Bedding large slab of bluestone to concrete for woodstove platform

Jakee

New Member
Oct 27, 2020
90
New Jersey
I finally got my 30" x 30" x 3" thick piece of blusone off my truck and into the shop. I'm going to bed it and I'll only have one shot at it (it's heavy)
I'm reading about dry pack mortar, but I think that's mainly meant for outside patios. I think thinset maybe too thin according to what I've been reading.
I'm just concerned that the weight of this will squish out much of what I put down if I mix mortar too thin. I'm going to add a slurry to the stone's base before dropping it on.
I have one shot and don't want it to turn into a fiasco.
Any advice?
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
23,037
central pa
I finally got my 30" x 30" x 3" thick piece of blusone off my truck and into the shop. I'm going to bed it and I'll only have one shot at it (it's heavy)
I'm reading about dry pack mortar, but I think that's mainly meant for outside patios. I think thinset maybe too thin according to what I've been reading.
I'm just concerned that the weight of this will squish out much of what I put down if I mix mortar too thin. I'm going to add a slurry to the stone's base before dropping it on.
I have one shot and don't want it to turn into a fiasco.
Any advice?
It is thick enough as long as it sits level you wouldn't even need anything under it
 

Highbeam

Minister of Fire
Dec 28, 2006
18,336
Mt. Rainier Foothills, WA
The thinset isn’t meant to hold the stone up above the slab. It’s meant to fill the areas under the three points of the stone that don’t make contact with the slab. The extra will spooge out the sides.

I set my last shower base in a bed of thinset mortar for the same reason. Support.
 
  • Like
Reactions: bholler

Jakee

New Member
Oct 27, 2020
90
New Jersey
The thinset isn’t meant to hold the stone up above the slab. It’s meant to fill the areas under the three points of the stone that don’t make contact with the slab. The extra will spooge out the sides.

I set my last shower base in a bed of thinset mortar for the same reason. Support.
I've been thinking about it and your right. Mortar is not necessary, I have small variations on the concrete surface. 32nd or so in a particular area. I'll just thinset the whole base and let it do it's thing. The stone is massive but I just don't want to take the chance of a crack from the stove weight (325lbs. with bricks)

Once it's down I just have to figure out how to get the stove up on it, in it's exact spot without messing the stone surface up.

Thanks
 

jetsam

Minister of Fire
Dec 12, 2015
4,957
Long Island, NY
youtu.be
Thinset, mortar, sand mix, whatever. It's not gluing the slab down, it's filling all the air holes so the stone is evenly supported underneath

For fine tuning the stove location with one person, you could probably put the stove on 2 or 4 pieces of plywood , slide it into place, then tilt 2 legs up, remove plywood, tilt other 2 legs up, remove plywood.

If it was me, I'd template the legs and the flue pipe with a piece of cardboard, then use the template to mark the stone, then level the stove by drilling some shallow leg holes (put 4 evenly sized pegs in the holes, lay plywood on the pegs, see how level it is, adjust hole depth if needed). That way if the stone ends up a little uneven, the stove doesn't, and the stove can't be slid out of position.
 

Jakee

New Member
Oct 27, 2020
90
New Jersey
Thinset, mortar, sand mix, whatever. It's not gluing the slab down, it's filling all the air holes so the stone is evenly supported underneath

For fine tuning the stove location with one person, you could probably put the stove on 2 or 4 pieces of plywood , slide it into place, then tilt 2 legs up, remove plywood, tilt other 2 legs up, remove plywood.

If it was me, I'd template the legs and the flue pipe with a piece of cardboard, then use the template to mark the stone, then level the stove by drilling some shallow leg holes (put 4 evenly sized pegs in the holes, lay plywood on the pegs, see how level it is, adjust hole depth if needed). That way if the stone ends up a little uneven, the stove doesn't, and the stove can't be slid out of position.
Thanks.
It's a Napoleon pedestal base so the slide portion happens with a larger area than 4 feet. I've already marked exactly where the stone and stove will go with a plumb line hanging also marking the center of the pipe opening.
I'll figure something out as far as getting it on there.
 

rwhite

Minister of Fire
Nov 8, 2011
1,756
North Central Idaho
Super sliders. Lay some 2x4s or wood in front of the stone so it's up to level. Finagle it on the boards and get sliders underneath the slide it across. My 70 yo neighbor lady was able to move an entire king bedroom set with sliders and one of those air bag foot pump deals.
 

xman23

Minister of Fire
Oct 7, 2008
2,217
Lackawaxen PA
Hi Jakee
I to have a project that I want to do with a large blue stone slab. Where did you get yours? I'm in North Jersey and PA. PA has loads of blue stone but I haven't seen large pieces.
You have one large stone, which I assume your setting on a wood floor. I agree, you need to just fill voids between the floor and the stone.
Not your scenario, but. What I did, as a base on the wood floor, and mitigate flex in the floor. I cut and nailed down stucco screening. Then troweled out thin set, let it dry. Used mortar mix to set a number of stones for the hearth.

Jetsan, excellent description of mapping out the stove position. I did the cardboard temple position the stove. Multiple pieces of plywood to slide the stove on the stone, then tipping the stove and pulling them out is a great solution.
 

Jakee

New Member
Oct 27, 2020
90
New Jersey
Super sliders. Lay some 2x4s or wood in front of the stone so it's up to level. Finagle it on the boards and get sliders underneath the slide it across. My 70 yo neighbor lady was able to move an entire king bedroom set with sliders and one of those air bag foot pump deals.
I have a set that I bought as a possibility, I may be doing it that way, thanks.
 

Highbeam

Minister of Fire
Dec 28, 2006
18,336
Mt. Rainier Foothills, WA
For sure slide the stove into place on something. I have had good luck with two strips of plywood even under a pedestal stove. Then remove the strips.

Rather than grind the stone to make the stove level just add shims to keep it from rocking. Almost impossible to get it perfectly level but rocking is lame and will lead to it walking across the hearth with heat cycles. Under the whole rear of my BK pedestal is a flat bar of steel two feet long and1.5” wide by 1/8” thick to bring the back up. Raising the rear is easier to hide.
 
  • Like
Reactions: Mech e

Jakee

New Member
Oct 27, 2020
90
New Jersey
Hi Jakee
I to have a project that I want to do with a large blue stone slab. Where did you get yours? I'm in North Jersey and PA. PA has loads of blue stone but I haven't seen large pieces.
You have one large stone, which I assume your setting on a wood floor. I agree, you need to just fill voids between the floor and the stone.
Not your scenario, but. What I did, as a base on the wood floor, and mitigate flex in the floor. I cut and nailed down stucco screening. Then troweled out thin set, let it dry. Used mortar mix to set a number of stones for the hearth.

Jetsan, excellent description of mapping out the stove position. I did the cardboard temple position the stove. Multiple pieces of plywood to slide the stove on the stone, then tipping the stove and pulling them out is a great solution.
Mine is going on concrete in my shop. I'm in 'Mid' Jersey. There's a landscaping place that I ordered it from. It actually comes from your area. Get in touch with a high end landscape/stone place. They can order it for you. Mine cost $240.
For sure slide the stove into place on something. I have had good luck with two strips of plywood even under a pedestal stove. Then remove the strips.

Rather than grind the stone to make the stove level just add shims to keep it from rocking. Almost impossible to get it perfectly level but rocking is lame and will lead to it walking across the hearth with heat cycles. Under the whole rear of my BK pedestal is a flat bar of steel two feet long and1.5” wide by 1/8” thick to bring the back up. Raising the rear is easier to hide.
My leveling situation is not so extreme. I'm all set to mix the thin set, I have the stone in place. I've raised and lowered it myself 3 times now. It's standing on it's edge, in place, the problem is that if I lower it by myself I'd have to stand right in the middle of the thinset to not hurt myself lowering it. I've thought it out frome every angle and I need someone to help me lower it standing off to the sides so as not to be stepping in the cement. It's frustrating because I'm all set to do it.
 
  • Like
Reactions: Highbeam

rwhite

Minister of Fire
Nov 8, 2011
1,756
North Central Idaho
Mine is going on concrete in my shop. I'm in 'Mid' Jersey. There's a landscaping place that I ordered it from. It actually comes from your area. Get in touch with a high end landscape/stone place. They can order it for you. Mine cost $240.

My leveling situation is not so extreme. I'm all set to mix the thin set, I have the stone in place. I've raised and lowered it myself 3 times now. It's standing on it's edge, in place, the problem is that if I lower it by myself I'd have to stand right in the middle of the thinset to not hurt myself lowering it. I've thought it out frome every angle and I need someone to help me lower it standing off to the sides so as not to be stepping in the cement. It's frustrating because I'm all set to do it.
I'm no Mason so take this suggestion accordingly. Can you butter the stone rather than the floor so you don't have to step in it?
 

Highbeam

Minister of Fire
Dec 28, 2006
18,336
Mt. Rainier Foothills, WA
I'm no Mason so take this suggestion accordingly. Can you butter the stone rather than the floor so you don't have to step in it?
Just don’t lose your grip on the buttery slab and drop it on your foot. Get that bucket of clean water and a sponge ready. You don’t want thinset to set up on the outside.

the only trick with backbuttering this slab is that it’s vertical so your thinset may run. You don’t want really dry mixed stiff thinset since the goal isn’t to level the stone but to fill the voids. Too dry and the slab may float like a puck on an air hockey table.

Have a level handy and be ready to jump up and down on the high side.

Setting tile (or slabs like this) can be very stressful for perfectionists.
 

rwhite

Minister of Fire
Nov 8, 2011
1,756
North Central Idaho
My other thought would be to leave a space free of thinset on the floor where you need to put your feet and lower down on a 4x4 block. Then reach underneath and put thinset down?
 

Jakee

New Member
Oct 27, 2020
90
New Jersey
I'm no Mason so take this suggestion accordingly. Can you butter the stone rather than the floor so you don't have to step in it?
THAT is a stroke of genius.
I have a kid from the hardware store coming on Sunday to help me. I'm reconsidering whether I should return the thinset and go to mortar. The only reason being The floor is not perfectly level there as it slopes a little. I'm thinking how important is it that it be perfectly
Just don’t lose your grip on the buttery slab and drop it on your foot. Get that bucket of clean water and a sponge ready. You don’t want thinset to set up on the outside.

the only trick with backbuttering this slab is that it’s vertical so your thinset may run. You don’t want really dry mixed stiff thinset since the goal isn’t to level the stone but to fill the voids. Too dry and the slab may float like a puck on an air hockey table.

Have a level handy and be ready to jump up and down on the high side.

Setting tile (or slabs like this) can be very stressful for perfectionists.
Just don’t lose your grip on the buttery slab and drop it on your foot. Get that bucket of clean water and a sponge ready. You don’t want thinset to set up on the outside.

the only trick with backbuttering this slab is that it’s vertical so your thinset may run. You don’t want really dry mixed stiff thinset since the goal isn’t to level the stone but to fill the voids. Too dry and the slab may float like a puck on an air hockey table.

Have a level handy and be ready to jump up and down on the high side.

Setting tile (or slabs like this) can be very stressful for perfectionists.
I am stressed about it as the floor does slope slightly. As I replied below, I have help coming Sunday, that will take some of the edge off. I'm hoping to build up a bit more on the low end to make up for it.
I'll go check again for the 80th time how much slope there is.
 

Jakee

New Member
Oct 27, 2020
90
New Jersey
Just don’t lose your grip on the buttery slab and drop it on your foot. Get that bucket of clean water and a sponge ready. You don’t want thinset to set up on the outside.

the only trick with backbuttering this slab is that it’s vertical so your thinset may run. You don’t want really dry mixed stiff thinset since the goal isn’t to level the stone but to fill the voids. Too dry and the slab may float like a puck on an air hockey table.

Have a level handy and be ready to jump up and down on the high side.

Setting tile (or slabs like this) can be very stressful for perfectionists.
Since you guys have been such a help, here is the amount that it’s off.
The slab is going diagonally in a corner. It’s 30”x30” and measured diagonally is 42” and is off being level by 5/16” (diagonally)
So it’s either I go with it using thin set, or build a frame and pour self leveling in there. I don’t know if it’s with all that for 5/16th over 42”.

I would value your opinions on this set of circumstances as I’m driving myself crazy
 

Jakee

New Member
Oct 27, 2020
90
New Jersey
Just don’t lose your grip on the buttery slab and drop it on your foot. Get that bucket of clean water and a sponge ready. You don’t want thinset to set up on the outside.

the only trick with backbuttering this slab is that it’s vertical so your thinset may run. You don’t want really dry mixed stiff thinset since the goal isn’t to level the stone but to fill the voids. Too dry and the slab may float like a puck on an air hockey table.

Have a level handy and be ready to jump up and down on the high side.

Setting tile (or slabs like this) can be very stressful for perfectionists.
Since you guys have been such a help, here is the amount that it’s off.
The slab is going diagonally in a corner. It’s 30”x30” and measured diagonally is 42” and is off being level by 5/16” (diagonally)
So it’s either I go with it using thin set, or build a frame and pour self leveling in there. I don’t know if it’s worth all that for 5/16th over 42”.

I would value your opinions on this set of circumstances as I’m driving myself crazy
 
Last edited:

xman23

Minister of Fire
Oct 7, 2008
2,217
Lackawaxen PA
So close here, I'm feeling your pain. Someone chime in with the how to do this, and why.

Gut feeling is with a large slab and a concrete, there not machined surfaces. They will be voids and potential rocking, it needs to st in mud. what mud is the question. A bit off level stove can't be any issue, my opinion.
 

rwhite

Minister of Fire
Nov 8, 2011
1,756
North Central Idaho
Since you guys have been such a help, here is the amount that it’s off.
The slab is going diagonally in a corner. It’s 30”x30” and measured diagonally is 42” and is off being level by 5/16” (diagonally)
So it’s either I go with it using thin set, or build a frame and pour self leveling in there. I don’t know if it’s with all that for 5/16th over 42”.

I would value your opinions on this set of circumstances as I’m driving myself crazy
A pic would be really helpful. Are you saying its sloped 5/16" or is one corner off by 5/16"? 5/16 is quite a bit and anything other than thinset will be hard to hide. The problem is that it probably won't rock. The weight of the stove will hold the slab and if you get a couple of heavy dudes drinking beer in front of the stove its liable to snap. It's a pain but I think thin set is the way to go.
 

Jakee

New Member
Oct 27, 2020
90
New Jersey
A pic would be really helpful. Are you saying its sloped 5/16" or is one corner off by 5/16"? 5/16 is quite a bit and anything other than thinset will be hard to hide. The problem is that it probably won't rock. The weight of the stove will hold the slab and if you get a couple of heavy dudes drinking beer in front of the stove its liable to snap. It's a pain but I think thin set is the way to go.
The slab itself sits solidly on the concrete surface with only 32nds or so variation underneath where you wouldn't even know it. I only know because I took a straight edge to the area in all directions. The grade of the floor slopes down 5/16th on that diagonal measurement.
I'm certain thinset would grip to all the surfaces once the stone is in place. The big question is if it's not perfectly level would it be a no-no asctetically? Meaning really noticible. Or if I should just go ahead and put a mortar base to it and level the stone that way.

Just using thinset at this point would be the easy out for me.

As far as being stepped on, it only sticks out 8" from the front of the pedestal.

I appreciate all the responses and solutions.
 
Last edited:

brenndatomu

Minister of Fire
Aug 21, 2013
5,803
NE Ohio
How thick is this slab?
If its thick enough, I'd think it could be shimmed up to level/stable and be good enough?
 

Jakee

New Member
Oct 27, 2020
90
New Jersey
How thick is this slab?
If its thick enough, I'd think it could be shimmed up to level/stable and be good enough?
It's 3 inches. If I shimmed it up to be level there would be a visible space which I definitely don't want. Plus the stove is about 350 lbs. w/bricks so I wouldn't trust the structural integrity
 

rwhite

Minister of Fire
Nov 8, 2011
1,756
North Central Idaho
Slide the stove into place on just the concrete and see how far it is off level. If it was 1/8" or less and I could lay the slab down with out it rocking then you could probably get away with no thinset. If it rocks any at all I'd thinset it.
 

Jakee

New Member
Oct 27, 2020
90
New Jersey
Slide the stove into place on just the concrete and see how far it is off level. If it was 1/8" or less and I could lay the slab down with out it rocking then you could probably get away with no thinset. If it rocks any at all I'd thinset it.
Good point. I had the stove on the concrete for the purpose of lining up exactly to my ceiling support box. I didn't notice any crookedness to the stove. I moved the stove out of there to preparing for the slab and only noticed it when measuring and taking a straight edge to the floor.
 

xman23

Minister of Fire
Oct 7, 2008
2,217
Lackawaxen PA
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.