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Unread 05-03-2020, 12:08 AM   #1
Susanne
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What type of underlayment for slate tile on 1 1/8" OSB T&G, & deflection L/1601?

I have 6" x 24" x 3/8" thick slate tile that I would like to install in a herringbone pattern in my den. My subfloor is 1 1/8" T&G OSB with a joist deflection of L/1601. The house was built 17 years ago here in the Pacific NW, and hopefully any subfloor swelling & settling that is going to happen, already has, and it has no squeaks or weak spots in it. I would like the finished tile floor to be as close in height as possible to the adjacent 3/4" hard wood floors, and that is where my dilemma comes in. Which underlayment to use? 1/4" HardieBacker is thicker than 1/8" DITRA, but the DITRA instillation requires a double layer of wood with natural stone tile. What do you guys & gals recommend?
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Unread 05-03-2020, 08:27 AM   #2
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Welcome, Susanne,

I'd be curious to know what your joist structure looks like and is made of in order to produce such a large L value. Pretty stout, that.

Nevertheless, everything I've read says stone needs two layers of ply, regardless of the underlayment used.
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Unread 05-03-2020, 08:42 AM   #3
Davy
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Hi Susanne. I just want to make sure, this is actual slate and not a porcelain tile that looks like slate?
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Unread 05-03-2020, 01:29 PM   #4
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Essentially, there are two different components to deflection. To get that stiffness, you must have some engineered lumber, trusses, or steel as your joists. But, along them is one thing, in between them is another. The joists (or I-beams, or whatever) only account for one direction. The second bit is in between the them. So, that's where the two layers comes into account. Tests have confirmed that where the ends of the sheets of subflooring come together that a load in between the supports will cause them to lift the ends up slightly. Tile can't handle that, especially natural stone. The second layer provides two things: first, improve the deflection number between joists, and second, shield that vertical movement from the ends of the panels.

Now, 1-1/8" panels are pretty robust. Not sure that is sufficient justification to avoid the second layer. FWIW, that's an industry standard that is not unique to Ditra.

What I suggest you do is to call Schluter and see what they think.

Also, need to verify that you've calculated your deflection properly. Normally, the only way you'd get to that number is either by error, or to have something other than solid 2x material. Note that any span is between supports, not the size of the room.
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Unread 05-03-2020, 01:48 PM   #5
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Thank you guys for responding so quickly! Thank you Jim for explaining the reasoning behind the double layer!
The L value is so high due to it being a first floor level with 4x4 support footings six feet away from the concrete foundation wall on one end and 6'10" away from the support beam on the other end, they basically run down the middle of the room under the joists. I initially thought they were 1x10" joists but now that I'm digging through the floor plans it says they're 4x8" joists. I double checked by going in the crawl space and sure enough they're 4x8" doug fir joists, which would make the deflection value even higher. I was hoping someone might be able to double check my calculations, but it wouldn't let me include a link in my post to a picture of the floor plans. If I'm using the "Deflect-O-Lator" correctly: I have Doug Fir joists in good condition, 8 inches tall, 4 inches wide, 24 inches on center, and (rounding up to) 7 feet long between supports which would give me a deflection of 0.035, this translates to a deflection of L/2411.
Maybe it should be the room we keep the elephants in.

It really is a natural stone slate tile.
I like your suggestion Jim on calling Schulter. Hopefully they're open on Monday.
With a deflection of L/2411 and the OSB is 1 1/8", do you guys think that I'm taking a huge risk not doing a double layer in order to keep the finished height down?
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Last edited by Susanne; 05-03-2020 at 03:44 PM. Reason: Included more information
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Unread 05-03-2020, 03:20 PM   #6
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Welcome, Susanne.

Yes, that would be correct. There's a good article in our Liberry showing what I consider the very best method of installing the second layer of subflooring.

My opinion; worth price charged.
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Unread 05-03-2020, 04:06 PM   #7
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Thank you CX!
I had read the article and I'm glad they actually explain why the double layer recommendation. The more I can learn the better!
My dilemma is between taking the risk of cracked tile and grout vs. the risk of tripped over edge at the six foot long entrance where it meets the hardwood floor.
With everything being shut down on residential construction here, I haven't been able to consult a pro in person, so I really appreciate everyone's help on here!
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Unread 05-03-2020, 04:19 PM   #8
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The 24" OC is going to make the deflection between joists a bit higher. Most (all?) of the guidelines do not recommend stone with that spacing. The 4" wide joists will decrease the gap some, but not sure that's enough. Are they actually 4" wide? Normally, they're that size when they initially mill them, but they get cut down as they're smoother...that's why a typical 2x material is actually only 1.5" thick, not 2".
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Unread 05-03-2020, 05:22 PM   #9
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You're right Jim, the joists are actual 3.75".
Looks like I'm going to have to double layer or risk broken tile and grout.
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Unread 05-03-2020, 06:12 PM   #10
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Industry guidelines call for that second layer to be a minimum of 3/8" (nominal) thick, but it's usually hard to find any that thickness that isn't shaped like a pretzel, so thicker is usually the only end result.

FWIW, it sounds like you have Advantec which is good stuff, but it's tough to get a screw into easily. You don't want a screw that has threads the entire length, as it would be then easy to get what's sometimes referred to as screw jacking of the panel...where the screw makes at least a partial turn before it digs in and then sinks into the second layer. IF there are threads the entire length, that will end up forcing the second layer to stand proud of the first...it must be in firm contact. Having the screw with a smooth shank the depth of the top layer means it can pull that layer down tightly without issue. Otherwise, you end up trying to ream out the top with those threads, and potentially stripping the head or breaking the screw off in the hole and still have a gap.

There are ways to make a ramp between the two floors.
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Unread 05-03-2020, 09:18 PM   #11
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I really don't want to go thicker than 3/8".
That's really good to know about the screws. Would deck screws be the way to go then?
With a double layer then 1/8" DITRA seems to be the best option to keep the height down after that. How much more height should I expect the mortar to add to the total finished height?
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Unread 05-04-2020, 08:04 AM   #12
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While the difference between 3/8" and 1/2" ply doesn't sound like a lot once you handle the material in person you'll definitely notice the difference. But regardless of which you use be sure it's not less than B/C grade, exposure 1.

And regardless of which you use 1 5/8" construction or deck screws will do the trick.

I think Ditra is installed using a 1/4" notch trowel (but double check the install instructions) so you would have a bit less than 1/8" of mortar under the Dirta.

If the back of your slate is completely flat you might be able to get away with a 3/8" notch trowel. Might. So a bit less than 3/16" of mortar build up.
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Unread 05-04-2020, 10:19 AM   #13
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From a strength viewpoint, 1/2" is definitely stronger than the 3/8" (if you can find some that is flat!), but with the 1-1/8" stuff underneath, you're more interested in shielding the tile from the short joints over the joists. With a 3-1/2" wide flat space, that will lessen the lever effect, too. I wouldn't bet the farm, but am pretty confident that should work for you.

FWIW, the Ditra trowel is slightly smaller than a 1/4" square notched trowel, and will end up with less buildup, but I think they list that as an alternative. If you do it right and have the thinset the right consistency, the actual buildup isn't all that much because a good quantity of the thinset actually flows around the fleece, meaning there isn't that much actually built up on the floor once you're done installing it. The stuff needs to be fairly fluid, but (barely) still able to hold a notch. IF the instructions give a range in acceptable water to add to the mix, use the maximum. It DOES make a big difference if you mix the thinset per the instructions...it can change how it looks and feels a lot at the end, versus the beginning. Keep in mind, you have to mix and wet not only the cement, but the modifiers. The slaking period is critical as is the remixing after the wait. You want all of the modifiers to become their liquid state, and then mixed so that they can cover all of the cement and sand in the mix to a consistent mix...that takes time, the right paddle, and the right speed. Too fast, and you can introduce air in the mix like whipping egg whites. Not what you want! Trying to wing it, and you will not get the best results. It doesn't hurt to use an egg timer so you aren't winging it during the mixing and slaking periods.
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Unread 05-04-2020, 11:03 AM   #14
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The Ditra trowel is 11/64", so try to stay as close to that as you can. In this case, more is not really better.
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Unread 05-05-2020, 12:24 PM   #15
Susanne
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Thank you guys for all of your help!
That's really good to know about the thinset.
It looks like I've got a lot of work ahead of me.
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