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Unread 09-26-2017, 03:09 PM   #1
36lpi
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Subfloor Thickness - The Classic Question

Hello, and thank you in advance for any thoughts or suggestions.

Here's what we're starting with:
  • A small 5' x 8' bathroom on the second floor
  • There's a supporting wall just below it and right in the middle of the space
  • House was built in 1979
  • Floor joists are 2x10 and 16" OC
  • Existing subfloor is 19/32"
  • We removed a large section of the existing plywood (not all of it), because we had to reroute some of the plumbing
  • Added 2x4 blocking every 12-14" between existing joists
  • Replaced the removed section of plywood with new 19/32"

Question:

We'd like to use a larger format tile (maybe 12"x18"), and was wondering if we should glue and screw another layer of 1/2" plywood over the the new 19/32" and then a layer of 1/8" Ditra. We're trying to get as close to the 3/4" height of the adjacent hardwood floor as possbile, but are willing to accept being a little higher, than having a weak floor, and cracked tiles.

-OR-

Can we do without the additional 1/2" plywood, and just use 1/4" or 1/2" backerboard?

Thanks again,
Corey
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Unread 09-26-2017, 09:17 PM   #2
Tool Guy - Kg
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Welcome to the forum, Corey.

Get ready to read a book. These are my thoughts off the top of head, so they might not be in perfect order or address all your questions in perfect order.

Tiling successfully has a lot to do with how well each component ultimately performs in contribution to the "whole". Because it's not likely that everyone is going to start with perfectly uniform materials...or perform each individual task perfectly, some "elbow room" is built into the process. And that's done in terms of trying to "overbuild" each component or process along the way to help the finished product perform as expected. So, the joists, subfloor, subfloor seams, tiling substrate, bonding mortar, tile material, grout material, and skill to install each of them all come into play.

In the case of tiling, it's good to know that the tile is brittle. It doesn't have much strength on its own. It relies on support from below. In this case, plywood is one of the biggest factors as it provides the subfloor's strength and stiffness. Cement board doesn't play into structural values (hence the reason it doesn't help to install thicker cement board over thinner cement board). So, the plywood is key. Now, the question is this: can 5/8" plywood in excellent shape and installed properly (strong axis of ply perpendicular to the joists and fastened with a proper nailing schedule) perform as a good substrate for tile, providing that it's installed over sufficient joists....and a proper tile substrate is installed over it? Yes, it can. So, we got our answer, right? No, we don't. You have to look at the whole.

Let's look at your subfloor. Will a floor that's got a lot of plywood pieces fitted together (you said you've got some plywood pieces fitted together) act the same as a single sheet of ply? No, it won't. It's not unified. Why does this matter? Because a lot of individual pieces of subfloor will act like a bunch of tiny islands all smooshed together rather than a single land mass. Each "island" expands and contracts on its own. And each island doesn't support the next. So, if you look microscopically, you'll see that the dynamics of subfloor movement (flexing and expansion/contraction) are changed when the subfloor isn't unified....or to use a term you might hear a lot in tiling substrates, it isn't "monolithic". It doesn't act as one.

So, that brings up the question of a second layer of ply. A second layer ties all the "islands" together into a much more monolithic substrate. So, is it needed, or is it overkill? Well, to answer that, you need to look at what the first layer is and how well it's supported. From your description, it initally sounds to me like you've got lots of support with the 2x4 cross members. So, can we assume your cross members are doing enough to eliminate the additional flex normally encountered with a chopped up plywood subfloor? We'd need to go even deeper and understand microscopically what excessive movement we're in danger of encountering that might cause the tile to crack. And the fact that you've got a supporting wall right below this may prove a curse if you've got a plywood seam parallel and close to it. You see, that seam, being atop the wall support, will want to split open when loads are applied to the floor on either side of it. That is a force that will work to crack the tile over this seam. But this is a difficult measurement to quantify.

But one such measurement of movement that we can define is "deflection". This is the amoung of bending (vertical movement on a floor, for instance) that can occur to the entire tile assembly (tile/subtrate/subfloor/joists) without causing the tile assembly to fail (crack or lose bond). You see, vertical movement that occurs beyond a mathematical curvature of more than 1 part of vertical flexing for every 360 parts of horizontal run, you're in danger of a tile failure. To bring this into easy numbers, you could have 1" of downward drooping deflection of the tile assembly in a tile floor that was 360" long without the tile being in danger of it failing/cracking. You'll hear this expressed as "deflection not to exceed L/360". For man made tiles, the second number should be at least 360. For natural stone tiles, this number needs to be at least 720. The higher this number, the stronger/stiffer it is. Take a look at the Deflectolator. It's a quickie calculator that gives you a quick reference for how stong/stiff your floor joist structure is. There are plenty of houses built where tile isn't a good idea because the joists are too flexy. This L/360 deflection is a very typical minimal standard the building industry. Empirical data shows that if you're installing tile over a floor at least this stiff, you can install man-made tiles without the joists being too flexible for the stiff, brittle tile. And it's important to note that deflection occurs both along the joists (what the Deflectolator will tell you), and "between joist" deflection...that is, the amount the plywood bends in the span between the joists. This between joist deflection is mostly a product of the plywood thickness. The thicker the ply, the less the "between joist" deflection. The industry standard of using a minumum of 5/8" plywood subfloor thickness is due to the between joist deflection.

Ok, let's get back to the cross members. Since you've apparently got them all over the place, their intended function is two part. One, to support the otherwise unsupported plywood seams. The second will be to reduce "between joist" flexing that otherwise occurs without them. Alright...now let's get super critical and look at how the cross members were installed. We could look at how tight they're fitted between the joists...how well they are fastened....how dead flat are they in relation to the joists....But it might be best to sum it up in this question: Are the cross members joined in such a way that they are "one" with the joists, or is there "give" that would allow the plywood to flex?

Cripes! What's all this mean?

If all 9 aspects of the installation are true, you're a candidate for a single layer 5/8" floor. But realize, this is the industry's bare bones, dead minimum. There's no wiggle room. You're right on the edge of success and failure.
IF:
1) You're joists are at least L/360,
2) Joists aren't spaced more than 16" O.C.;
3) You don't have a plywood seam parallel and relatively close to the supporting wall below,
4) Your plywood is installed with its strong axis perpendicular to the joists and with a proper nailing schedule,
5) Your cross members are installed well enough to eliminate all the flexing that occurs due to multiple joints withing the subfloor,
6) You install any of your tiling substrates properly (whether it be Ditra or otherwise) properly and follow ALL their rules,
7) You use a good quality bonding mortar appropriate to the tiling substrate and follow all the rules on mix, slake, pot life,
8) You clean the backs of the tile, as appropriate, and burn a coat of bonding mortar to the tiles immediately before installing,
9) And you grout properly.
Then yes....you can use a single layer of 5/8"
That being said, a single layer of 3/4" subfloor provides a lot more strength to tile assembly and would be an upgrade. I'd prefer removing the 5/8" layer in favor of a single layer of 3/4" any day. But the configuration of your joists in relation to the walls and tub or whatever else may make this impractical. As an alternative, a second layer of 1/2" ply added to your 5/8" (Installed properly and WITHOUT construction adhesive. Please ask if you intend to use an adhesive and we'll steer you right) is even better because it adds not only extra thickness (strength), but also unifies the subfloor and second layer of ply into a more monolithic layer.

P.S. You still awake?

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Unread 09-27-2017, 06:44 AM   #3
36lpi
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Thank you, sir! I really appreciate the thoughtful answer and all of the details.

I wanted to remove al of the 5/8" ply, but there's a section of it that covers an area we spray foam insulated. It's spray foamed, because it goes out over the entry way below. If I removed it, I'd more than likely pull a lot of the spray foam below, along with some electrical.

I think a sheet of 1/2" ply over the top, and 1/8" Ditra is the way to go. My blocks are tightly placed and fastened, and I think this will make a solid floor.

Out of curiosity, why do you say NOT to use glue when attaching a layer of ply? And am I correct to say that you shouldn't attach the additional layer of ply to the joists, only to the layer of ply below?

Thanks again,
Corey
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Unread 09-27-2017, 02:14 PM   #4
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The only way gluing is acceptable in that scenario is if you spread the glue fully over the entire sheet with a trowel. Most people would squirt out lines of glue over the floor, causing little voids between the lines of glue. Those voids could translate into vertical movement, which you don't want.

Just screw the top layer into the bottom layer only by either mapping out the location of the joists and intentionally avoiding them, or using screws that are no longer than the thickness of both plywood layers
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Unread 09-27-2017, 02:52 PM   #5
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With lots of blocking, it probably isn't an issue, but when patching plywood, your piece will ideally attach to at least three joists so the middle is supported...otherwise, it will be relying entirely on the fasteners to keep it from moving towards the middle when weight is applied.
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Unread 09-29-2017, 05:53 AM   #6
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Thanks, everyone!
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Unread 09-30-2017, 02:51 PM   #7
36lpi
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If you're still with me, I have one more question:

The corner of the bathroom, over by the toilet, isn't level. It drops about 1/2" +/- in a span of 36" x 36". It dips towards the corner just after one of the floor joists.

Should I put leveler down on top of the existing 5/8" ply, and then add the layer of additional 1/2" plywood, or after the 1/2" ply? Again, I'm adding a layer of 1/2" ply before I put down the Ditra. If I do if after, I'll be adding the Ditra on top of the leveler.

Thoughts? Thanks in advance!
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Unread 09-30-2017, 05:06 PM   #8
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Level the floor after you've added any subflooring you want. Unless you can level the joists before any subflooring is installed. The levelers may not hold together or may debond when you run a screw through them to anchor the top layer...it must be on top.
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Unread 10-01-2017, 06:54 AM   #9
36lpi
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Thanks, Jim.

What about the toilet flange? The hole hasn't been cut yet (it is measured though)... If I add leveler in that area, the toilet flange will have to sit on top of it. That's fine, but we'll then have to drill through the leveler to fasten the flange.

I had an idea of of using some Laticrete 255 Multimax as a leveler in that corner. Again, it's only about 3' x 3' max with the lowest point being about a 1/2". This Laticrete allows for a 1/2" bed, and I was going to trowel that out, and then lay my 1/2" ply over the top while it's still wet. I was then going to screw it down like you would hardi backerboard. Again, while it's still wet.

I thought this would be a good approach, because it would give me a solid surface to attach the flange and start my tiling on.

Bad idea?
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Unread 10-01-2017, 01:56 PM   #10
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All toilet flanges are designed to sit on TOP of the FINISHED floor. Yes, there are lots of them attached to the subflooring, but that's not how they were designed. So, when working with it new, I'd plan to install it there. When cutting your tile, you'd want to notch the tile for the screws so you don't have to try to drill through it later (but it can be done, but usually works best with a diamond core bit - a tool you may never need to use again).

IMHO, the reason so many toilet flanges are installed on the subflooring is because it is cheaper to have the plumbing done as much as possible when building a house, and then, they don't have to call the plumber back for that task and maybe mess up the finished floor, or worry about timing. IOW, it's a convenience, but not how they are intended to be used. FWIW, do not use an all plastic flange...you want one with a metal ring made out of SS.

Is the drain pipe 3" or 4"? If it's 4", you could use an internal mount flange. That means, you don't have to leave a gap around the pipe to be able to slide the flange hub onto the pipe...it glues on inside. In that case, you can leave the drain tall while you work on the floor, then cut it off after the floor tile is installed. While they make them, it is not recommended to use an internal mount flange on a 3" pipe - there are a few toilets that actually have a 3" throat on the outlet...using that type of flange cuts the ID by 0.25x2 or a half an inch, then, trying to make that last turn to the drain can often cause clogs or fit problems.
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Unread 10-02-2017, 05:45 AM   #11
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Thanks again, Jim! I really appreciate your time and knowledge.

I love doing these types of projects, but I obsess over doing them the best way possible. It seems that a lot of projects present their own unique challenges, and often require a unique solution.

After a little more research, I have two concerns around using leveler and then tiling over it.

1. I have to level around the toilet, and my understanding is that leveler doesn't create the best bond with plywood, and tiling over it could be problematic if there's the tiniest bit of flex in the floor. If there's ever going to be the slightest movement, it's probably going to be around the toilet.

2. If I did use leveler, I would have to screw the flange through it, and as you said, the leveler will break with screws.

I'm still still trying to figure out a good way to level that area before attaching the additional 1/2" ply. If I can do that, then the next layer of substrate (Ditra or Hardi backerboard) will have direct contact with a solid surface.

I was planning to use Ditra because I figured I'd save about an 1/8 of an inch in additional floor height, but I wonder if I should just use 1/4" Hardi, and then adjust the floor height difference with a little extra thinset underneath it.

I also thought about ripping a bunch of shims to place under the 1/2 ply to level it out.

Considering all of this, what would you do?
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Last edited by 36lpi; 10-02-2017 at 06:07 AM.
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Unread 10-02-2017, 06:49 AM   #12
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Level Before or After additional 1/2" Ply

First off, many thanks to Tool Guy - Kg and jadnashua on a previous post.

We’ve gutted our 5’x8’ bathroom on the second floor, and reinforced the existing 5/8” subfloor as best as possible (cut our old 5/8", added new, and added a bunch of blocking between joists). For extra support and a more monolithic layer we’re also adding a layer of 1/2” ply before tiling. Note: We want to use large format tile.

During this process we found that the corner of the bathroom, over by the toilet, isn't level. It drops about 1/2" in a span of 45” x 30”. It dips towards the corner just after one of the floor joists.

I’m reluctant to use leveler before tiling, because my understanding is that it doesn’t have the best bond with plywood, and if there were any flex, it would start to break. It would also mean we’d have to screw though the leveler for the toilet flange, and the leveler would more than likely break.

My ideal solution would be to level that corner before adding the 1/2” ply. That would give my tiling underlayment (Hardi or Ditra) a nice surface to bond to, but I’m not sure what the best method would be.

Here are some ideas?

1. Use 1/4” Hardi and compensate for the dip with a little extra thinset in that corner.

2. Rip a bunch of shims to place between the exiting 5/8” and the additional 1/2”

3. Trowel out a small bed of Laticrete 255 Multimax in between the 5/8” and the additional 1/2”, and then screw the 1/2” down into it.

Thoughts? Thanks!
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Unread 10-02-2017, 06:53 AM   #13
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Hi Corey,

Just make very sure there are no voids at all between the two layers of plywood. None.
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Unread 10-02-2017, 07:32 AM   #14
36lpi
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Thanks, John!

I'm just trying to figure out the best way to avoid the voids. What do you think about my Laticrete 255 idea?
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Unread 10-02-2017, 10:22 AM   #15
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Level Before or After additional 1/2" Ply

Option 2 and 3 is good. Use the shims for spacing and lay your plywood on top. Check grade. When you have it right then remove the top layer of plywood and fill with the laticrete so you have a fill with no voids. Reinstall the top layer of plywood. P.S. I often use sheetrock screws for height gauge. Put the screws in every 6 to 12 inches. Screw them up and down as needed to get the top layer of plywood right then fill in with your laticrete. That way you will not have to worry about the shims moving and the screws are fast and easy to adjust in height. Screw down the top layer of plywood when done while the laticrete filler is still wet.
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