Torsion Box Under Stone Slab for hearth

[Lilacsmokestack]

Hi All,

I live in an old house dating to 1801 with a softwood floor in the living room that sits over the top of some much older wide plank floorboards, which then sit on some log joists. Parts of this floor deflect a bit when you walk on them, though the corner where I am planning to put a stove is fairly stiff.

I'm planning to have a hearthstone craftsbury installed. My wife and I think a single slab of stone would look good, and further we think it would be nice if the hearth was very low to the floor. Flush to the floor would be even better if there were a way to do it. Seems most people like to build up the hearth, but this is going in a fairly small room and the hearth protrudes into it a bit more than we'd like, the lower it is, the less intrusive it will feel I think. The stove only requires ember protection so R value is not an issue. This would be a fairly large slab, shaped like a home plate with dimensions around 50" deep by 58" wide (see attached pic).

I'm worried that the stone slab will crack and I've been googling this subject on and off for months (the subject of supporting stone/tile over a wood floor). I have been getting lost in threads about modified mortars, schuter-ditra membranes, full mortar beds, thin-set, re-enforcing wire mesh, wire gauges, water content of the mix versus tendency to shrink and crack, etc. I have no confidence that I'll get the details right with putting a mortar bed on the floor to support the stone slab. I can see that stone cracking in my mind right now when the stove guys come to install it.

Then it occurred to me - granite countertop folks do not deal with mortar beds. They take a huge unwieldy slab of granite and put it right onto some plywood and hold it in place with a few dabs of caulk and its own weight. Right? So I'm thinking I'll build a torsion box, maybe half inch ply for the top and bottom skins, and then a web that's about an inch thick on whatever spacing looks appropriate as I build it. The webbing or honeycomb would probably be half inch ply too. I'm not sure if I would just let the box float on the floor, or try to rigidly fasten it to the floor in the hopes that the floor would stiffen the box and the box would stiffen the floor.

Thoughts on any aspect of my approach would be appreciated.

Thanks,

LilacSmokeStack

 

[cableman]

Whats below this section?
I would have a 2" piece of bluestone cut. If you want it flush as possible, the floor would have to be cut out and some how framed up from below.
I would just drop down a layer of durarock leveling it the best you can with thinset, then drop the bluestone slab on top of that with a layer of thinset. Should be strong enough as long as the below structure can handle the weight.
My corner bs slab is roughly the same size. Stuff is pretty strong. Mine was layed on a leveled raised mud section on top of a slab not wood floor.

 

[Nick Mystic]

I'd be concerned about the weight of the stove and big stone hearth you're planning on installing. The stove is over 300 lbs. and the stone must be pretty heavy, too, depending on how thick it is. You said the floor deflects a bit when you walk on it, so that's a signal that you could have some weak floor joists. A lot could be going on in a home over 200 years old. Can you get under the floor in the area where the stove is going and add some support posts?

As for your torsion box idea I think you might be overthinking things. If the stone sits flat on the floor I think you would be okay just laying it in place and shimming where

 

[Highbeam]

Throw out the torsion box idea.

I have similar floor with regards to beam supports on 5 foot centers and then 1.5" plank subfloor. Get under the floor and add support to the stove area from beneath. Then make it look nice on top. The stove, hearth, wood, and even a couple of big dudes warming their butts will only add up to 1000# or so. No problem for two added 4x6 beams, held up with 4- 4x4 posts, setting on adjustable saddles, on concrete pier blocks.

 

[Lilacsmokestack]

Thanks for the replies guys! I do have a tendency to overthink things, and often think myself into dead ends. I will try to get a photo of what the floor looks like from underneath and post that. I have received other votes (from my Dad) for just laying the stone down on the floor directly (possibly with some thinset to level things out) - the thought being that a 2" thick piece of stone is going to be very strong on its own and should resist cracking without additional enforcement. I guess all the noise on the internet about tile being applied over wood floors is maybe not applicable here, since tile is so much thinner than what I'm planning to use.

Incidentally, we took a trip to the stone yard this weekend and the piece of bluestone we're likely to go with is not blue at all, it's brown and it is 'natural cleft' meaning it was split out from the quarry without any further finishing or polishing. We need a piece that is 50" by 50" minimum in order to cover this corner installation, and that severely limits our choice of stock.

 

[bholler]

I am just curious how you plan on moving a 50" by 50" by 2" piece of stone into the house

 

[cableman]

I am just curious how you plan on moving a 50" by 50" by 2" piece of stone into the house

Its not too bad as long as its not going on a 2nd floor anyway. Mine was the same size, its about 400lbs.
We were able to stand it up onto a dolly then roll it off dolly at back door and into place. Just have some man power available!