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Installation of Cultured Stone Hearth over Hardi backer board

Post in 'DIY and General non-hearth advice' started by Give it a shot!, Dec 8, 2013.

  1. Give it a shot!

    Give it a shot! New Member

    Joined:
    Dec 8, 2013
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    Michigan
    I am building an elevated hearth for a gas fireplace. I built a wood frame covered in plywood then 1/2" Hardi backer board. I plan to use this as a base for the 18x18 cultured hearth stone (rough back).

    Last night I mixed up some Quickcrete Mason Mix (Type S) but it did not seem to want to adhere to the Hardi backer board. I laid by 5 hearthstones (Boral Cultured Hearth stone) using a 3" wide strips of mortar with 1" gaps about 1/2" to 3/4" thick. After a few hours it was clearly not sticking so I took it all off. It looked like I did not add enough water to the mortar mix. So, after cleaning everything up I am ready to give it another try.

    I checked some forums including this one and found tips to make the mortar a little wetter (clearly mine was too dry), use a bonding agent and wet the backs of the cultured hearth ston. I don't see any mentions about installing over backer board only installing over a lath/mortar bed.

    Do I need any type of special surface prep like bonding agent, metal lath or can I use Mason Mix type S mortar right on top of the backer board?

    Thanks for your ideas

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  2. ironpony

    ironpony Minister of Fire

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    Although Quickrete mix is a spec mix, per masons I know I add some type s mortar and some stucco lime to make it sticky. About a cup each per bag. Using it straight out of the bag is very difficult. Installing stone over cement board is not recommended however I have done it many times inside and outside with great success. Once you stick the stone, do not mess with it, leave it until it is dry the next day. it will come apart if you mess with it but once it is dry it is there to stay. Patience an practice.
  3. Give it a shot!

    Give it a shot! New Member

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    Should I make a mortar bed with metal lath? of do you think a scratch coat with mortar would work? I am also thinking about a scratch coat using the Type S mortar mix and Quikrete Bonding Adhesive
  4. ironpony

    ironpony Minister of Fire

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    I do not think you need to. You might want to try a tile store and get there advice being a horizontal surface. I did use a tile mortar for one project, it has been there 7 years now. My outdoor kitchen is all over cement board, no issues.
    biggest thing I found was mortar mix. Also do not want stone or board to dry, it will suck the moisture out of the mortar causing it to dry to fast.
  5. Hogwildz

    Hogwildz Minister of Fire

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    Not sure what stone your using, but the Owens Corning Cultured stone I used called for mesh when installing over cement board.
    You should download the manufacturers installation manual as it will give you a lot of info.
    You can do either a scratch coat over the mesh along with buttering the back of the stone, or you can put a wet coat on the lath and also butter the back of the stone.
    I did some scratch, which for me did not work as well as a wet coat.
    I also used the masons mix and added nothing, it worked great as is.
    Yes wet the backs of the stones prior to buttering, and mixing it a tad wetter does help, but be very careful not to go too wet with it. When I first started I thought it was going to be too dry and lumpy prior to getting it mixed real well, then added too much water. Practice does make perfect.
    If you are doing the scratch coat and then letting it dry, you will have to wet the scratch coat prior to settimg your buttered stones on. The additives may help in this situation.
    Ironpony is also right about not messing with the stones one you set them, while your setting it is is fine, but then leave it alone.

    Oops, for some reason I was thinking vertical wall, never mind LOL. You should be fine without the mesh for a flat, horizontal surface.
  6. Frozen Canuck

    Frozen Canuck Minister of Fire

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    That's where you need to begin. With so many makers of cultured stone there are many methods (some proprietary) for installation. Devil is in the details as they say & those are in the manual for the product you purchased. They should also spec any additives they recommend if they say mortar base is OK.
  7. Give it a shot!

    Give it a shot! New Member

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    Thanks everyone for the advice. The Mfg has very detailed instructions for the verticle stone but only a a couple paragraphs about the hearth stone. They do suggest type S or N mortar and a metal lath bed. I plan to call them tomorrow for additional info.
  8. bsruther

    bsruther Minister of Fire

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    Northern Kentucky
    I'm in the process of rebuilding my fireplace using cultured stone. For me, type S mortar does not bond to backer board very well, the moisture gets sucked out of it, before it has a chance to bond.
    I found a polymer modified mortar at HD that so far seems to work pretty well, it's called Custom Marble, Granite and Travertine.
    Found the spec sheet for it online and it specifically said it could be used with stone veneer.

    Haven't set any stone with it yet, but yesterday, I did use it to fill some gaps and attach a piece of backer board to the bricks on the front of the hearth.
    I was really happy with it's texture and consistency and how easy it was to apply. It easily adheres to vertical surfaces, the brick and backer board didn't suck the moisture out of it either. It seems to dry a lot slower than any thinset or regular mortars I've used.

    Checked my work this morning and it looks good. the backer board on the hearth is rock solid. The gaps I filled on the outer edge of the wall backer board were 1/2" wide in some places and there is no shrinkage whatsoever, that I can see anyway. This mortar dries white, so I'm going to have to be extra careful not to get any on the stone faces.

    I don't know how much difference it will make, but I also heavily scored all of my backer board, including the hearth with the carbide tipped backer board tool.
  9. Give it a shot!

    Give it a shot! New Member

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    I called the company where I purchased the cultured stone and they confirmed that this would be an appropriate mortar to use. I also checked the spec sheet for the hearthstone and it lists this as well. They only specify polymer modified but don't clarify if a thin-set or medium-set is preferred. Based on the size and reviews I plan to pick-up a bag of this product on my way home and try it tomorrow. Thanks!
  10. Wooden Head

    Wooden Head Member

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    I've set cultured stone inside and outside using thin-set mortar. The same used for setting tile. I used cementboard on one project. The trick with cementboard is to wipe the surface of the board area ( your going to set in the next 15 min.) with a wet sponge or spray with a water spray bottle. I've used metal mesh directly over plywood and thinset to it. Don't use either method in a wet area.

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