Tile over plywood over timber subfloor

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Old 12-27-12, 09:14 PM
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Tile over plywood over timber subfloor

I am getting ready to start tiling one of my bathrooms, plumbing has been modified and hvac relocated. So I now have a patch of 3/4 ply where the old wood subfloor was located. I was walking over the patched spot the other day and noticed a little bit of flex in the plywood. I plan on laying another layer of 1/2" ply over the entire room and tiling over that. I want to make sure I am doing it right. So below I will list the layers that I am envisioning please let me know if I am on the right track or way of base. any suggestions are welcome but i want to stay away from tile board if at all possible because the layout is going to be a pain with 3x5 sheets. Way to many cuts to make.

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1. Tile (Ceramic or Porcelain, is one better than the other for this situation)
2. Modified Thinset
3. Schulter Ditra ( I would like to skip this layer if its not necessary, its kind of expensive)
4. Modified Thinset
5. 1/2" Plywood Screwed into floor joist (24" c/c)
6. Modified Thinset
7. Roofing Felt (this one is not my idea, read it on another site and wasnt sure.)
8. Original Subfloor/Patch


Any suggestions are greatly appreciated. Thanks in advanced.
 
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Old 12-28-12, 04:24 AM
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If you have flex in the subfloor adding 1/2" of plywood will probably not solve that. Neither will CBU. If you have flex you'll probably have problems with the tile.

You need to figure out what is causing the flex. If the supporting joist structure is adequate then my guess is that the plywood patch is moving along the unsupported edges.
 
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Old 12-28-12, 05:38 AM
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the patch only covers one bay of joists, and the only flex is at the unsupported seam, like you had mention. So I willtry to find a way to better support the seam. Once that is solved am I on the right track with the layers I listed? here is a link to a picture of the area in question. https://www.dropbox.com/s/v9q7deljpj...%2032%20PM.jpg
 
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Old 12-28-12, 05:59 AM
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#5 no. Don't screw the second layer into the joists. Intentionally miss them to provide a slip.
#6 no. Thinset is not needed between the layers of subflooring.
#3 You'll either need to use a product like ditra or concrete backer underlayment 1/4"
 
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Old 12-28-12, 06:06 AM
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I should have been more clear #6 would only be in the areas of the patch . It is about 1/8 lower than the original.
 
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Old 12-28-12, 06:19 AM
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If you have access to the joists from below you can add suppoort blocking underneath the seam patches. If the joists aren't accessible you can remove the patch and add the support from above and then lay the patch back down.

Where were you going to put the roofing felt?
 
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Old 12-28-12, 06:24 AM
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I read somewhere about puttin it down over the original subfloor before any mortar. But I am thinking it is not necessary.
 
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Old 12-28-12, 06:51 AM
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You've got two differing opinions here, on how to install your tile. You'll need to pick and choose. You'll need to install an isolation membrane such as Ditra or cbu over the plywood. Don't install the tile directly over the plywood. Thinset can't be used as a leveling agent for your subflooring. You should use a self leveling compound and don't drive screws/nails through it. Is the patch under where the sink cabinet goes? Don't sandwich the subflooring with the tar paper. It could cause moisture problems between the sheets down the road.
 
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Old 12-28-12, 10:31 AM
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You will probably have problems if you lay your tile directly over plywood using a thinset mortar without using CBU or Ditra. It's not recommended by the mortar manufacturer. I have not used Ditra but if I remember correctly (I checked it out a while back) the manufacturer requires modified thinset mortar as a bed and unmodified to set the tile.

As far as I know none of the thinset mortar manufacturers recommend setting tile directly on plywood. You can use a Type 1 mastic as the adhesive. Plywood is one of the recommended bonding surfaces. I've done two rooms and a couple of countertops that way without having any issues but the tile guys will disagree with that. If you use plywood and mastic make sure the ply is exterior grade AC.

Whatever way you go, the flex in the floor has to be eliminated.
 
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Old 12-28-12, 12:27 PM
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If you patched with 3/4" ply and it is 1/8" lower than the main floor, then you have 2 layers of 1/2" ply down in the main section. Since you are planing on putting another layer of 3/4" down I would do the following: Remove top layer of 1/2" ply, remove 3/4" patch. Use the residual 1/2" ply you removed to re-fill in the patch making it the same level as the balance of the bath (install cross blocking at the seams in the patch). Then go over the whole floor with your 3/4" making sure to start at the patch wall and work out to make that first joist span as strong as possible.

Once the subfloor is correct, I would then use either Ditra or 1/4" cement board (cheaper by far) set in a mortar bed. Then tile over everything. Don't let the number of cuts scare you away from cement board underlayment. You simply mesh tape the seams with thinset and you are good to go. It will cut easier than you think. Once you score the embedded mesh on the cement board you bend and it snaps along that line. Cuts very similar to drywall but you will go thru a few more blades. Hardibacker is a little more challenging to cut but certainly within your skill set.
 
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Old 12-28-12, 02:05 PM
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You don't say what your original subfloor is. Is it dimensional lumber/planking? Tell us what it is. You have 24" oc floor joists, yes. If so, you will want to add a layer of minimum 1/2" plywood over the original subfloor to avoid any defletion issues between the floor joists. You need to remove the 3/4" patch and replace it with a material of the same thickness as the original subfloor. At that time, install blocking between the joists to support all the square cut edges of the patch. For 24" oc joists you may want to consider using ditra XL.

Dont use any roofing felt between subfloor and underlayments. Dont use thinset to install a second layer of plywood, just use screws and lots of them. Use modified thinset to install the ditra and unmodified (dryset) mortar to install the tile.

Tell us the details of your existing structure. What size are the joists, what is their unsupported span and on center spacing? Identifying the species and grade if possible would also be helpful. What do you currently have for a subfloor? Type of material and size.
 
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