# Setting a Limestone Hearth Slab



## climb.on (Dec 16, 2011)

Long time lurker, first time poster...

The HO talked me into finishing their new fireplace with thin veneer and setting a limestone slab hearth and mantle (not normally my thing, but, I've done a couple lick and stick jobs now). Everything was framed out with metal studs and covered with cement board. The limestone slabs are substancial at 20" x 65 x 2" & 14" x 65" x 3" that both sit on solid bases covered with cement board. At first I figured I would just use an S-Type Motor (same as for the thin veneer) to set them but the slabs being so large I don't think I'll be able to level it easily on a solid bed of motor. I also don't think a notch trowel work with well S-Type Motor. Is a modified thinset right for the slabs? Something else?

I'm not too worried about the hearth going anywhere, but the 300+lb mantel does give me a bit more cause for concern (it has a 5" overhang from it's 9" deep base). I want to be sure it has a real good stick.


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## JD3lta (Nov 22, 2009)

The hearth is on cement board? Anyhow, it sounds like a nice inside job. I just suggest an acrylic additive for the mortar. It will help your consistency some, you can take some of it though and paste the back side of the stone using it like Elmer's glue to get a real good bite.


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## dom-mas (Nov 26, 2011)

First off if you want to set the hearth in type S that should be fine. Use a stiff mix nd level the bed that you put it on. Use a margin trowel or parging trowel and get it the proper thickness and level. That way when you install there's very little adjusting to do. You could also set it on "buttons" or spacers then fill in with the type S. remove the buttons and give a few taps to set the slab and your done. 

As for the mantle, no matter what anyone tells you, you don't want to rely on maortar to stick anything together. mortar is a gasket, that's all. PL premium sticks things together great.


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## JD3lta (Nov 22, 2009)

Also, just a friendly reminder that sponging the piece with water slightly is optimum for a standard decent bite.


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## JBM (Mar 31, 2011)

I have drilled out the back side of bluestone and put epoxy with rods into the framing behind it.


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## JD3lta (Nov 22, 2009)

If it's dead-nuts level where you're setting the stone and you want to avoid mud. Landscape Block glue binds very well.


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## climb.on (Dec 16, 2011)

dom-mas said:


> First off if you want to set the hearth in type S that should be fine. Use a stiff mix nd level the bed that you put it on. Use a margin trowel or parging trowel and get it the proper thickness and level. That way when you install there's very little adjusting to do. You could also set it on "buttons" or spacers then fill in with the type S. remove the buttons and give a few taps to set the slab and your done.
> 
> As for the mantle, no matter what anyone tells you, you don't want to rely on maortar to stick anything together. mortar is a gasket, that's all. PL premium sticks things together great.


Lot's of great advice - thanks. Good to know my first thought was the right one. Thanks for the confirmation.



JD3lta said:


> If it's dead-nuts level where you're setting the stone and you want to avoid mud. Landscape Block glue binds very well.


The bases are both perfectly level. I did consider PL but I worry over time the effect that the heat will have on it over time. I'm not familiar with landscape block glue, but I would have the same concern. This is a wood stove insert and sounds like they plan to do a majority of their heating with it. If it was a simple gas fireplace, I'd be using adhesive for sure.



JBM said:


> I have drilled out the back side of bluestone and put epoxy with rods into the framing behind it.


Great idea. I'll use some pins in conjunction with the type s. Then I'll have no worries about the damn thing ever coming off.


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## stonecutter (May 13, 2010)

Be sure to clean the limestone before you fix it to the fireplace or nothing will adhere to it very well.


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## JBM (Mar 31, 2011)

What is this 300 pound mantel going to rest on?


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## JD3lta (Nov 22, 2009)

Having a stone 'pop' is also of utmost concern for me. I understand it is involved in a fireplace application. I have set 8 foot treads and believe mortar is not the issue. Look at buildings with veneer larger than the sizes originally specified. I am concerned that a landscape block glue might 'pop' if only fashioned to concrete board. I would advise having a mason perform this job unless screwing it up isn't a concern.


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## dom-mas (Nov 26, 2011)

JBM said:


> What is this 300 pound mantel going to rest on?


I think he said steel studs. Hopefully they're structural.

Landscapers here use PL for setting their blocks all the time so I think that block glue and Pl are about the same thing.

Is this a sawcut slab or is it naturally bedded (the side that goes down is maybe a bit smooth but has some irregularities) Mortar needs a porous material (which liestone is not) or some decent irregularity or both for a good bond. Either way, unless there is some kind of key, say a sort of dovetail, cut into the slab I wouldn't trust mortar to keep it there. If there is 1/3 of the mantle overhanging you only need a little over 100lbs (say some teenager wants to jump up and sit on it) to overcome the balance. 

When I install mantles I want them well cantileverd, attached mechanically somehow or to have the wall above sitting on them 

Good luck

Edit. didn't notice in your earlier post that you're using pins. Good idea. that should hold great


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## dakzaag (Jan 6, 2009)

The hearth will set just fine on mortar, wet the stone a bit before you set it and you should have time to adjust before it sets up. Gravity will hold it in place just fine. 

The mantle is a little more tricky. I have drilled holes before, but I have had the stone break also cause the holes weaken the stone. You could consider a notch or key way in the veneer to slide the stone into. This would give the stone additional counter weight. 

I am partial to some sort of cantilever but sometimes that isn't what is wanted. 

Since you have no idea what might end up on top of the mantle, a little overkill is probably a good idea. If a TV is going above the mantle then you know not much else will be placed up there.


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## climb.on (Dec 16, 2011)

dom-mas said:


> I think he said steel studs. Hopefully they're structural.
> 
> Landscapers here use PL for setting their blocks all the time so I think that block glue and Pl are about the same thing.
> 
> ...


Yep, steel studs and they are structural. You could park a truck up there. Yes, saw cut slab. It's pretty smooth on the back. Not sure there is much I can do. I suppose I could make a few saw cuts?

Here's a few marginal pictures I've been grabbing with my phone.

The thin veneer will sit on top of the back of the mantel so that will help lock it in too. Between the veneer, mud, and pins, I should be good.


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## dom-mas (Nov 26, 2011)

I think you'll be just fine, but for some cheap easy insurance you might want to put some steel shims between the upper framing and the mantel. that way if there is any down pressure at the front of the mantel, the back won't have anywhere to go.


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## climb.on (Dec 16, 2011)

dom-mas said:


> I think you'll be just fine, but for some cheap easy insurance you might want to put some steel shims between the upper framing and the mantel. that way if there is any down pressure at the front of the mantel, the back won't have anywhere to go.


Great idea. I didn't exactly shim it, but I cut some angle brackets to keep the back of the mantle from ever tipping forward. For the pins I just used 1/4" SS bolts up through the framing of the mantle base. I drilled 3/8" holes in the bottom of the mantle. It was easy and worked great (drill bit is in the hole upside down). Here are some pictures of the pins and brackets I used. Thanks to everyone's input here, I feel very comfortable with method and the result.


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