Stone Flooring Help Needed

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Old 02-16-13, 08:33 AM
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Stone Flooring Help Needed

I recently installed a new wood stove in my circa 1929 home. The stove sits on the existing rough field stone stone fireplace hearth that is flush with my hardwood flooring. In order to meet code, I am required to extend the hearth out by 12 inches. I chose three-quarter inch thermal Bluestone - three pieces 12"x24" - because it matches my bluestone mantle. I did not want to use a stone tile because i could not find anything that looked quite right. So I chose the 3/4" bluestone because it would be flush with my hardwood after installation and I was able use 3 pieces instead of 6-one square foot tiles.

When I remove the hardwood floor I discovered the 3/4" subfloor needed to be replaced which I did with three-quarter inch interior AC plywood. It was suggested to me, given the fact that i want to keep the new bluestone hearth at the same height as the existing hearth, that I could use a Masonary adhesive to apply the stone to the new subfloor providing the subfloor is flat and solid, which it is. The new 1' x 6' section of subfloor sits on four two by eights (they are full 2"x8s") and I attached it with construction adhesive and screws so it it very solid.

The issues that concern me are applying the stone to the sub floor directly and given the fact that I used interior plywood, what can I do to fill in the joints between the stones - four- 1/4+" joints. I can't put down a backer board because it would bring the new mantle up over 1/4" and would be a trip hazard (elderly patents live with me) and would not look good. It was suggested I could use a Polymer mortar mix (generally used on patios because it flexes) but I'm afraid of getting the plywood damp when I lightly mist the Polymer mortar mix in place. I suppose I could apply some waterproof caulking on the floor between the joints the stones to avoid the minimal bit of water getting in contact with the plywood.

Do you see any problem with this approach? I understand this is not the ideal way to do this but I am trying to work under the constraints of this old house so if you don't see any glaring issues, I'd be grateful to do it this way.

Thanks in advance for your help.
 
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Old 02-16-13, 09:13 AM
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Since this is not a wet area (tub/shower) and you are only spanning a 1' area without foot traffic, I do not see any problem in adhering the stone using regular thinset mortar. No matter that your subflooring a an interior grade of plywood, the mortar will dry and that's it. Yes, ideally you should have a vapor barrier such as a cbu or ditra, etc., and normal advice would be different. I do understand your need to keep it the same height. You have done a lot of work to make this right. You could coat the plywood with Zinzser BIN primer to help alleviate moisture problems while the thinset "sets".
 
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Old 02-16-13, 11:32 AM
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Thanks for your advice. What's the minimum effective thickness of thinset I could get away with using under my 3/4" bluestone. Speaking of a primer on the plywood prior to thinset install, I already have some Kilz Original oil primer - would that work? What is your thought about the masonry adhesive instead of thinset - do you see a downside to this? The stone yard where I purchased the stone recommended this approach since I didn't want to bring up the floor level.
 
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Old 02-16-13, 01:35 PM
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Dave,

Keep in mind that installing natural stone. (esp. a slab), over a single layer of ply is all wrong and is liable to crack. But since it's too late now, you may as well continue. Doing it right would probably mean ripping out the rest and starting over.

Jaz
 
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Old 02-16-13, 02:40 PM
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Jaz, given that I only have 3/4" of depth to make a new hearth flush with the old one, what would you reccomend?
 
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Old 02-16-13, 03:14 PM
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Jaz may have an alternative way of doing it, but you may be able to use a mastic product under the stone to adhere it. It isn't the way I would install stone, but given that you already have a minimal clearance to work with and no foot traffic, it may work. Just a word of caution, as Jaz mentioned, this is not a normal way to install the stone, as some sort of underlayment is always used.
 
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Old 02-16-13, 04:24 PM
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Stone tiles require a double sheet subfloor/underlayment as the base. Then either a membrane such as Ditra or one of the Noble products, or a concrete backer.

I think you'll find a thicker base under the original stone. I've seen hearths built with 8-10" of concrete under them. To do it right you'd have to do some work underneath, which is why I said just go ahead. It may have a good chance or working for you if it's otherwise solid.. Otherwise there's nothing wrong with the hearth being an inch or 2 higher than the floor. People will get used to it. It's actually normal.

Jaz
 
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Old 02-16-13, 07:05 PM
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What's the minimum amount of thinset I could get away with using effectively?
 
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Old 02-16-13, 08:30 PM
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If the substrate is flat, you can get away using a 1/4x1/4x1/4" trowel. This should give you about 3/32" thinset thickness which is the recommended minimum for proper bond and cure. This of course assumes that the tiles are perfectly flat and uniform thickness.

Jaz
 
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Old 02-17-13, 03:21 AM
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I may have misunderstood, but is the end result needed to ensure the stone matches what is already there, and the floor level at present? I agree that hearths usually stick above the existing hardwood flooring, but I was under the impression it also had to meet the existing stone, since it was an extension of matching stone.
 
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Old 02-17-13, 04:46 AM
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Yes, the desired result is for the new 1'x6' hearth extension to be flush with the existing fireplace hearth which is flush with the hardwood floor. I agree, a hearth that is 2-4" higher than the floor is fine and common but it would look strange in my application to have an elevated platform that is higher than the rest of the existing fireplace hearth and flooring.

Jaz - I watched a video on Ditra and it seems like a great product. To clarify, I wasn't opposed to using a stone tile; in fact, I looked for some but couldn't find a natural stone tile that looked like my existing 2" bluestone mantle or or my field stone hearth and surround. Based on what I see on-line, landscaper's bluestone is very different than flooring bluestone. The material I am trying to match is the blue/slightly greenish stone used in paving, stair treads and patio applications.

I realize a stone tile install is the way to go providing I can find the right material. Are any of you in New England with suggestions where to purchase material - or an on-line source?

Thanks.
 
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Old 02-17-13, 04:53 AM
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Would it be possible for you to take a couple of pix of what you have (not closeups) so we can see what you see? Thanks. http://www.doityourself.com/forum/el...your-post.html
 
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Old 02-17-13, 08:02 AM
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Good suggestion with adding pics.

First pic shows structure after removing old subfloor; second shows new 3/4" ac plywood subfloor and third shows the 3/4" bluestone I'd like to use - it is just sitting on the floor not attached yet. As you can see, I need to keep the same level as the hardwood and old hearth.

Thanks for your help and suggestions, I appreciate them all.


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Old 02-17-13, 08:33 AM
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Maybe Jaz will have a differing view, but I don't see anything wrong with what you have, now. I like the extension of the breadboarding on the edges.....classy. Although it is not an "accepted" method, and I concede to that, I would adhere the stone to the plywood with mastic and grout it. You are going with rough stone to smooth stone, so I would look to the wood to be my determining factor.
 
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Old 02-17-13, 03:24 PM
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Thanks, Chandler.

Enough has been said to still obviously make me very nervous about putting down my stone directly on plywood using adhesive. I might have found some 1'x2' slate tiles that might work and only require a one box purchase, and not a huge order. The slate tile is listed as .4 inches. If I were to do this job "more properly", by using the 1/8" Ditra underlayment and the two layers of thin set, this should bring me right in the 3/4" thickness I do not want to exceed to keep all my floor surfaces the same height.

I say this method would done "more properly" because the Ditra website and Jaz indicate when applying stone to plywood, two subfloor layers are required/recommended. But since I don't have that kind of depth, I'd have to use my existing one layer of interior 3/4" AC.

A side question regarding the use of Ditra: it appeared to me that the installers in the video were applying thinset directly to interior plywood - is that correct? I ask because I have read elsewhere that thinsets are to be applied only to exterior grade plywoods.

The more I read, the more confusing it gets! Thanks again for your assistance to my newest round of questions.

Dave
 
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Old 02-17-13, 03:53 PM
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I know it was a tough decision, and I didn't really want you to do it that way, but you know....rock and hard place. Trying to get it done with as little lift as possible and still have a solid floor. I agree wholeheartedly with the suggestions on the Ditra and you using a narrower stone. It should look great. Keep us posted on the results.
 
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Old 02-17-13, 04:28 PM
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Chandler, is it ok to put thinset on interior plywood?
 
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Old 02-18-13, 03:04 AM
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Not the best situation, and since you are replacing the plywood with a thinner sheet, I would opt for PT plywood. Interior plywood, just doesn't have the glue needed for the moisture you will introduce and can delaminate.
 
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Old 02-18-13, 05:14 AM
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I wasn't planning on (and really hoping i wouldn't have to) removing the interior plywood. Will it really be difficult to remove after using construction adhesive? Is there anyway I could use it - treating it, putting something down before putting down the thin set?
 
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Old 02-18-13, 05:04 PM
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Zinsser BIN primer will help with this.
 
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Old 02-18-13, 08:00 PM
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When we say to use exterior plywood, we do NOT mean pressure treated. You NEVER want pressure treated indoor or to bond things to. Exterior ply refers to the glue used to bond the 5 plies together to make the sheet.

Note; Ditra is 1/8" installed, total. The thinset adds nothing. So, .40 slate + thinset, .125 max and Ditra .125 is less than 3/4". Plus keep in mind that 3/4" ply is not 3/4", it's 1/32 less.

Jaz
 
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