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Unread 06-06-2020, 11:11 AM   #16
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Rick, Schluter does not require an unmodified thinset mortar for installing Ditra over a plywood subfloor. They require a thinset mortar "suitable for the substrate." When bonding anything directly to plywood in a ceramic tile installation the ANSI standards require the use of a thinset mortar meeting the requirements of A118.11. There are no exceptions to that. If you wanna bond to plywood with a thinset mortar, it must be a modified mortar meeting that specification.

My opinion; worth price charged.
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Unread 06-07-2020, 06:53 AM   #17
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Thanks Dan.

Anyone hear of RDT plywood? Local box store ran out of BCX grade and wanted to see if I used this on part of the underlayment would be ok.
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Unread 06-07-2020, 07:07 AM   #18
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A little Googling and I found this:

An RTD is a resistance temperature detector. It is essentially a thermocouple, designed to provide real time, very accurate temperature readings. RTD plywood appears to be exposure grade 1 plywood manufactured using RTD technology to control the temperature of the glue during application and curing. Presumably this makes the plywood superior to CDX plywood, which by the way stands for C face on one side, D face on the other side, exposure grade 1.

CDX is NOT exterior grade plywood. Exterior grade plywood uses superior glues to CDX. Since CDX is exposure grade 1, it is suitable for use as a substrate in damp environments, such as sheathing for roofing, or underlayment for tile. RTD plywood appears to refer to the manufacturing process; the only way to know the exposure class on RTD plywood would be to check the rating on the sheet, it should be stamped on the sheet.

And, unfortunately, this:

I bought some of thaRTD at HD and "Ready to Delaminate" seems most accurate. Several 4 X 4 pieces I had stored in a covered protected outdoor area warped badly and started to delaminate after a rain. They did not get wet, only exposed to high humidity. Some scraps I left out in the actual rain completly came apart !! I am not impressed with this stuff. Guess thats why its cheap at 13+ $ a sheet.
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Unread 06-07-2020, 03:03 PM   #19
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Thanks guys. I researched the RDT as well and not getting good vibes on using it. I'll get more BC grade and use that to be sure.
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Unread 06-09-2020, 03:40 PM   #20
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Flat underlayment question

I put down 1/2 " ply over existing OSB and it is flat in most areas. I have a small section where I see a small gap under my level. Largest gap height is about 1/8" inch. Will this be okay to move forward? Im thinking maybe the board is slightly warped. Any idea how to fix the low area to slightly raise it? Check out the photo and you can see the slight gap.

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Unread 06-10-2020, 07:01 AM   #21
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1/8" is quite a lot, Rick, especially if you intend to use large-ish floor tile. I don't recall exactly what the industry standard is, but it's something like no more than 1/16" of deviation from flat over 2', and 1/8" over 10'. Pretty flat.

Yer gonna want to address that. But first, what do you intend to use over the plywood to serve as your tile underlayment?
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Unread 06-10-2020, 10:12 AM   #22
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Thanks Dan. I have 3/4 OSB with 1/2 plywood over that. I plan on using Ditra over the plywood then install 12 x 24 porcelain tiles.
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Unread 06-10-2020, 11:08 AM   #23
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In that case, Rick, you might consider using an SLC product to flatten things out. The big box stores have them, and they will probably require the use of a primer over bare plywood. Be sure to look for an SLC that allows for a feathered edge.

Once the SLC has cured you can install the Ditra over it.

How large are the low spots? 1 S/F, 2 S/F?
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Unread 06-10-2020, 02:16 PM   #24
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Thanks Dan. I have 2 areas, one is about 1 sq ft and about 1/8" dip and another near the toilet drain that is considerable, about 1/4" dip around the drain about 1 soft on each side of the drain.

With the SLC on top of the plywood, can you use the same thinset to adhere the ditra over both the SLC and plywood?

Thank you!!!
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Unread 06-10-2020, 03:01 PM   #25
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Can't see it from here but you may need to build a dam around the toilet drain to contain the SLC.

And yup, you can use the same thinset mortar on toppa the SLC as you use on toppa the ply, and that mortar will need to be a modified. Lifted this directly from the Schluter site:

"The type of thin-set mortar used to install DITRA depends on the substrate material. For example, to set DITRA over plywood or OSB, a modified thin-set mortar meeting the requirements of ANSI A118.11 is used."
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Unread 06-10-2020, 05:12 PM   #26
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Thanks Dan. I added a pic and as you can see, it considerable as you get towards the corner. I'll look into building a dam like you suggested.
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Unread 06-10-2020, 06:14 PM   #27
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SLC question

Was wondering, with the SLC, should I need to caulk or do anything with the gaps in-between the 1/2 inch plywood I added to the OSB? Also, is it bad if I only apply the SLC to the 2 sections effected and feather it out?

Thanks again for the help!
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Unread 06-11-2020, 07:52 AM   #28
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I put a screw driver in the direction where I'm noticing a hump in the OSB subfloor. Behind the toilet drain, it actually dips closer to 1/2 inch in the back corner. I do notice that a joist runs under the hump. What's the best option to resolve the hump:

1. Use a planar or sander to grind down the hump?

2. Try to remove a section of the OSB that spans the hump?

3. Use SLC over the floor to even out the hump? If I go this route, is it better to use the SLC over the OSB and then put 1/2 plywood over that or put the SLC over the staggered and slightly gapped plywood?

Thanks and sorry for so many questions. I just dont want the tiles cracking for the time I'm investing in this..
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Unread 06-11-2020, 08:30 AM   #29
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1. You do not want to flatten the subfloor by removing so much of the subfloor material that you compromise the integrity of the subfloor.

2. Perhaps. The optimal method of repairing that area is to determine the reason for the humps or dips and correct that at the joist-top level.

3. If you do that you must pour the SLC over the top layer of subflooring unless you plan to use a CBU as your tile substrate in which case you'd pour over the CBU.

As for filling gaps, you want to consider that you'd be pouring water on that subfloor and fill any gaps that would allow water to escape.

My opinion; worth price charged.
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Unread 06-11-2020, 11:03 AM   #30
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Thanks CX. I was planning on using a layer of ditra on top of the plywood for the tile. I believe the ditra can be installed ontop of the SLC.
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