Bedding large slab of bluestone to concrete for woodstove platform

[Jakee]

Just saying how ever it's done, don't leave the middle with nothing under it. Recipe for a crack someday.

I'm in the thinset camp, 1/2 notched trowel where you need it to a skim coat where you don't need it.

I think that's what I'm going to do and disregard the slight level issue.

 

[RandyBoBandy]

Is this bluestone thermal treated or left natural? If it is thermal treated it should be pretty straight when sighting down the edge.( best done when it’s standing on edge). Natural stone usually will have a twist or curve to it when sighting down it. If it is natural and has a twist you might be able to use that to your advantage with the concrete floor. Than just shim it and point it with some mortar. This is a really thick piece of stone so I would not worry about it cracking unless you drop it. It won’t even know the stove is on it. If you are going to mud it in I would get yourself some type S mortar and plan on putting down about a 1” mud bed. This will give you plenty of room to tap it into place with a dead blow hammer. Now remember this stone is heavier than hell at 3” thick so it will squeeze the mud out very easily. So be careful to not tap it down to far to where you can’t level it out. It will be damn near impossible to break the suction on it being that thick and heavy. And don’t rush it. You will have plenty of time to slowly work it down before your mud sets up. Keep a bucket and sponge close by to wash the stone down as you go. You will also want a 1/2” margin trowel, 1-1/2”-2” margin trowel and probably a small pie trowel. These will help you point and clean up your edges. However if you are going as thin as possible these tools might be if no use. I would still butter the stone but that doesn’t mean you need actual mud on it. Just get the stone wet with a sponge and use your large pie trowel to smear mud on the back. The slurry left on the stone will be sufficient.

Last but not least are the edges of the bluestone thermal treated or left with the smooth saw cut look? You can reface your stone with an acetylene torch to give it that natural stove look again. Hope this helps more than confuses. Good luck and keep us updated.

 

[Highbeam]

The way you deal with the level issue is you set the slab down dry and drive composite door shims under it on the low side to level it. Until you’re happy. Then lift the slab without upsetting the shims. Tape the shims in place and trim them away from the edge. Slather the whole thing with wet thinset and drop the slab down. It will sink down onto the shims and spooge out the extra thinset. It won’t drop too far because of the shims. Verify level again and clean the spooged thinset. The thinset wall at the slab edge will look like grout, don’t try to dig it out from underneath.

Absolute worst case is the slab cracks some day. It’s still safe.

 

[xman23]

The way you deal with the level issue is you set the slab down dry and drive composite door shims under it on the low side to level it. Until you’re happy. Then lift the slab without upsetting the shims. Tape the shims in place and trim them away from the edge. Slather the whole thing with wet thinset and drop the slab down. It will sink down onto the shims and spooge out the extra thinset. It won’t drop too far because of the shims. Verify level again and clean the spooged thinset. The thinset wall at the slab edge will look like grout, don’t try to dig it out from underneath.

Absolute worst case is the slab cracks some day. It’s still safe.

HA! Excellent procedure I thought, until got to it cracking someday

So how do we lift and drop this 325 LBS into a corner? or did I miss that.

 

[Highbeam]

HA! Excellent procedure I thought, until got to it cracking someday

So how do we lift and drop this 325 LBS into a corner? or did I miss that.

Not going to be easy without pinching fingers. I set a granite countertop on a bed of silicone in a corner and wanted to drop it straight down gently. It was tough.

Have you considered laying three or four pieces of 1/2” pipe on the ground in the thinset, then position the slab and try to pull the pipes out? I didn’t realize you were in a corner.

 

[rwhite]

Use your felling wedges if you have any. That would be the easiest to remove. If I can ask, Why are you putting bluestone down over concrete in a shop? I'm imagining a spic and span shop with glossy floors. Lol Nothing wrong with it, just curious

 

[Jakee]

I appreciate all the advice from everyone.

Here's how I'm going to deal with it tomorrow. I already set the stone on edge, on the high side of the grading. I will tape off the concrete floor edges so as to make clean up easier. I'll sponge wet the area on the floor as well as the bottom of the stone.
I'll mix the thinset to a peanut butter consistency then apply it to the floor evenly, then add extra on a third of the base on the low side.
I will gently lower the stone with my helper in the expectation that having the stone lowered from the high side of the grading to the low side, the high side will have started squeezing from the moment of compression and the low side where I've added a bit more thinset, will float just a bit to help to compensate for at least a part of the leveling issue, if I can take out half of that 5/16th I'll be happy. Maybe it won't work that way, but that's the plan.
I'd rather be sure it has support in all areas than go a little crazier making it perfectly level.

If it's off level a little, so be it. I didn't notice it when I had the stove sitting there by itself.

And yes the stone is thermal treated except the bottom which is super flat.

Thanks everyone.

Even at this point I'm thinking about the 1" of mortar and tapping it into level as Randy pointed out.

 

[Jakee]

Use your felling wedges if you have any. That would be the easiest to remove. If I can ask, Why are you putting bluestone down over concrete in a shop? I'm imagining a spic and span shop with glossy floors. Lol Nothing wrong with it, just curious

 

[Jakee]

When I put the stove into place for measuring it looked like it needed something under it. Purely for aesthetic reasons.
The floor is unfinished concrete at this stage.

 

[Jakee]

Not going to be easy without pinching fingers. I set a granite countertop on a bed of silicone in a corner and wanted to drop it straight down gently. It was tough.

Have you considered laying three or four pieces of 1/2” pipe on the ground in the thinset, then position the slab and try to pull the pipes out? I didn’t realize you were in a corner.

It's not backed into a corner, it's going to placed diagonally in the corner, fairly close but away from the walls cement block and walls.
I have it on small diameter PVC pipe and plywood now in order to roll it away when I had to move it for the bluestone.

 

[RandyBoBandy]

Keep some shims handy. If you go the route of a thick mortar bed you can leave the shims in for a few hours. Come back in a bit when the bed has set up and remove the shims and point where they were. If you use the premix bag of mortar from the box stores they will have sand in it. So when you sponge up your mess you can dab at the final cleanup and leave a nice exposed aggregate look to your grout line. You will probably need to go back in a day or so and clean it all with muriatic acid to get the haze out. Are your edges thermal treated as well?

 

[Jakee]

Keep some shims handy. If you go the route of a thick mortar bed you can leave the shims in for a few hours. Come back in a bit when the bed has set up and remove the shims and point where they were. If you use the premix bag of mortar from the box stores they will have sand in it. So when you sponge up your mess you can dab at the final cleanup and leave a nice exposed aggregate look to your grout line. You will probably need to go back in a day or so and clean it all with muriatic acid to get the haze out. Are your edges thermal treated as well?

Thanks
I may still do the mortar route. I have a lot more experience with mortar than thinset. And the edges are thermal treated.
Do you think I could accomplish that with thinset as I described in another reply I made today? We're talking a 1/4+" drop over 48" .

 

[RandyBoBandy]

Thanks
I may still do the mortar route. I have a lot more experience with mortar than thinset. And the edges are thermal treated.
Do you think I could accomplish that with thinset as I described in another reply I made today? We're talking a 1/4+" drop over 48" .

I can’t speak for thin set. I’ve never really worked with it enough to be of help. I’ve worked with type S quite a bit over the years so this would be my approach to it. A thick bed of mortar gives you plenty of workability to get it right. At least in my mind and experience.

 

[Jakee]

I can’t speak for thin set. I’ve never really worked with it enough to be of help. I’ve worked with type S quite a bit over the years so this would be my approach to it. A thick bed of mortar gives you plenty of workability to get it right. At least in my mind and experience.

I think your right.

 

[RandyBoBandy]

Do you have a welder? When I go outside later today I will upload a picture of my bluestone hooks I use on patios.

 

[weatherguy]

Love bluestone, my mantle above my stove is bluestone, 6'x2'.
Just curious if you're putting over concrete why you would need any type of adhesive?

 

[RandyBoBandy]

Love bluestone, my mantle above my stove is bluestone, 6'x2'.
Just curious if you're putting over concrete why you would need any type of adhesive?

To take any wobble out and fill the voids. Plus I believe the OP has a 5/16 gap when the stone is level so that would be a big enough gap to where I would want it filled.
I’ve been mentally planning a new hearth for a couple of years now. I have about 7 years worth of bluestone leftovers from doing patios so I have a good amount of veneer that I think I can do my whole back wall out of bluestone.
Post a pic of your mantle please.

 

[RandyBoBandy]

If you could quickly weld something like this with a much longer shank you could use it to lower down the side against the wall. I also use them to help get my fingers out of the way in the last couple inches of lowering a piece down.

 

[Jakee]

Love bluestone, my mantle above my stove is bluestone, 6'x2'.
Just curious if you're putting over concrete why you would need any type of adhesive?

There are minor variations in the cement floor under where it will be (32nd to a 16th) and I don't want to take the chance of the weight of the stove (330lbs approx) cracking it.

 

[Jakee]

Do you have a welder? When I go outside later today I will upload a picture of my bluestone hooks I use on patios.

No welder yet, someday...

 

[Jakee]

If you could quickly weld something like this with a much longer shank you could use it to lower down the side against the wall. I also use them to help get my fingers out of the way in the last couple inches of lowering a piece down.

Thank you for the pictures, that's a really good idea Mine is away from the corner (diagonal to the corner).
I'm all set to do the mortar bedding tomorrow with type S, I've never dealt with such a heavy slab and I'm trying to think of just how stiff to mix it.
I know that a difficult thing to try to communicate. 30x30slab 3"thick. What do you think?

 

[Jakee]

Is this bluestone thermal treated or left natural? If it is thermal treated it should be pretty straight when sighting down the edge.( best done when it’s standing on edge). Natural stone usually will have a twist or curve to it when sighting down it. If it is natural and has a twist you might be able to use that to your advantage with the concrete floor. Than just shim it and point it with some mortar. This is a really thick piece of stone so I would not worry about it cracking unless you drop it. It won’t even know the stove is on it. If you are going to mud it in I would get yourself some type S mortar and plan on putting down about a 1” mud bed. This will give you plenty of room to tap it into place with a dead blow hammer. Now remember this stone is heavier than hell at 3” thick so it will squeeze the mud out very easily. So be careful to not tap it down to far to where you can’t level it out. It will be damn near impossible to break the suction on it being that thick and heavy. And don’t rush it. You will have plenty of time to slowly work it down before your mud sets up. Keep a bucket and sponge close by to wash the stone down as you go. You will also want a 1/2” margin trowel, 1-1/2”-2” margin trowel and probably a small pie trowel. These will help you point and clean up your edges. However if you are going as thin as possible these tools might be if no use. I would still butter the stone but that doesn’t mean you need actual mud on it. Just get the stone wet with a sponge and use your large pie trowel to smear mud on the back. The slurry left on the stone will be sufficient.

Last but not least are the edges of the bluestone thermal treated or left with the smooth saw cut look? You can reface your stone with an acetylene torch to give it that natural stove look again. Hope this helps more than confuses. Good luck and keep us updated.

I'm re-reading your excellent advice for tomorrow. Thank you very much.

 

[RandyBoBandy]

That is a heavy a$$ piece of stone so don’t be shy on laying down a thick bed. You will squeeze a lot out in the initial set down. You will want it thick with the way you are lowering it. It’s gonna want to shoot it out on the side with least resistance. So it’s best to have a thick bed so you can float some mortar back to the wall side of the stone. Be patient. I would mix it to the consistency of soft serve ice cream. Maybe on the thicker side considering the weight of the stone. But if you go to dry it’s going to fight you when trying to tap it down into place. Shims will save you. I can’t stress that enough. If you let it sack out to far it will turn into a nightmare in a matter of seconds. Thick bed, patience, shims, patience. You got this.

 

[brenndatomu]

Shims will save you. I can’t stress that enough. If you let it sack out to far it will turn into a nightmare in a matter of seconds. Thick bed, patience, shims, patience. You got this.

Yup, if it doesn't hit the mud almost perfectly flat, it will want to squirt out on the side that hits last, and be high on that side too...I agree, mix the mud on the thick side, and use shims...at least on the side that needs to end up 5/16" off the existing concrete.

 

[RandyBoBandy]

Here’s another pro tip. Don’t mix the whole bag of mortar. Save some dry in case you get to wet in your initial mix. You can always add dry. You can’t take out water.