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Unread 12-19-2020, 09:27 AM   #1
m.t.fagan@comcast.net
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Looking for input on subfloor decisions - LFT gauged slate

Hi! Looking for some extra grey matter to review and comment on decisions around what to put below my LFT 12x24 slate.

Want to thread the needle between putting down the lightest floor I can but also create acceptable deflection for long term life.

Considering installing a 1/4" CBB and then Ditra membrane

Or can I sneak by and go directly to Ditra and then my slate over 5"8 decking over a fairly rigid joist structure?

Alternately, is the 1/4" CBB heavier than needed to stiffen inter joist deflection on the decking so should I use a 1/4 or 3/8" luan ply instead?

Really appreciate in advance anyone taking a peek at the drawings and including your thoughts on how you might tackle.

All the best!
Mike
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Unread 12-19-2020, 10:58 AM   #2
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Welcome, Mike.

While I can't read much of your PDF, it appears you have two layers of 3/8th" (or nominal 3/8th") subflooring on 16" joist spacing. I'm guessing that's not correct, but...........

How 'bout you tell us just what you have for a joist structure - type, size, condition, unsupported span, etc.) and what you currently have as subflooring. Please be specific.
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Unread 12-19-2020, 11:21 AM   #3
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Bummer, was hoping the drawing would make it simple to analyze.

have 2x12 joists, old school built in the 70's.
12' supported span from mud-sill to steel I-Beam

Joists are in great condition. Not hogged up with any penetrations for plumbing/electrical/ducts-work.

5/8 decking, ext grade ply, ring-shank nailed (needs to be screwed in at least some sections), want to over-screw all of it.

Original floor had 5/8 subfloor over decking which I demo'd out. (prior bad tile job).
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Unread 12-19-2020, 11:32 AM   #4
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If the joist spacing is, in fact, 16" on center, your joist structure appears to meet the deflection requirement for a natural stone tile installation.

On top of your nominal 5/8th" subflooring you must add an absolute minimum of nominal 1/2" exterior glue plywood with no face of grade lower than C, then your tiling substrate of choice. Considering that your current subflooring of the absolute minimum thickness is old and damaged, I would recommend the second layer be nominal 5/8ths" as well. Here is a good article from our Liberry on how best to install the second layer (they refer to the second layer as underlayment, which it is not) of subflooring.

[Edit] Actually, I magnified your PDF and now see what you've got and what you intended. No, you cannot do that. Natural stone tile requires two layers of structural subflooring regardless the joist spacing. Your proposed CBU does not qualify as a structural layer.

You can, of course, tile over anything you want, we can only tell you what the tile industry standards require and where the smart money will be betting.

My opinion; worth price charged.
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Unread 12-19-2020, 11:57 AM   #5
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Thanks for the article link!

So per the article diagram and instructions, no quarter turn of the underlayment course? Run in the same direction as the decking?
And screw only not glue and screw?

Thanks
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Unread 12-19-2020, 12:24 PM   #6
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All structural subflooring layers must be oriented with the strength axis perpendicular to the joists.

I am a major fan of gluing all parts of any subfloor structure. One of the authors of that article has told me that with their increased fastener schedule they can get "very close" to the same rigidity as with gluing. And there is some information in the tile industry that indicates there may be some very small amount of uncoupling between those two layers of only mechanically fastened subflooring. Entirely up to you how you install yours.

The primary reason for the double layer subfloor requirement is to avoid any place where the gap between panels extends from the joist top all the way through the subfloor. Whatever you elect to do, just be sure you don't violate that requirement.

My opinion; worth price charged.
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Unread 12-19-2020, 02:29 PM   #7
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Thumbs up Thank you

Really appreciate the feedback and thoughts. Thank you!
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Unread 12-23-2020, 08:17 AM   #8
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Question on glue

I have an additional question on the glue to utilize to help adhere the underlayment to the existing subfloor.

Have researched other posts and have picked up that you do not recommend caulk gun glue delivery (I wouldn't either) and would be interested in a recommendation for product and application.
Assuming that I might find a roller applied product that I could roll out both the existing subfloor and the back of the underlayment sheet as each sheet goes down after testing for fit.
Have no care about time in terms of how long a recommended product may take to set before I should begin tile work. Could take a week or more and I wouldn't care as getting appliances will be dragged out into the new year.

Thanks!

Mike
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Unread 12-23-2020, 09:40 AM   #9
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I'll point out again that the tile industry recommends you do not glue the second layer of subflooring to the first, but that you instead use only mechanical fasteners for the second layer and that said fasteners not penetrate the joist structure.

If you wanna glue yours, my recommendation would be a full spread of Titebond II adhesive. A lot of glue is required and the process is not as easy as it sounds. A gallon of the glue, if memory serves, will do a little more than two 4x8 sheets. Haven't done one of those procedures for a long time.

My opinion; worth price charged.
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Unread 02-22-2021, 08:07 AM   #10
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Looking for input on transition - leveling - WWYD

Morning!

Am prepping a kitchen subfloor for LFT 12x24 slate install and am most of the way complete except for where I transition from the kitchen to the adjoining room.

At that transition, there is a dramatic depression in the decking (5/8" dip down over approximately 30")


My options as I see it are:

1. Leveling at the decking prior to installing the 5/8" subfloor. Potential negative, this will leave a large height difference between the finish kitchen floor and the adjoining room hardwood flooring (probably 1")
>> Assuming that if the best route is fully level I should NOT make up the 5/8' delta with leveling mix alone but potentially build up with underlayment (luan or play and then use leveler over that?)

2. No leveling under the subfloor, instead allow the finish flooring to drop down as a 'ramp' in a sense to transition the finish kitchen flooring into the adjoining room. Potential negative, will introduce potential lippage for a few tiles where I allow for this transition.

3. Split the difference, potentially level away about 1/2 of the difference trying to minimize both the lippage and the drastic difference in finish floor heights.

Some of the reasons that make me lean towards either no or just a little leveling. The transition is constrained mostly to a 'entryway' of sorts between the two rooms. Entryway is approximately 25" long by 30" wide. I *could* simply allow this section to dip down and introduce a cut in the tiles at that entryway line.


Have included pics to hopefully illustrate what I am faced with.

What would YOU do?


Thanks in advance!


Mike
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Unread 02-22-2021, 09:45 AM   #11
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A 5/8" difference is quite a lot, Mike. What is the thickness of the existing subfloor?

The short of it is that you can't do any leveling between subfloor layers. You either level at the joist tops, or on top of the subfloor.

If you are using natural stone you do need the 2nd layer of ply, and it needs to be at least 1/2" nominal - assuming your first layer is at least 5/8".
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Unread 02-22-2021, 10:03 AM   #12
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Welcome back, Mike.

I've combined your new thread with the original thread on this project so folks can see what you're working on and what's been previously asked and answered. It'll help if you'll keep all your project questions here.

[Edit] Which Dan hadn't seen when he posted.

Seems we've got some semantic issues here. I believe what you're calling "decking" is actually your first layer of subflooring and it appears that's all you have over your joists at this point. Is that correct?

You know you need a second layer of subflooring and you know how that must be installed prior to installing whatever tiling substrate you intend to use for your stone tiles, right?

Your tiles care not a whit about your substrate being level, they care only about flat. And those large format tiles care a great deal about being flat. The industry standard is no deviation from intended plane of more than 1/8th" in ten feet nor 1/16th" in two feet. That's a very, very flat subfloor and you'll be glad to have it come time to install those large format tiles.

The best place to level or flatten a wood framed subfloor for a tile installation is at the joist tops. Doesn't appear you're inclined to do that. Second best place in your application is on top of your second layer of subflooring. You do not want anything between your two layers of structural subflooring.

The only two ways I know to achieve that flatness is the use of a mortar bed or a self-leveling compound. Since you don't need level, the use of such materials, well, similar materials, can be executed on an un-level surface, but that can get complicated. There are some cementitious patching materials out there, some of them also SLCs, that can be used without reinforcing mesh on parts of your subfloor to make it flat. I have zero experience with them, but we've got folks on here who have used them.

As for your transition to the adjacent flooring, make your subfloor suitable for your tile installation, then make transitions as necessary.

My opinion; worth price charged.
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Unread 02-22-2021, 12:53 PM   #13
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Thanks for the review and input

Had me guessing what happened to my post CX!

Thanks and I assume it does help to review the issue in context.

Sorry for the wobbly semantics, to help.. yes my first subfloor (what I am referring to decking, directly attached to joists) is 5/8" ply.
My subfloor that I am putting down now is 5/8" A/C ply. Screwing that as per guidance on the edges and in the field of the sheets.
Has gone down well and has resulted in a very flat and rigid subfloor. Very happy with how it has come out.
This is all prep for Ditra mat and then LFT 12 x 24 slate.

So if I read the replies back correctly, I should pursue one of two tacks..

1. Perhaps the 'best', pull up the first subfloor in this section where I have the large deviation from flat. Shim on top of the joists so that the initial subfloor is flat, then install my second subfloor over that like I have done in the rest of the kitchen.

2. Only other alternative, go down with my second subfloor on the area in question and THEN level using a self leveling material to get that area to 'flat' with the rest of the floor, then install my Ditra mat and proceed with tile.

Guess I just came up with a 3rd albeit admittedly wonky solution. I could try an carry the other room flooring (parquet oak) into that entry way and get away from putting tile down in that one troublesome section.


Hmm, more to think about.

Thanks for taking a moment to think on it and provide some guidance back! Truly appreciated!

Mike
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