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Unread 05-21-2022, 12:14 AM   #1
Kevinjm4
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Penny install over iffy subfloor

To be able to install penny rounds over Ditra heat since they normally don’t allow install of less than 2”x2” I got a unique warranty from Schluter if I followed their recommendation.

Flooring is a single layer of 3/4” non t&g plywood on 16OC joists with 2x backing at every break in the plywood. Span of 2x6 joists is 6’. Plywood is probably from 1994 from the last major reno.

They are having me (to maintain warranty):
- self level (Henry liquid backerboard) which will be 1/8”-1/2” give or take...
-ditra heat, and fill the voids in the ditra with all set, then set kerdi sheet over that.
-then the porcelain penny rounds

As I was prepping to pour the self leveler I noticed there were a few spots inbetween the joists that frankly just don’t feel like 3/4” plywood should feel. Unless I’m going crazy, but I could cause about 1/16” deflection by pressing down with my foot in that spot and to a lesser degree a few other specific spots. The rest of the floor felt great... hard to explain this over a post... don’t quite me
On the 1/16” hard to measure...

Should I...
-Leave it because all those layers add stability?
-Add 2x backing right under the specific spots that need it?
-install 1/4” hardibacker
-add 3/8 plywood?

Combination of those or something else?

Ideally I need to keep floor height at a minimum. Obviously would love to not have to add anything if not needed.

Any help much appreciated. Thanks
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Unread 05-21-2022, 02:25 AM   #2
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Welcome to the forum, Kevin!
Quote:
Originally Posted by Kevinjm4
…Should I...
-Leave it because all those layers add stability?
The extra layers are merely spreading the point load from small penny rounds to a slightly larger “footprint” so that they don’t collapse into the air spaces of the DitraHeat.

The industry standard for how much the overall floor can bend, or deflect, is L/360. That translates to 1 unit of measurement of bending down for every 360 units of measurement of its length. Doing the math, you initially might start asking yourself if this weak spot is spread over at least 22.5”. But, you need to step back a second and realize that this appears to be a weak bendy spot in a floor. Localized deflection between joist bays is a big culprit of tile floor failures.

Quote:
Originally Posted by Kevinjm4
-Add 2x backing right under the specific spots that need it?
That depends. It’s time to figure out what is causing this localized deflection and correct it. It may be a weak spot in the plywood (like it was damaged from a heavy point-load blow before or after installation). It may be an air gap under the ply along the joist. It might be an air gap under the ply at the blocking of one of the seams (it’s very easy to have an air gap here). It may be a joist connection that is not dead-solid. This is a structural issue of the plywood’s strength integrity or the underlying support. Merely adding 1/4” thick Hardiebacker will not add any strength to the floor. Strength comes from the plywood and the supporting structure below. And while adding plywood is usually a very good thing to reduce the localized effects of bending at the plywood seams of a single layer subfloor, I encourage you to focus on correcting your localized weak spot at the root cause before moving forward. Correcting whatever is weak at its oven level is also very likely to eliminate additional layers that you’re working so hard to keep to a minimum.

When figuring stuff like this out, I find that it’s best to start simple. Let me ask you how your plywood is fastened down. If you’ve added any fasteners recently, what were they and how did you install them? (Like: deck screws, 2” long, and pre-drilled)

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Unread 05-21-2022, 07:17 AM   #3
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Welcome, Kevin,

Have to admit that I'm not quite sure what the purpose of the self leveler under the Ditra Heat is. In my thinking the point loads are on top of the DH. Perhaps Schluter is more confident of the bond between the DH and SL than between the DH and plywood. I don't know that a 1/8 to 1/2" thickness of SL would lessen the plywood deflection between the joists.

Regardless, if Schluter is signing off on it...

Because the pockets and channels of DH are relatively deep it is likely that the mortar used to pre-fill it will shrink, leaving little divots at each pocket. Mine did. Because you'll want it as flat as possible, and because you'll want to use a small notched trowel to set those penny's to avoid mortar squeezing up between them, you might consider pre-filling a second time if you notice that the first fill did shrink.
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Unread 05-21-2022, 08:56 AM   #4
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Bubba, the weak spots im speaking of are in no mans land (no joists under, no backing, not at a seam, just the middle section of a sheet of plywood). You may be right, could have been prior damage.

I glued and screwed with 2” and 3” deck screws the repairs that were made as I opened up the floor for access. Everywhere else (along all joists and seams and backing) I screwed without pre-drilling 2” and 3” deck screws.

Dan, thanks for the tips on the install. Yes, Schluter signed off on it but they didn’t know about theissues with the plywood. They assumed it was structurally sound, I’m assuming. The SL was merely at my request I don’t think it has any structural part to play in all this,unless it does. That at least wasn’t Schluter’s intention with the SL I can tell you that much.
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Unread 05-21-2022, 10:07 AM   #5
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Is your abbreviate of SL short for ‘self leveling cement’?

And when you say you opened up the floor for repairs, is the single layer of plywood a single piece of plywood spanning multiple joists or is this a small piece? If you’ve got a picture, that might be helpful.

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Unread 05-21-2022, 10:23 AM   #6
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Yes SL self leveler. I’m not sure if it’s cementitiois but it’s Henry’s liquid backerboard.

The floor has been pieced back together jigsaw puzzle style. Probably 8 individual pieces total in varying sizes make up the floor for this 8x8 space. Not one full piece. I cut a few holes strategically (along joists) for access for plumbing and just replaced those with either new pieces cut to fit or put the old one right back in and glued and screwed.
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Unread 05-21-2022, 11:15 AM   #7
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Those joints over the joists are weak spots. They allow more downward diflection because there’s nothing to resist the counter joist bay’s upward deflection.

Image for the sake of this discussion that you’ve got a room with bare joists. Now lay a 1’ wide x 48” long piece of 1/4” plywood that spans exactly three 16” O.C. joist bays. It’s there with no fasteners. Step into the middle joist bay and it deflects down under your weight and the two end rise into the air unrestrained (this is almost how your little patches of plywood are acting that only span a single joist bay). Now let’s do a second experiment and fasten the same piece over the the three joists. Now stand in the middle and you’ll see that it deflects down less because the continuous piece of ply counteracts the downward deflection of the center joist bay through its resistance to upward deflection of the two outside bay. Also, when we compare to the first example, we see that a lack of tensile strength through the seams (because it’s a joint) contributes to its downward deflection through the center bay.

….AND, to top it off, the relatively severe bending that is concentrated over the plywood seam above the joist (due to live and dead loads) has been a huge contributor to tile cracking on a single subfloor tile assemblies. A second layer of plywood with staggered seams reduces this type of deflection. But before we run off and tell you to add a second layer of plywood, it would be smart to see what you’ve got. With your 8’ x 8’ area, that would normally require only 2 or 3 pieces of plywood to cover everything and you’ve got 8 pieces! I’d hate for you to throw good money after bad if your single layer is so chopped up.

Can we see a pic?

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Unread 05-21-2022, 11:50 AM   #8
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Penny install over iffy subfloor

Here’s a pic. Joists running L to R. Don’t have one prepatch.. but I’m sure a pro can see what’s going on. Also 8 pcs an estimate


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Unread 05-21-2022, 02:31 PM   #9
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ID: Subfloor exterior grade

I can’t find written anywhere whether my subfloor is exterior grade or not. I checked top side and underside... No symbols numbers letters logos etc. is there another way to tell?

My point with asking this is I was told in order to use a self leveler the subfloor needs to be cdx(?). But even if it isn’t exterior grade it can be primed to overcome that. I have Henry 542 (cementitious) liquid backerboard I would like to use for my self leveler, and Henry 554 level underlayment primer. I have already applied Henry 549 feather finish to any screw/nails and seams/voids before I realized I should check on if my plywood is ext..

Upon checking tds it doesn’t mention anything about needing to be exterior plywood, nor anything about if the plywood isn’t exterior that this primer will protect it.


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Unread 05-21-2022, 03:54 PM   #10
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Hey, Kevin. Welcome to the forum.

Let's keep all questions related to this project on this thread so that questions and answers aren't duplicated, and the history is in one place.

I can't tell where all the seams are, but at least the plywood appears to be oriented properly.

Although unlikely, I could see how there could be up to seven different 4x8 sheets in that space.
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Unread 05-21-2022, 04:21 PM   #11
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Sorry I was trying to post about the plywood grade on a new thread... I’ll try again


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Unread 05-21-2022, 04:22 PM   #12
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ID: plywood exterior grade?

I can’t find written anywhere whether my subfloor is exterior grade or not. I checked top side and underside... No symbols numbers letters logos etc. is there another way to tell?

My point with asking this is I was told in order to use a self leveler the subfloor needs to be cdx(?). But even if it isn’t exterior grade it can be primed to overcome that. I have Henry 542 (cementitious) liquid backerboard I would like to use for my self leveler, and Henry 554 level underlayment primer. I have already applied Henry 549 feather finish to any screw/nails and seams/voids before I realized I should check on if my plywood is ext..

Upon checking tds it doesn’t mention anything about needing to be exterior plywood, nor anything about if the plywood isn’t exterior that this primer will protect it.


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Unread 05-21-2022, 05:12 PM   #13
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Hi Kevin,

Kevin (Kman) was trying to tell you to keep your questions together. He merged your two threads into one because that’s our policy on this forum. You might not realize how certain things are related, but we’ve come to understand that having the whole project as a single thread is far easier for the all-volunteer staff to grab background info. In most cases, this is very true. In this case, there isn’t much background info to obtain on the ply. But in the same line of thinking, if Schluter knew how patched together this single layer floor was going to be, they wouldn’t have bestowed their blessing on it for the penny rounds over DitraHeat.

The floor is far too segmented for me to consider tiling over it with a single layer. If keeping height to a minimum is an important feature, I’d replace all those pieces with 2 large sheets of tongue & groove 3/4” plywood. If you want to proceed as is, it’s your house and your money to make the final call.

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Unread 05-21-2022, 06:50 PM   #14
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Well that is disappointing but I appreciate the honesty. I may still go forward... But there’s another small format tile that is a bit larger (2x2) which is an option... link below.

If this is also too small, do I basically need to just install a larger format tile? Or is it the case that no matter what tile is used the floor is compromised and needs to be addressed regardless of tile choice. If that’s the case what are my options to keep it at a minimum thickness added.. 3/8, 1/2” max..

https://www.homedepot.com/p/Daltile-...C1P2/100556713


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Unread 05-21-2022, 07:34 PM   #15
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All tile, irrespective of its facial dimensions is brittle and will crack if not supported from below. The size of the tile is not the main problem. The chopped up single-layer subfloor is the main problem.

Send Schluter the picture from post #8 and draw dotted lines at all the plywood joints and watch them freak out and rescind their special warranty. They will likely be okay if you add 3/8” or 1/2” ply over everything if installed to their specs.
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