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Davidjames1 05-03-2021 09:33 AM

Another Tile on Plank Subfloor!
5 Attachment(s)
Hi all, Im newly registered but have gained lots of advice over the years via this forum!

Here is my situation, Im in the middle of a full remodel of my 40s house in Oregon. Doing as much of the work as I can myself. Im a woodworker by trade, but also really have a passion for all trades and a massive respect for pros who are in the trades.

About 8 months ago I hired a close friend who is a liscened contractor to help with installing durock in my kitchen in preparation for tile. To be honest I wasn't well versed in tile substrates at the time and trusted the contractor to choose wisely when working ( :bonk: ) 8 months later (yesterday) and Im cutting tile and getting ready to instal 12x24 porcelain, and I keep having this nagging thought of "is my substrate sound for tile?". Short answer of course it wasn't... I just didn't catch it in time.

Here Is what he did on instal of the durock. Stripped all existing flooring down to the 1x7 plank flooring, re-screwed plank (I think...) and I remember buying him the thinnest for the under the durock. He troweled the mortar and nailed the durock to the plank flooring. If you go under the house in maybe a few spots you can see some mortar that seeped through the t&g...
He never taped the seams, just mortared them. So a few days ago I taped them.

My floor in this particular room has a deflection of L458
2x10 x 13' Span, 16" on center. Really no noticeable deflection when giving floor a hard stomp with heel.

The room beyond through the smaller door opening was added on later on to the house and is osb with durock on it (not an ideal substrate either) but I'm less worried about that.

So my question, am I totally screwed here? Will the expansion of the plank in the long run give me cracked tiles and even more of a headache? Everything I read on this forum leads to "YES" I really am of the mindset of doing things right, and it really keeps me up at night when others don't :drool2:

cx 05-03-2021 10:04 AM

Welcome, James. :)

If you don't add that geographic location to your User Profile the information will be lost before we leave this page.

You apparently know the technically correct answer to your question. USG - may they rest in peace - would not sanction the installation for their Durock product. The diagonal installation of the subflooring boards is also not in your favor. That said, it's all a matter of risk tolerance. Is your floor tile installation more likely to fail than it would over a proper subfloor and substrate? Yes. Is failure of the tile installation guaranteed? No.

I'll point out that one CBU manufacturer (James Hardie) once recommended installation of his Fiber/Cement panels over sawn board subflooring, similar to your situation. I've never seen that in writing, but I trust my source. Point is, James Hardie no longer recommends such an installation for Hardiebacker, nor do the manufacturers of any other CBU of which I'm aware. I figger there's likely a reason for that.

The lack of the appropriate fiberglass mesh tape on the CBU seams can also be problematic, 'specially with the installation over the sawn board subflooring.

Originally Posted by James
The room beyond.....is osb with durock on it (not an ideal substrate either)

Not sure what your problem is with that installation. If the joist structure is adequate, particularly the spacing, and the OSB style and thickness are adequate, and the Durock was properly installed, I don't see a problem there at all. What am I missing?

[Edit] Now that I see the rest of your photos, the grade of those joists is a bit suspect to my eye.

My opinion; worth price charged.

Davidjames1 05-03-2021 10:43 AM

Thanks for you reply, I know your opinion is highly valued on this forum.

When the contractor installed the boards he didn’t tape, but I came back and taped them, just to be clear.

Are you concerned with the knots in the joists? I noticed that to and though of maybe sister-ing a joist to it.

Regarding osb in the other room, I believe I just read to many forums comparing ply to osb, I’m glad to hear it is of no concern to you.

cx 05-05-2021 09:42 AM

Depending upon just how the CBU joints were initially filled, the after-the-fact taping may or may not be effective.

Yes, the knots are problematic, both from their size and location on the joist. And we can see only a small sample of the entire structure.

I make no distinction between good quality OSB and plywood for subflooring unless you are trying to bond something to it, in which case I always favor plywood.

My opinion; worth price charged.

Dave Gobis 05-05-2021 09:53 AM

When cement board first came out they also recommended over board floor but discontinued the recommedation after a high failure rate. We did some and one had a minor problem. So failure is not a given but a risk.

Davidjames1 05-05-2021 12:12 PM

One concern of mine is also the transition into the next room. It is osb. Wondering if the slight expansion/contraction through seasons will be different from the two and possible cause issues in that door way.

I’m leaning toward cutting it and the plank all out and laying osb. Sadly it’s $81 a sheet now GAH

Davidjames1 05-07-2021 02:06 PM

Ok so here is my plan thus far, I would love advise

I’m going to rip out this floor, take plank flooring out. and lay 3/4 osb, and 3/8” plywood atop following the information in the subfloor pdf posted here. 3/8” sheet would be laid following the 1/4” point from joist. And lastly I’d lay 1/4” hardi on top. I already have a small slop coming out of the next room so having the extra 1/8” of height will help in my favor.

Is the 3/8” ok to use? I know you normally recommend 1/2” atop the 3/4 sub

Lastly I normally use adhesive for the osb to joists. Anything recommended between 3/8” and 3/4” osb other than screws?


cx 05-07-2021 03:16 PM

Why not consider adding nominal 1/2" plywood to the entire floor, both sections, just as they stand now, David?

Whatever you choose, you don't wanna use any sort of adhesive between the additional plywood and either the board subfloor or an OSB subfloor. Just screws.

I don't favor using the nominal 3/8ths" plywood in any case. Too difficult to find any that's sufficiently flat and then too difficult to get it fastened flat to the first layer of subflooring.

My opinion; worth price charged.

Davidjames1 05-07-2021 03:24 PM

Yeah after researching 3/8, your right, hard to find!
1/2” on 1/2” and 1/4 hardi will keep my height. And will have a solid subfloor. Thanks!

CX you suggest external ply/rated glue with both sides C minimum right?
Would you still suggest adhesive to the joists on bottom sheet?

Lastly a modified thinset 1/4” trowel and taped seams for hardi on subfloor?

cx 05-07-2021 03:28 PM


Originally Posted by David
...1/2” on 1/2” and 1/4 hardi will keep my height.

If I'm understanding that, please don't. I would never start a subfloor with anything less than nominal 3/4" material regardless what I'm putting on top.

My suggestion was to put nominal 1/2" plywood on your board subfloor and on your existing OSB subfloor.

Are we communicating now?

Davidjames1 05-07-2021 03:39 PM

I’m sorry, you lost me!

We’re you saying once ripping out the the plank flooring, still laying down 3/4 osb to the joists and then 1/2” ply atop?

Might I add going into the next room, the floor deflection was better due to floor joists (was built after house on a slab, likely the garage) and as I mentioned earlier we put down 3/4 osb below the durock. It’s solid.

It’s just this section in the kitchen that is original plank. Under the durock

All that to say, I can get 1/2 CCX locally. Would that be a good way to go? 3/4 osb on the joists. 1/2 ccx on top screwed spaced correctly. And 1/4” hardi on top of the 1/2”

cx 05-07-2021 05:30 PM

Perhaps someone else will wade in and unconfuse us, David.

If you remove the sawn board subflooring and replace it with nominal 3/4" T&G plywood or OSB, you technically don't need the second layer of plywood. That presumes your joists are spaced no wider than 16" on center.

My suggestion for the nominal half-inch plywood was if you do not remove the sawn board subflooring.

Davidjames1 05-07-2021 05:57 PM

Understood, I think I follow now.

Because the durock is mortared to the sawn board, I’m leaning toward ripping it all out and laying the 3/4 osb. And maybe just going back to 1/2” durock.

Sorry for the confusion.

Kman 05-07-2021 09:27 PM

Unless you just need the extra 1/4" height, there's no advantage to using 1/2" cbu instead of 1/4" cbu.

Davidjames1 05-12-2021 12:12 PM

Sorry for the changes.

So I started the tear out of my plank flooring and cbu

What I’m replacing it with is 3/4 osb to the studs and possible 3/8” ply on top and than cbu. I need the extra height which is why I’d stack the 3/8”, I would glue and screw but not into the studs.

Would a ACX 3/8” plywood be ok on the osb? And than cbu over it with thinset?

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