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Old 09-08-2018, 06:53 AM   #1
Longshot_HN
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Floor Deflection Question

So I've demolished my bathroom and now I am looking at the floor. This is my first DIY bathroom. The good news is that it is pretty flat and level because it was constructed with engineered beams. Right now there is some 3/4 inch T&G Plywood on the floor a little gouged here and there, but mostly in okay shape.

I am working with a GC to help me in certain aspects of the process and he said even if the floor is a little less than perfect with some gashes and gouges, work with it because it is probably glued to the engineered beams and damaging one of them is going to cost me a lot. However, I am trying to figure out deflection and these beams are Georgia Pacific GPI 25 Engineered beams 11/ 7/8 deep, 16" o.c. It looks they were designed with a L480 deflection goal for installation. I found some literature at https://sherwoodlumber.com/wp-conten...ists-Specs.pdf but it does not list a GPI 25 product, but I'm guessing this is some variation on the 11 7/8 GPI 20 that is listed in the brochure.

The total span of the room is 11'. I will be installing large format wood look ceramic tile so I don't need the deflection requirements of a natural stone thank god.

Now, I know that is a perfect world I could glue and screw a 1/2 piece of plywood to the floor and everything would be plenty solid, but the build-up will likely be a problem for a freestanding floor tub filler placement (too much floor build up, can't attach fixture to valve. What those engineered beams give me in floor flatness and levelness they take away by being a pain to work with and being just in the way.

So what I am hoping I could do some build-up(which is aesthetically desirable in meeting the floor of the ajoining room) by either adding 1/4 cement board or 1/4 ply, then 1/8 Ditra, then tile. Would this be adequate to reach above the minimum deflection I need for LFT Ceramic or would I be taking too big a risk of failure. And do you think plywood or cement board for that extra floor thickness? I just want to make sure I'm comfortably above the L360 deflection minimum.
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Old 09-08-2018, 07:34 AM   #2
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I think you're mixing your deflection requirements, Aaron. There is a requirement for joist deflection and a separate requirement for between-joist subfloor deflection of the subflooring. Fixing one does not necessarily fix the other.

The size of the room to be tiled is of no consequence at all in determining the joist deflection. It's the unsupported span of the joists that counts. It would be rare for an engineered joist system not to meet the minimum L/360 deflection minimum requirement, and it's more common for it to be designed to meet L/480, especially if longer spans are involved.

Adding subflooring does nothing to improve the joist deflection, but it makes a major difference in the between-joist deflection. While your current subfloor design meets minimum standards for any ceramic tile substrate with which I'm familiar, a second subfloor layer would be prudent to compensate for the damaged condition of your existing subflooring. Your 1/4" plywood would not be acceptable and your suggested CBU adds no structural strength at all. The absolute minimum plywood you can add is nominal 3/8th" and I would recommend you go no lower than 1/2" exterior glue plywood.

I would not find it reasonable to limit your subfloor thickness to accommodate some particular plumbing fitting. You can either get the necessary fitting extension from the fitting manufacturer or a different fitting. Absent that, you could drill adequately sized holes in the existing subfloor before installing the additional subflooring to accommodate the fitting(s) and the required fasteners.

As for the overall floor height, make your subfloor suitable for the ceramic tile installation first, then make transitions to adjacent flooring as necessary.

My opinion; worth price charged.
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Old 09-08-2018, 08:54 AM   #3
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Yeah, I was mixing the 2 deflection requirements. So with 3/8 vs 1/2 inch ply, I assume I'm not going to know how big of a risk 3/8 is unless I wanted to try it and find out? I'll have to see about the plumbing to see if it can be extended.

There's some gouges in the 3/4 ply that's not already down. A couple inches here and there nothing of any particular large size or depth, should I fill those in with a feather finish product or something else before adding the additional layer of ply just to eliminate any possible voids. Then glue and screw, put on the Ditra and then tile?

Thanks for the help, I'd rather here bad news now rather then find out the hard way later.
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Old 09-08-2018, 12:12 PM   #4
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Quote:
Originally Posted by Aaron
There's some gouges in the 3/4 ply that's not already down.
A bit confusing, that.
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Old 09-08-2018, 12:57 PM   #5
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Sorry

"There's some gouges in the 3/4 ply that's not already down."

should be...

"There's some gouges in the 3/4 ply that's already down."

The 3/4 is in okay but not perfect condition. I wanted to take it up to get a pristine working surface, but because of the risk to the joists of taking it up, the GC I'm working with on my bathroom project thinks I should leave it alone and it would be plenty strong enough for ceramic tile. He does tile and probably what he does will work most of the time just fine (decades of experience), but I don't think he is always TCNA compliant. So while his advice is probably reasonable and he's been a good reliable GC, I don't know if I'm sure it's optimal because he wears so many hats. I did not have an in-depth conversation about deflection, just enough to get the feeling that he was not worried about it.

The Cement board over the 3/4 plywood was his suggestion, filling in the gouges with thinset as I lay the cement board. He didn't think the gouges would be significant enough to matter.

If it was pristine, would it be sufficient without the 1/2 inch? Is the damage which is relatively light why I need the 1/2 inch in your expert opinion? And if that is the case, the more damaged the more I need the 1/2 inch?

I'm leaning towards just doing the 1/2 inch at this point and making everything work around that. Would I fill the gouges with anything or just lay 1/2 inch on top and glue and screw?
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Old 09-08-2018, 01:36 PM   #6
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Aaron, you need to understand that the 3/4" plywood over 16" joist centers is acceptable to all tile substrate manufacturers, but all the testing for that is done with new material, in perfect condition, perfectly installed over joists with zero deflection. And the material needs pass the test only once.

If you're comfortable installing a CBU over what you've got, do that. I'd personally want the second layer of subflooring.

I would recommend you fill the larger damage areas before installing the second layer of plywood. I can see the damage, but I'd likely use something like water putty. I would not recommend you glue the second layer, but if you do it must be with a full spread of wood glue, not something out of a tube.

My opinion; worth price charged.
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Old 09-08-2018, 03:09 PM   #7
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I kinda had a gut reaction to the CBU that made me uneasy and brought me here. I think it would probably work with the CBU, but I'd feel alot better with a layer of plywood. So although my GC may be correct in his assessment that the floor is probably sound enough, I'm also going to take your advice and sure it up a bit just to be safe.

So I'll do the 3/8th, something like this https://www.homedepot.com/p/23-32-in...9323/303564747, which will be marginally thicker than the 1/4 CBU thinsetted to the floor. As you suggested, I'll fill in the gouges that are most offensive with the water putty. I always thought you were supposed to glue this 2nd layer to prevent squeaking as well... have you found the glue to be unnecessary?

Also when installing the second layer of plywood, I found information that I should use screws every 6 inches, not screw into the joists but just into the existing subfloor and offset the seems between the 2 layers... sound right?

Would 1.5 inch GRK or Spax construction screws be acceptable to join the 2 layers together? Just want to use the minimum fastener possible that will effectively join the layers without putting holes in anything important that might not be visible from above. Shouldn't happen with 11 7/8 deep joists but I don't want to find out that things aren't as deep as they should be the hard way.

Thanks CX
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Old 09-08-2018, 03:24 PM   #8
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1. Finding a sheet of 3/8" that isn't shaped like a potato chip can be hard. Most times I just up it to 1/2" (15/32")

2. If you follow the proper screw pattern suggestions the 2nd layer won't be the one squeaking.

3. 4' edge of the sheet gets started at the 1/4 point of the joist bay, avoid joists, 6" in the field, 4" on the edges.

4. A coated or stainless steel screw will work at that length.
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Old 09-08-2018, 03:59 PM   #9
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What Ryan said.
Quote:
Originally Posted by Aaron
...which will be marginally thicker than the 1/4 CBU thinsetted to the floor.
You do understand that the plywood is the second layer of structural subflooring and you'll still need the CBU or a tiling membrane of some sort on top of that, yes?

While I'm a large fan of gluing every part of a wood framed subfloor structure, there is actually some data in the ceramic tile industry suggesting that there may exist some beneficial uncoupling action between the two layers of plywood subflooring. Not sure they'll ever quite sell me on that, but we have very reliable information that just using the proper mechanical fastener schedule can give you want you need. Check out this good article from our Liberry for reference.

My opinion; worth price charged.
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Old 09-08-2018, 05:27 PM   #10
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Always planned on using Ditra as the uncoupling membrane and already accounted for it in my build-up.

The article was perfect CX. What I found online was close but it's nice to have it spelled out by the TCNA.

Thanks for summarizing rmkee.

Hopefully, I can find some non-potato chip 3/8, I won't need many. 4 sheets would do me. If I can find them then I'm going to be good on all fronts, if not then I'm going to be within a 1/8 or so of my plumbing fixture being an issue and will have to see if I make it or find a way around that. A half inch may work but it will be real close and a little too much thinset between the layers of Ditra and plywood or Ditra and tile could put me over. Too close for comfort, but if I have to deal with it, then I will.

I think I'm good and I know what I got to do. Thanks so much for your guidance on this.
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Old 09-08-2018, 06:15 PM   #11
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Just a bit of clarification: While the TCNA and several of its published methods are referenced in that article, Aaron, the article is not the work of the TCNA nor does it purport to speak for that organization.

The two gentlemen who authored it have plenty of credentials on their own, though.
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Old 09-08-2018, 07:06 PM   #12
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Fair enough. With all the references to the TCNA I thought it represented their thoughts secondhand so to speak with the authors elaborating beyond the basic TCNA guidelines.

Either way, I still feel comfortable following this guidance for my install.
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Old 09-14-2018, 08:03 AM   #13
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Aaron's 1st Bathroom Remodel

Hello Everyone,

I am in the process of my first bathroom remodel and I thought I'd start a thread for questions as I get going. I've asked a few questions here already but I keep finding myself back here for advice.

Attached is the overhead view of the plan for the bathroom. Right now I am in the process of figuring out my shower area and I ran into a couple of issues I need to figure out.

My first issue is that on the wall with the fixtures (shower valve and head), I planned on wet-shimming 1/2 inch kerdi board for to make things easier to work around fixtures and to be able to tie into a niche on the adjacent wall seamlessly. I framed out the wall outline in place, but did not add the studs in the middle at the plumber's request. He wanted to add them where he needed so he could easily center the mixing valve. The total length of the wall across is only 36 inches. I did not notice at the time, but to install the valve he put the studs in the center 23" apart o.c. I don't know alot about plumbing, but I can't figure out why such a distance. Especially, since most substrates in a shower require 16" o.c. stud spacing. It's not load bearing and I could rig some blocking to make up the difference, but it will be a bit of a hack job with all the piping and wood cross pieces to work around. Should I be calling him to move the studs closer or just add some wood blocking as best I can to support the 1/2" Kerdi Board? Is there another material I could use that's 1/2" thick that would bridge that gap.

The second issue is I am making a bench at the back of the shower. It will only be attached at one side wall, then it will be 36" across 17" high and 15 -17" deep. I want to make this out of 2" Kerdi Board and thinset so I can customize it to my exact dimensions. I've seen the videos by Schluter showing how to do this, but the long-end back of their custom built benches are always abutting a wall. In this case there will be no wall behind it. I am going to be putting in frameless shower doors and they are going to run and corner over the bench. Will a bench made of Kerdi Board support this kind of continuous load? My other option is to frame something out of wood and encase it in Kerdi Board. Has anyone used 2 inch Kerdi Board in this kind of application?

Thanks in advance.
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Old 09-14-2018, 09:55 AM   #14
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If you tried to post a pic it isn't there.
1. I've never understood why plumbers can't grasp simple framing...you need to add studs to keep the spacing a 16"oc. Its pretty easy to block in between studs to mount valves...
2. A little fuzzy on the bench/door configuration. A pic or sketch would help. Kboard is very strong when assembled, but again I'm having trouble visualizing your design.

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Old 09-14-2018, 10:33 AM   #15
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Sorry forgot to click upload before.

See attached.
Attached Images
File Type: pdf 18.04.09 - Bathroom Layout.pdf (85.9 KB, 14 views)
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