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Unread 09-23-2020, 03:15 PM   #16
weronika
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Thanks for more tips. The previous layers were 7/8 inch thick in total, if I add 1/2 inch plywood that will take me to 1 1/8'' plus thickness of two layers of mortar (how much thickness would that account for?). I think the hollow core doors can only be cut 1/4 inch from the bottom so I worry the extra plywood would take me over that?

I also wanted to ask if putting in a layer of plywood is a complicated job that requires a lot of tools? Will they be able to cut the plywood to size for me at Home Depot/Lowe's?

If I do put in the extra plywood, do I still need to worry about deflection? There's a tiled bathroom very close to the laundry room, I thought that would indicate that the house structure is good for tiling?
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Unread 09-23-2020, 03:45 PM   #17
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I'm thinking that's a typo on Ali's part. The ceramic tile industry requires that joist deflection not exceed L/360 for a tile installation. That's 1/360th the length of the unsupported span of the joists, measured in inches.

The part about being "without risk of failure" must be his own personal warranty, but I can tell. You might wanna give him a call, Veronica.
Hahaha definitely a warrantless warranty with $0 face value.

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Unread 09-23-2020, 04:18 PM   #18
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Originally Posted by weronika
If I do put in the extra plywood, do I still need to worry about deflection? There's a tiled bathroom very close to the laundry room, I thought that would indicate that the house structure is good for tiling?
The extra plywood will play little into deflection.
Determining if the "structure is good for tiling" is a room for room thing.

What you might not realize is that when you step into a room, the weight of your body bends the floor joist just a little bit. This is know as "deflection".

For a given weight, the primary factors that determine how much your floor deflects are:
* type of wood (some woods are stronger than others),
* size of the joist (typically 2x8, 2x10, or 2x12),
* distance between the walls that support the joist,
* spacing between joists (typically 16" or 12").

For that part about "distance between supports", you have to think of your floor as a bridge. For a given type of wood, joist size, and spacing; when you stand in the middle of your room (i.e. bridge), the middle of the floor will sink farther down if the joist supports are 12' apart (say for a bedroom) compared to 6' apart (say for a laundry room). But you can NOT go by the walls above the joists (i.e. you can go by the size of the room). You have to know the structure below the floor that supports the joists, because the wall that separates a laundry room from a hall might all be sitting on the same "bridge".

This WILL NOT be consistent thru out the house.
I'll use my own house as an example. The floor joist under our bathrooms (that have tile) are 2x8 on 16" centers and are about 8' long. But my living room, which also uses 2x8, is 14-1/2' long. That span is so long that the builder had to space floor joist every 12" (and at that I think he was stretching building code requirements). When you walked across the living room, you could hear the entertainment center shaking for the floor bouncing excessively. So where tile could be install in the bathrooms, the living room has a "deflection" that far exceeds what is acceptable for tile.

That number that CX thru out... 360... That means for a "bridge" to support tile, that bridge should not deflect by more than the length of the bridge divided by 360. So if your bridge is 360" long, the center can't sink by more than 1" when you stand in the middle of the room (actually, I think it's based on some 'standard' weight... obviously each person weights differently and therefore different people would make it deflect a different amount).

In the blue bar above, there is a link labeled "Deflecto". You can use that page to enter your data on the floor joist supporting a particular room and it will run the calculations to tell you how much deflection the floor has. It needs to be 360 or greater for tile.
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Unread 09-23-2020, 04:49 PM   #19
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Joseph, thank you for the detailed explanation.
What does the extra plywood achieve, then?
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Unread 09-23-2020, 08:33 PM   #20
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Veronica, there are two different and distinct areas of deflection in a wood framed subfloor. The first is the joist deflection, which we have been discussing. That is based upon the design of your joist system and can be calculated once you know the necessary details of your joists.

The second, and frequently most important area of deflection, is the subflooring between the joists. The requirement there is also L/360, but the subflooring requirements are determined in the ceramic tile industry are determined by a test (ASTM C-627) commonly known as the Robinson floor test.

The manufacturers of different tile underlayment products determine their minimum subfloor requirements using that test and publish their requirements as type and thickness of subflooring material. Each manufacturer wants his product to be used in as many applications as possible and will, therefore, publish the absolute minimum subfloor requirement. Keep in mind that this test is conducted using new material, in near perfect condition, near perfectly installed over joists with zero deflection and the test needs pass only once.

The additional layer of subflooring, especially over old, damaged subflooring, not particularly well installed, just makes good sense to me. Over 16" joist spacing a single layer of nominal 5/8ths" plywood can be made to pass the test sometimes. A single layer of nominal 3/4" plywood will pass the test most of the time. A double layer of plywood with a bottom layer of at least nominal 5/8ths" and a second layer of at least nominal 1/2" will pass every time.

The two deflection standards are independent of one another and each must be met for the best chance of a lasting ceramic tile installation. Fixing or improving one does not improve the other. One must also consider any specific or unique loading that might apply to a specific subfloor.
Quote:
Originally Posted by Joseph
That number that CX thru out... 360...
CX didn't just throw out that number, the L/360 floor joist deflection requirement is from the ceramic tile industry standards. It is also, by the way, the building code requirement in all residential jurisdictions of which I'm aware.
Quote:
Originally Posted by Joseph
So if your bridge is 360" long, the center can't sink by more than 1" when you stand in the middle of the room (actually, I think it's based on some 'standard' weight... obviously each person weights differently and therefore different people would make it deflect a different amount).
The joist deflection is based upon a uniform loading, not a particular weight in the center of the floor. The standard is 40psf live load and considering a 10psf dead load in most applications. Our Deflectometer is more conservative (because most of its use is for remodeling application and not new construction) and is calculated upon a total load of 50psf.

My opinion; worth price charged.
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Unread 09-25-2020, 01:59 AM   #21
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Thank you for the detailed explanation.
Would you be able to give me advice regarding which type of plywood to purchase?
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Unread 09-25-2020, 07:41 AM   #22
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The plywood you'd be looking for will have a face grade no lower than "C", and should be "Exposure 1" (indicating the glue used to hold the plys together will tolerate some moisture. At HD you'll probably find something that is marked B/C, Exp 1, or close to that.

And yes, as long as the HD you go to has a panel saw, and it's working, and the people who are trained to use it are working, they'll cut it to size for you. Sometimes they charge a buck per cut, sometimes they don't.

Attaching the ply will require at least a drill/driver or, better, an impact driver.
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Unread 09-25-2020, 09:21 PM   #23
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Thanks, Dan.
Should I also be looking for plywood with T&G or is that not important for this second layer?
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Unread 09-25-2020, 09:22 PM   #24
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Not necessary for the second subfloor layer, Veronica.

My opinion; worth price charged.
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Unread 09-25-2020, 09:41 PM   #25
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Does this look like a good option?
https://www.lowes.com/pd/AC-Plus-15-...4-x-8/50100640
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Unread 09-25-2020, 10:02 PM   #26
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Better than you need, but if it says it's an exterior glue panel, it would work for your application. Most such letter-graded panels are exterior glue, but I can't tell what they're selling you there. You'd wanna see a grade stamp on it saying either EXT or Exposure 1.

They should also have a BC plywood and it should cost less than $36 dollars for a half-inch sheet.

My opinion; worth price charged.
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Unread 09-25-2020, 10:30 PM   #27
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Thanks, CX! In the specifications it says "Weather Exposure: Exterior" which I'm assuming is good. Just to double check the 15/32 thickness should be all right? I doubt the 1/32 of an inch would make much difference, but many things I thought about tiling turned out not to be true so best to check

More questions: if I'm putting in this second subfloor layer, should I still fill in holes in the bottom layer? With a brand new layer, is it less likely that I will need to level the floor or should I still pick up some self-levelling products?
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Unread 09-25-2020, 10:43 PM   #28
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Unfortunately, Veronica, the 15/32nds is the standard thickness for "nominal" 1/2" structural plywood these days. And they (manufacturers) get another 1/32nd as a tolerance. Similar to nominal 2x4s being only 1 5/8ths by 3 1/2 inches in reality.

Yes, the exposure rating of "Exterior" is just what you're looking for.

In your application I would likely fill all the larger surface voids in your existing subfloor with water putty or similar before installing the second layer.

As for leveling, your ceramic tiles don't give a rat's patooti whether the floor is level, they care only about flat. I would recommend you install your second layer and see where you are in that regard. If you need to flatten (or level if you just want level) it will depend upon your choice of a tiling substrate when you'd do that.

My opinion; worth price charged.
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Unread 09-25-2020, 11:34 PM   #29
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Thank you! This is all amazingly helpful.

This is my shopping list for this weekend with some extra questions. Please let me know if I'm forgetting anything!

1) PLYWOOD
Cut to size - remember to include 1/8'' gaps between panels and 1/4'' gaps at perimeter walls.

2) DITRA SHLUTER ROLL

3) TROWEL for Ditra
The instructions say "For DITRA use a 1/4" x 3/16" (6 mm x 5 mm) V-notched trowel, 5/16" x 5/16" (8 mm x 8 mm) V-notched trowel, or the DITRA trowel, which features an 11/64" x 11/64" (4.5 mm x 4.5 mm) square-notched design, is recommended.". Would using the Ditra-brand trowel, priced at $27, make it any easier for a complete beginner like myself? Or should I just get a normal 1/4" x 3/16" V-notched trowel like this one https://www.lowes.com/pd/QLT-by-Mars...wel/1000204159

4) THINSETS
Earlier on this thread Ali recommended the following thinsets: Custom's Flexbond (Home Depot) for use under the ditra and Mapei's Kerabond T (Floor and Decor) over the ditra.
For Custom's Flexbond - should I pick up 25lb or 50lb bag for 6x7.5ft room? How about "Schluter All-Set"?
For Mapei's Kerabond T - any product recommendations for something I can pick up at HD/Lowe's?

5) GROUT
I'd like to buy grey grout, any recommendations?

6) SCREWS
What type of screws should I buy for securing the plywood on top of subfloor?

7) TROWEL FOR 8x8 TILE
1/4'' x 3/8 U or Square notch

8) TILE SPACERS 1/8''
1/8'' grout joint recommended by tile manufacturer

9) TILE CUTTER

10) TILE NIPPERS

11) BUCKETS for mixing thinset

12) Grout HAZE CLEANER

13) CHALK LINE

14) Plywood PATCHING COMPOUND
https://www.lowes.com/pd/DAP-32-oz-G...xoCirMQAvD_BwE

---
Already have: drill, grout sponges, tiles
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Unread 09-25-2020, 11:53 PM   #30
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Mapei makes a decoupling membrane mortar which is unique to Lowe's and has similar specs to Kerabond T (although they still list Kerabond T as a superior product on their website).

As for grout, spectralock is my favorite and has a number of different grays - you can also order color samples from Laticrete directly to help visualize, I haven't used anything else since I tried spectralock because it's so easy to use and very durable. There are some good topics on it here and a tutorial on making smaller batches on the floor elf site.

As for trowels, I'd use one of the recommended measurements and order one on Amazon - I've had good luck with Marshalltown trowels. U notch is easier to use that square notch to get 100% coverage.

I use the GRK 2" #9 screws at HD for anything subfloor, they do a really good job of tightening up loose subfloor panels and should work well for your application too. For your application, a 1.25" screw should do the trick since you only want to secure subfloor to subfloor.

Skip the tile nippers and grab an angle grinder from Harbor Freight + a good diamond blade and you can do all your intricate cutting/shaping with it. Practice caution with the angle grinder, make sure you have a solid grip on the grinder and wear respiratory/eye protection.

Good luck!

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