be a percentagesign20 in the gap between "Hardibacker" and "Brochure". If anyone knows how to fix this link, let me know and I'll re edit it!

HearthKB said:

offroadaudio said:

Well, there is no real way to know with out knowing the specs for his stove.
If hearth requirements are a simple non-combustible floor covering, then he is ok. If he needs an R value of 1 or more, than this will not meet the requirement.
That 3/4" plywood is the nearest combustible under the stove, he must have the required R value between the stove and the plywood.

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His stove requirments are 3/4" plywood, Thinset, 1/2 Durock, 1/4" Tile. My question was since he screwed the Durock to the Plywood with Thinset in-between, will this effect the R Value? Your earlier post said not to screw the Durock to the Plywood.

He is laying more Thinset and installing the required 1/4" tile on top of the Durock.

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It certainly creates the possibility of heat transfer through the screw to bottom layers.
I personally would avoid that. 2 sheets of durock thin-set together basically become one - no need for screws with that superior type bond at work.
Do I have data to support this? Yes. A very clinical test. I tried like hell to break apart two pieces that were joined as mentioned - I tried with big hammers, pry bars etc....It can't be done. You can smash it all to hell and back, but the 2 layers will still be together.

Also, check the Wiki here about an NFPA wall shield and notice that it specifies NOT to have screws behind the stove for that reason.

Just wanted to make a note about the Micore300. That is what I really wanted to use. I tried the local Lowes, and they never heard of it. I tried Home Depot and all of the hearth shops within 2 hours of my house with the same result. I went to the USG web site and found the distributor was in northern Virginia. I called them and they told me the closest place I could get it was at Lowes in Roanoke, VA (about 2 hours from here). I called that Lowes and was told I had to get a minimum of 6 sheets and pay about $50. shipping to get them to special order it (I still had to drive to Roanoke to pick it up). I sent a letter to USG and ask if they knew of anywhere I could get just one sheet. After several weeks and a few phone calls, I was told they might get a single sheet in Baltimore MD. I finally just gave up.

BeGreen said:

Seems like a bit of overkill to me. The article says don't place the screws directly behind the stove body, especially the flue pipe, but that is a precaution for exposed screw heads. I personally have never heard of a pyrolysis fire from screw heat transmission and would like to see some evidence of this ever happening. Personally, I think it's a total non-issue, especially with a layer of tile diffusing heat before it hits the screw head, under the stove.

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Fair enough. I get a lot of grief from friends about building things overkill. All I know is that I can build a fire ON my hearth if I want and it will not hurt a thing.
So to the guy who screwed to his 3/4" ply - no, I would not re-do. If you like to go the extra mile for safety & peace of mind, do it that way next time.
BTW HD has some slate tile on sale cheap - I just finished my hearth (minus grout) this weekend - I'll start a new thread with pix.

Thanks guys.