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Unread 09-03-2007, 05:16 PM   #1
Ebonfyre
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Renovation gone wrong, advice badly needed.

I understand that there may not be an answer to this, but I figured if there was that it would be here. Let me try to describe the situation, and then ask about possible solutions.

I began a massive renovation to a house circa 1830 over a year ago. I'm an electrician by trade so am at least competent with tools, but am not as knowledgable about structure so I hired a carpenter to work with me for much of the early stages of the renovation.

We completely tore out and installed new floor joists in the area that is to be the kitchen so that I could install slate flooring on top of it. The area below it is a shallow crawlspace with 6x6 beam sills on top of rough hewn granite of varying thickness. It is no longer accessible to get below it, crawl space is too small. Due to the granite we had to use 2x6 joists. The room is 15' x 15', we put a 4x6 down the middle with a cement pier dead center to help carry the load, and then ran 2x6 joists from the 4x6 center spline to the 6x6 sills, 16" on center. I was assured this would be sufficient.

I understand from the Deflecto calculator on this site that it is not, which I just found a year later.

We glued and nailed 3/4" Advantech on top of the joists, and then glued and screwed 3/4" Plytanium Sturd-I-Floor Plywood to create the subfloor. I had a 280 pound friend jump on the floor and can see the floor flex some at the midway point of the 2x6's.

The slate is already bought and has been sealed, this step happening before I thought there might be a problem with the floor. I'm stuck with a brand new subfloor that can't hold slate I've already bought and can't return.

At this point my hope is that a non-traditional method might allow this to work out. Does anybody have any thoughts on how I can salvage this project? I'm open to perhaps using flexible grout or mortar products if that might work, regardless of how such grout may look finished. Really anything that might work would be considered.
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Unread 09-03-2007, 10:46 PM   #2
onereelbigfishy
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Hey Jeremy..

Sorry to hear about your troubles. The guys here are sticklers for the rules as far as doing things by the book which it doesn't look like your going to be able to do. That might be why you havn't had a response yet.

Before I found this forum I had learned everything I knew about tiling (which wasn't a whole lot) from a few remodel carpenters I'd worked for. I did a couple of (what I now realize) were sub standard slate installations. Those installs are over a year old now and still look great. Not to say that they will last forever but what I'm trying to get at is since you have already done the sub floor work more or less the best you could I think you have a couple options at this point:

1) Since the slate is already paid for you can go ahead and install it and hope for the best. It might fail down the road. But you might get lucky and get 20 or 30 years out of it.

2) Sell the slate and replace it with ceramic tile instead. Ceramic is not as delicate and will hopefully last better on your less than ideal sub floor. Actually the deflecto gives 494 for your sub floor so that should be suitable for ceramic.

Good luck!
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Unread 09-05-2007, 08:47 AM   #3
Ebonfyre
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The fact that the deflecto does say it is fine for ceramic is why I am even considering still doing the slate and asked for some advice. If I had simply spaced the joists 12" on center it says I'd be fine. I'm curious what the different factors are between slate and ceramic - seems to me one rigid brittle surface should be just as likely to crack as another rigid brittle surface. From the visible deflection I can see when a heavy person literally jumps on the weakest spot I don't think the slate itself will crack, it's the grout I'm concerned about hence my query about possible flexible products.

Perhaps simply asking if in this situation certain things would be more advisable to do than other would be better received? I do understand that professionals may not be willing to give "less than best practice" advice so simple yes or no, better or worse type answers would at least help.

Should I keep the weight down and not use cementboard?

Would a product like Ditra or Easymat help or worsen my situation?

Is there a non-cement based adhesive that would function in place of heavier and more brittle mortar?

Is sanded caulk an option for the grout lines?
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Unread 09-05-2007, 08:56 AM   #4
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More plywood?

I'm no expert, but can you glue and screw another 3/4"? Is the added weight too big a concern? If your 6x6 sills are sitting on solid granite, and the 4x6 down the middle of the room is sitting on a concrete pilon, it seems that you wouldn't have anything to worry about in terms of weight. glued and screwed plywood would tend to make the floor stronger.
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Unread 09-05-2007, 09:01 AM   #5
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Hi,

I have seen in stores thinset that claims it's more flexible. It says on the bag that it lowers the required stiffness of the floor to tile on. I think it's called flexbond(not exactly sure). But I do know those types of thinsets are definitely out there. Not sure if they work well. Wait for the moderators to give better advice.
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Unread 09-05-2007, 09:03 AM   #6
ddmoit
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Welcome to the forum, Jeremy.

I'm sure some pros will chime in soon - even if they don't have any good news for you.

To be clear, what numbers did you plug into the Deflecto? If I'm reading you right, it would be 2x6, 16" on center, 7.5' span.

What is the cost of your installed subfloor material in relation to your slate? I suspect that your floor can be made stiff enough for slate if you had access through the top again.
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Unread 09-05-2007, 09:05 AM   #7
Brian in San Diego
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Jeremy,

Sorry to hear of your predicament. The need for a beefier joist and subfloor system for stone installations is because of the fragile nature of stone. Man made tile can be made to certain strength characteristics that allow it to be more forgiving to deflection.

You have a couple of choices. Tile it as is and take your chances (like Marlin stated) or make changes to bring it up to the proper deflection characteristics. If pulling up the subfloor is out of the question, then I have nothing to offer. If you pull up the subfloor, you could sister the joists in this area and you would come up to the proper deflection.

Ditra won't fix any problems, but it is lighter than the CBU and it has less height when installed. You wouldn't want to use sanded caulk for the grout joints...especially in a kitchen. Thinset is not as brittle as you think. Once hardened it's stout stuff. The weak link in the equation is the slate. Is the grain of the plywood installed perpendicular to the joists? On the second layer did you carry the edge of the plywood past joists by 1/4 span? What did you use to glue the two together?

Brian
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Unread 09-05-2007, 03:07 PM   #8
Ebonfyre
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Thanks for the replies.

Fred - Adding another layer of plywood might be an option, but it would have to be 1/2" and would have to negate the need for cementboard as I only planned for another 1/2" of substructure. Anything higher than that would affect my entry door.

Gabe - That thinset product is exactly the sort of thing I'm also curious about whether it would help or not.

Dan - Yes, those are the dimensions I used. The cost of the slate is not worth tearing out the installed subfloor to me, if forced to I'll find another flooring type or more likely take my chances with this and cross my fingers. The floor does feel sturdy, but I understand that is not always enough and can see mild deflection under stress.

Brian - Yes the second layer of plywood was laid out to 1/4 past the joists and is running perpendicular to the joists. An entire tube of liquid nails subfloor adhesive was used on each sheet of plywood and spread with a V-notch trowel. They were screwed every 8" with outdoor coated deck screws.
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Unread 09-05-2007, 03:24 PM   #9
Brian in San Diego
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Jeremy,

I'm not a glue and screw convert yet, but I have read here that glue like TitebondII is the glue of choice...not a liquid nails product. I don't know what to tell you if that LN was spread over the entire surface. I read here that the TitebondII is more like squegeed over the entire surface. What you need to prevent is making voids in the plywood. I would say you are ahead of the game if it was troweled over the entire 32 sqft surface, but not positive.

Brian
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Unread 09-05-2007, 03:46 PM   #10
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Jeremy,

We need to get a professional ruling on your glue job. Glue applied incorrectly is worse than no glue at all, due to the voids that are created between the subfloor layers.

If your glue is OK, and you are not willing to tear up the subfloor to stiffen your joists, I would put that slate for sale on eBay, and install some porcelain tile that looks like slate.
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Unread 09-05-2007, 04:25 PM   #11
jadnashua
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Well, as I see it, you have three strikes against you: lower than design spec for joists, improper glue job (unless you used a trowel to get 100% coverage), and one of the more fragile and brittle stones. The natural imperfections in stone are the reason for higher strength floors verses ceramic, which is pretty consistent. A kitchen is a bad place to try to cut corners - people tend to congregate there, heavy appliances get moved around, and chairs concentrate loads. Unless the slate is ground and therefore smooth, it can be tough to keep clean in a kitchen and moving chairs over it can cause it to spall (flake off a layer). See what comments the pros bring to the discussion this evening, but it doesn't look ideal.
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Unread 09-05-2007, 11:18 PM   #12
Ebonfyre
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*sigh* This is the first I've heard that there might be a problem with the actual subfloor install... it figures.

The liquid nails adhesive was certainly troweled over the majority of the full 32 square feet, but by no means perfectly. It comes out in a tube so the adhesive was spread from squirt to squirt and if the distance between the squirts was not close enough there may be voids here and there - my apparently incorrect assumption was that the pressure from screwing every 8" would take care of that.
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Unread 09-07-2007, 02:43 PM   #13
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Any microscopic void left by the glue job will have zero effect in the grand scheme of things. Especially with a less than perfect subfloor. Although yours sounds better than a lot I've seen.

Personally I would go with Ditra and either go ahead with the slate you have or go find yourself one of those slate look-a-like ceramic tiles. That might be the idea solution although it would cost you some $$.

Good luck!
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