small clearance to install granite in floor

[ebeggel]

We are going to drop a piece of absolute black (67" X 17" X 7/8") into a hole in the floor for a fireplace hearth. There is very little room (1/8" clearance) to grab the piece and set it in (about a 13/16" drop at the long edge). ie our fingers will not fit.

Is there a good way to support it (with metals straps for example) to lower it into the hole? Or will this uneven support break it? We cannot lower it with a wood substrate beneath for fire code.

Thanks

 

[Leejosepho]

I believe I have seen some heavy-duty suction cups mounted on lifting bars for that kind of job, but I do not recall where I saw them. Maybe a stone shop could tell you how to set your piece.

 

[Sponsor]

 

[Jadnashua]

Definately rent, borrow, or buy some suction cups with enough capacity to hold that up. If it isn't polished (i.e., honed or flame finished), then you may not be able to get them to stick well. Make sure that you set the slab in mortar. Any small thing and pressure in the right place might crack it otherwise.

 

[Pewterpower]

This sounds interesting. Please post some pics when you are done.

 

[ebeggel]

one more question...

Thanks for the suction cup tip. A local tool rental has them.

We were set to pour quickcrete concrete today with some wire mesh for strength. I've gotten several opinions about the way to support this small slab (hole is 3" deep -- 17" X 68").

I am now thinking that it would be easier to forget the concrete and build a 2X4 frame, nailed to the floor joists (front and side), with a piece of durorock screwed on top and thinset between the durorock and granite.

Has anyone tried something like this? I am hoping to avoid possible cracking in the event of an earthquake.

Thank you for the advice.

 

[Geniescience]

curiouser and curiouser

Your First post said something about Fire Code -- and no wood !!

Tell us please what the floor is made of.
The subfloor and the finished surface.

David

p.s. EPS foam and XPS foam are always good to fill space as lightweight filler. My saying this is not a recommendation for you to do this, in your case; I need a lot more information first. But the best foams are strong enough to hold a slab in place..

 

[Jadnashua]

Depends on what is under that area you want to put the granite slab. But, assuming it is stable, I think I'd recommend a sand mix like that used in making a shower floor - 4-5 parts of sand to one part of portland cement. Mix with enough water to allow it to stay together when you grab a handful. You work it like beach sand as if you were building a castle - you can level the area easily, it's cheap, and will allow you to screed it perfectly level and flat. Once it sets up, use some medium set mortar (often called marble and granite mortar) to set the stone in.

 

[Markts30]

What about a base of SLC???
Then some thinset on top for the bedding?

 

[Jadnashua]

A bag of slc is around $25-35. A bag of sand mix, maybe $6 or so...many slc's have a limit of max thickness. Usually, if you want it as thick as it appears you want, they suggest using some pea gravel, yet more money. It would be quicker, and would work fine, though.

 

[ebeggel]

to curiouser and curiouser

First of all thanks to all who replied.

We hammer drilled out the bricks and mortar and left ourselves with a hole ~ 3"D X 17" X 65". On each side is a joist and a joist in the front.
To the rear are the bricks of the fireplace front.
The hole bottom is still mortar or concrete, and it is somewhat uneven but not bad.

We were thinking of building a frame of redwood with 1/2" duro rock on top.
The 1/2" durorock combined with a layer of thinset and a 7/8" thick piece of granite will give us the 1 1/2" clearance we need below the insert (away from combustibles).
Originally we had thought that we could put a bag or two of concrete tucked in below the frame (which would be screwed/nailed to the joists).

The idea of mixing sand with concrete brings up the possibility of throwing the broken bricks back in the hole with some concrete and welded wire...leaving the wood frame idea behind and being an economical way of looking at it. (Stable enough?)

I still cannot decide if just a wood frame affixed to the joists with a duro rock top would be stable enough in the case of an earthquake. I suppose that in the event of an earthquake the granite might crack no matter what.

We will not, as planned finish tomorrow so all options are on the board.
(cement or frame).

I thank you for your advice gentlemen.

 

[ebeggel]

photos

Photos might help in advising sand/cement vs. framing.

 

[Pewterpower]

It looks like you will have to frame that in, then use some concrete, or like was said earlier, the sand mix, to backfill it. The most important thing is that the frame be perfect. If one corner is the slightest bit higher (or lower) than any of the other corners, you'll crack it for sure, cuz I'm assuming this will be walked on. A perfect frame will allow you to float/screed the concrete perfectly level. Then you can hit it with some thinset and drop 'er in.

 

[Geniescience]

more concerns

hi ebeggel,

the pictures help a lot. A lot!

Your stone is 5' long and very thin and narrow. I might be worried about it cracking while being placed or if you have to pull it out to add more thinset... Do you know about trowel notches giving you a controlled thickness of thinset? Do you know the pillar method of setting stone? It gives you more leeway to play with (height) as you press it down into the right position.

I might thinset the stone onto a piece of Wedi board first, before placing it, since Wedi is reinforced with fiberglass fabric on two sides, and that makes it rigid. Then place the now-composite piece into the hole, and run less risk of cracking the piece now or later. Same principle as reinforced steel-and-concrete slabs, one component (which cracks but doesn't compress) on top, and another component, never cracking, on bottom, holding the upper component together. I have used Wedi, and steel too, to reinforce behind tiles.

You have enough extra space that you could put an electric heat cable under the stone. Very enjoyable. Cost is so low, it is less than a thermostat.

To hold the thing in place: if you use a little wood, as screed (and as frame) and a lot of sand mixed with cement as the real filler, you will have the best of both worlds.

I would not use Durock here, for a number of reasons. Doesn't necessarily reinforce the stone; might even make it more likely to crack. Using steel or Wedi will reinforce it.

When placing it, you could use "airplane" cable looped under it in two or three places to hold it and to pull up if ever necessary, and then pull the cable out when you are done.

DAvid

 

[ebeggel]

re: more concerns

No I have never heard of the pillar method...please explain. Also please explain the heat cable application.

I called around and no one in the immediate area has Wedi or has heard of it.

 

[Geniescience]

see the web sites, instead of calling around

In a search engine, use keywords
- electric floor heat cables
- Wedi tile

There are a few threads here that discuss these as well. Leejosepho and I have been posting on several recently.

About the pillars: I know I mentioned them to randyj a month ago when he posted about mortar under a tub.. you might find that thread easily by looking for randy's threads he started.

Basically, think of the wet cement mix (as if it were in a tube) acting like a pillar, and you squished down on it to get the level you needed. You could call it a dab or a spot too. Five pillars are enough for an average tile, one under each corner and one in the middle. Nine pillars is good too. Five-spot, Nine-spot are also terms used. In your case, many more than five or nine, that is for sure. A whole array grid of pillars, dabs, spots.

more later.

david

p.s. https://terrylove.com/forums/showthread.php?p=60573 about pillars

 

[Randyj]

I have this to say about that.... natural stone's biggest drawback is that it has fissures which can allow cracks quite easily. You want the support of the stone to be spread as much as possible and not too much in one area. My thinking is that a trowel with large notches using thinset is going to give maximum dispersion of support. Like you...wedi board is something that's not prevalent here and the box stores look at me like I'm stupid when I ask about it.

 

[Geniescience]

yes, natural stone has natural flaws

and like Randy says it needs support... And this raises questions as to what is under the hole we see in the photos. Solid concrete for 4" or more? Or something much less? Gypcrete? 2"? Is that floor going to flex a bit? Like L/720? Or L/360?

To get Wedi, what I have done is to call the main guy (or the next guy), whose numbers are on the Wedi web site, and let them tell me which SPECIALTY tile store carries it. There is no other way to get it.

I did this for someone who lives in another city too.

Each time, it took me a few seconds. I think that is normal for a special product.

Years from now, the product (and future knockoffs) will be so common that you can just call around, or drive around, and expect to see it somewhere. It'll will happen sometime this century.

David

 

[ebeggel]

our plan

It looks like we are going to go ahead and start this on friday (2/9).

I think the hole has some mortar (and possible something else as there are two different shades) on top of framing. I do not think that there is a slab under there.

As per the earlier suggestion we plan on pouring in a sand/concrete mixture with some broken up bricks mixed into the 'soup'. We'll probably put some welded wire in there.

Because there are joists (what I think of as framing) on both sides and in the front, we will add ledgers to those and a 2X2 or 2X4 in the back to form a screed line.

When we get it all in plane with the floor and it dries we are thinking that 3 sets of double cups should be enough to put the granite in place.

I did locate a Wedi distributor but everyone who has volunteered their 'hands on' help insists that the granite is not likely to break if handled gingerly.

If there are any last minute warnings I'd appreciate them. thanks again

 

[Jadnashua]

Forget the broken bricks. If you use something like a 2x4 as a screed, pull them out after and fill that area with the sand mix before things set up.

 

[ebeggel]

first step done

The hearth stone is in...now for the facing and surround.