Old slate blackboard for a stove hearth?

[g-hoppa]

Tomorrow looks like another snow day, but I need a volunteer to come up to the blackboard anyway:
The local salvage place has a couple full size sheets of slate for very cheap. They are old blackboards, 48" x 96", polished on one side (the writing side) and rough on the other. We are thinking of cutting one down to use as a hearth (rough side up). The slate is beautiful-- dark gray/black, and seems pretty tough. It has to be pretty old, I think, which I imagine means it's from the US. (I think I read that American/New England slate tends to be stronger than imported stuff).
Only problem I can think of is the thickness-- they are only about 3/8" or maybe 1/2" thick. My plan is to build up the hearth with 1/2" plywood, a HY-C micore pad, a sheet of 1/2" Durock, then use thinset to bed the full sheet of slate. That's more than enough R, and my main concern is keeping the slate from cracking under the weight of the stove (Jotul 3) and me standing on the hearth. The floor is supported below by a heavy beam, and shouldn't move or flex much at all. Does this seem like a good plan? Should I throw another layer of Durock in the pile to stiffen it even more?

 

[woodsmaster]

It seems like the plywood and durarock would make it strong enough. I have slate on the walls behind my stove and my insurance said it had to go. They said it can build up and hold to much heat. It had been there for over 20 years so I changed insurance and kept the slate.

 

[pastera]

My old hearth was black slate

It's beautiful when cleaned with lemon oil but high maintenance.

New hearth is travertine tile and is much easier to keep

Aaron

 

[firefighterjake]

I would think you would be fine . . . I have 1/4 inch slate tiles from Monson, ME . . . attached in a similar fashion . . . two sheets of plywood (for extra durability) and 1/2 inch sheet of Durock (didn't need the Micore since no R value was needed for my woodstove) . . . used modified thinset with latex to attach the tile . . . no cracking, no movement.

 

[VCBurner]

My father inlaw has a few slate chalk board he took from Holy Cross during a renovation. They were just being thrown away. The are a beauty and are perfect for a hearth. I have a couple here now. I wonder about the green paint that is on the smooth or chalk board side of it. It could be lead paint, I suppose. I think I'll steal a couple more and put them down in my basement hearth where my new stove is going to go. A couple of 4'x4' pieces for the floor and a few more for the walls around it. It'll be kiddy cornered. It will go well with the tyle floor that will be going in that part of the basement.

 

[Thistle]

In my shop against one wall I have 4 pieces of 3/8" Pennsylvania Slate 4 ft square salvaged when I was working in a suburb's 1940's junior high school remodel in 1999.Was in charge of demo'ing a couple rooms,one had the blackboards with oak trim,another included the outside brick wall with 5" X 7" x 10' sloped indiana limestone sill.Worked very carefully when prying them from the wall,1 piece still had the manufacturing/shipping label from a NE Penna Slate company on the back.Set them on a drywall/door cart,wheeled it out to my truck before someone else either grabbed it or tossed them in the dumpster.Limestone I've used in various landscaping projects,the slate makes good inset table tops (with 3/4" plywood underneath for support).Also I made a "baker's peel" from a smaller scrap piece last summer.Works great for sliding pizza crusts & loaves of bread in & out of oven.Penn slate is considered the best for the old roof shingles & other outdoor uses,it has more strength & resilience,resistant to weathering than other domestic or imported varieties.

 

[CountryBoy19]

What R-value do you need? If you can skip the micore and just do durock you may be better off. Are you talking about putting plywood on top of your subfloor? If so you could skip the plywood & micore and just do 3 layers (or more) of durock to get a good R-value.

 

[g-hoppa]

Well, I'm not done yet, but so far the slate is proving very easy to work with. A Quasar brand 'Turbo Rim' blade in my regular circular saw cut through the slate very easily. Dusty, though! I'm using a long 8" strip for wall protection (there's a tricky boundary between exposed brick chimney and drywall that's just inside minimum clearances). The main slab for the floor is 4'x4' and looks great cleaned up. A run-of-the-mill masonry bit zapped right through the 3/8" slate so I can attach the thinner strip to the wall using spacers.
The hearth is laid right on top of the oak finish flooring. 2 layers of 5/8" ply (as much for height as for strength), the micore (I had already bought a HY-C stove pad, and left the thin steel 'skin' on), then a 1/2" layer of Durock, and soon I will throw thinset and the slate slab on top. I'm a little concerned that the fortified thinset (fortified with latex, I believe) may off-gas when heated. Anyone know?

By tomorrow night I should have the whole hearth in place and the stove hooked up . . .

FirefighterJake- I just saw in your post that you used that fortified thinset . . . any issues with the heat? Sounds like if you didn't need any R, it might not be an issue for your setup anyway.

Pics when it's done.

 

[firefighterjake]

Well, I'm not done yet, but so far the slate is proving very easy to work with. A Quasar brand 'Turbo Rim' blade in my regular circular saw cut through the slate very easily. Dusty, though! I'm using a long 8" strip for wall protection (there's a tricky boundary between exposed brick chimney and drywall that's just inside minimum clearances). The main slab for the floor is 4'x4' and looks great cleaned up. A run-of-the-mill masonry bit zapped right through the 3/8" slate so I can attach the thinner strip to the wall using spacers.
The hearth is laid right on top of the oak finish flooring. 2 layers of 5/8" ply (as much for height as for strength), the micore (I had already bought a HY-C stove pad, and left the thin steel 'skin' on), then a 1/2" layer of Durock, and soon I will throw thinset and the slate slab on top. I'm a little concerned that the fortified thinset (fortified with latex, I believe) may off-gas when heated. Anyone know?

By tomorrow night I should have the whole hearth in place and the stove hooked up . . .

FirefighterJake- I just saw in your post that you used that fortified thinset . . . any issues with the heat? Sounds like if you didn't need any R, it might not be an issue for your setup anyway.

Pics when it's done.

No issues whatsoever . . . the tile looks just as good as it does the day I laid them down three years ago . .. well perhaps there may be a bit of chipping . . . but nothing major . . . just adds a bit of character. No cracking or movement and no issues with heat . . . I should point out though that the Oslo only has an ember protection requirement for the hearth . . . that said, it can get quite warm on occasion . . . especially in front of the stove.

 

[ironpony]

just a word of caution
some old"slate" blackboards are actually tempered glass
with a coating that resembles paint, like was stated above
they will not cut, but will shatter into millions of small pieces
I found out the hard way
positve it was slate, told the wife we can cut it to fit
got out the diamond blade, touched the edge
KABOOM glass all over, still finding it in the shop

 

[Sisu]

Another word of caution is that some old blackboards were made of asbestos cement board. Awesome fire-rating but not so good for you if you are cutting it up. Just verify that it is 100 percent slate.