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Unread 08-04-2020, 10:56 AM   #1
Shevaman
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1/4" hardibacker with I-joists on 24" O.C. II

Hi all, I am new here, see there's a lot of professionals here and need your advice please.

I just bought a foreclosed fixer upper in NJ and doing my first full renovation for myself (not for flip). Doing most of the work myself without any real experience other than smaller home projects and going through 3 complete renovations as a client paying contractors in another country (Ukraine).

I have the exact same situation as the original poster in terms of joists spacing (24" OC) and plywood (23/32 rated Plywood, Exposure 1). I am going to install 12x24 tile in my kitchen (above the basement) and in bathrooms.

I see this thread is from 2002 and looks like HardieBacker Cement Backerboard specifications got upgraded and now they recommend using 1/4 inch boards with up to 24" OC spacing (looks like was less before). This is from their installation guide:

We recommend 1/4 in HardieBacker® board for floor applications, unless 1/2 in thickness is needed for transition.

For all floors:
• Use minimum 5/8 exterior grade plywood or 23/32 OSB with Exposure 1
classification or better, complying with local building codes and ANSI A108.11.
• Joist spacing not to exceed 24 in on center.
• The floor must be engineered not to exceed the L/360 deflection criteria (L/720
for natural stone), including live and dead design loads, for the specific joist
spacing used.

Is it now acceptable to use 1/4 cement backer board over 24" span joists on 23/32 plywood? Looks like yes according to the manual, but I wanted to ask the pros. Or is 1/2 board a better option here?

Thank you in advance!
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Unread 08-04-2020, 11:44 AM   #2
jadnashua
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There is no structural advantage with any cbu in thickness on a floor. While most require 1/2" on a wall, Hardie allows their 1/4" stuff to be used on a wall. Hardie is classified as a fiber-cement board. The fibers are cellulose from their SDS (nominally, wood fiber). That does make it a bit stronger, but none of them specify a cbu as structural...it is there for compatibility to the tile and mortar.

Personally, I'd be a little leary about nominal 3/4" over a 24" OC structure. I'd want a second layer of plywood, then something compatible with the tile, either a cbu or a membrane. The I-joists provide the strength along them, and the subfloor in between them...24" OC is a significant distance to keep the deflection there in check.
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Unread 08-04-2020, 11:58 AM   #3
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Welcome, Viktor.

I've moved your post from that 18 year old thread to prevent confusion in both places.

As you may have noted in that old thread, ceramic tile industry standards make no provision for the use of CBU over single layer plywood subflooring over 24" joist spacing. Matters not what thickness of CBU you use because the CBU is presumed to have zero structural capabilities.

The Hardiebacker instructions you read are a bit misleading and always have been. Whether that is intentional on their part I don't know. But the salient portion of those instructions (I'm guessing you meant to quote the James Hardie installation instructions?) is this:
Quote:
Originally Posted by Viktor quoting James Hardie
• The floor must be engineered not to exceed the L/360 deflection criteria (L/720 for natural stone), including live and dead design loads, for the specific joist spacing used.
You can't meet that goal using a single layer of nominal 3/4" plywood over 24" joist spacing.

Keep in mind that subfloor deflection has two components: The joist deflection and the subfloor deflection between joists. While most engineered joist structures (you don't indicate that's what you have) exceed that deflection requirement, it's the between-joist deflection that's the problem here. That's not going to meet the L/360 requirement Hardie says you must have.

There are manufacturers of tile substrate products, well, one that I know of, that do actually recommend one of their "uncoupling membranes" for use in a single-layer subfloor over 24" joist spacing application, but keep in mind also that there is no standard in the industry for such products.

Up to you, of course, what you wanna tile over. We can only tell you what the tile industry recommends and where the smart money will be betting.

My opinion; worth price charged.
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Unread 08-04-2020, 04:38 PM   #4
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There's a specific test procedure used to evaluate subfloors and their suitability to support tile. It's called the Robinson Floor Test. Basically, it's a weighted roller that moves around a tiled section of flooring and keeps going, adding weight, until the flooring fails. That gets separated into different classifications from light residential to heavy commercial. Over a 24" OC subfloor with Ditra, it got a light residential rating, if I remember correctly. Might be sufficient for use in a bathroom since you're not going to have a party in there. You're much better off going to a second layer of plywood to lessen the deflection between those joists. That also assumes that everything else is done perfectly, including workmanship, and the stuff in the room within the design limits as to weight and weight distribution.
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Unread 08-04-2020, 07:13 PM   #5
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Jim, I believe it's only the Ditra XL that Schluter recommends over a single layer plywood subfloor over 24" joist centers. Regular Ditra still requires double layer plywood or OSB subflooring over 24" centers to the best of my knowledge.

Ditra Installation Handbook. Page 5.

And I still wouldn't do it.

My opinion; worth price charged.
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Unread 08-04-2020, 07:42 PM   #6
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I believe Blanke also approves Permat for use over a single layer subfloor 24" O/C as well.

I agree that using Ditra XL in this application is a bit nerve racking, but I've done it twice totaling over 2000 sq.ft. One was 6-7 years ago, I think , and one was about a year and a half ago. No call backs as of yet and these were the kind of people who will call back if they have a problem.
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Unread 08-04-2020, 10:02 PM   #7
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Hi guys,

Thank you so much for replying to me so quickly with so much info. I am a web-developer for 15+ years, but surprisingly never posted on a forum board before Thank you for moving it in a separate thread, don't want to confuse anyone of course.

Just to confirm - yes, my plywood subfloor sits on the engineered I-joists system. This is a townhouse in a community built in 1997, NJ.

Again, I never did tiling or any major construction, other then some finishing work - laminate, wallpaper, etc, some plumbing and electricity. So apologies if I misuse terms or say something silly.

What I am trying to achieve is the result that will hold and not crack or wobble for at least 5-8 years. I will probably only stay at this house for 2-3 years. I still want to do it properly. Some contractors suggested me to use PL glue under CB, but I quickly googled and saw in this forum too that it's a big no-no. So I will use a non-modified mortar under CB, and then Fortified Medium Bed Mortar under the 12x24 tile.

But you guys are saying CB is not even an option. Once I bought the house one of the first things I noticed was the softness of the floor. My idea was to try to strengthen from the below (I have an open basement). But I still have to research if this can even be done. I am starting to get a feeling that nobody does it and not a good idea. Another idea, as you also suggest, is to put another layer of plywood - I did not research that yet too.

I am not sure I want to go Ditra XL route as I was hoping CB will increase the stiffness of my soft floor by at least 30% (my made up number). With Ditra I feel it will be even less. But as you describe it may add 0% stiffness with either of those options.

Is extra plywood layer is really the only option here if I want to do tile? If so, what thickness I need to add on top of my 23/32?

P.S. I was thinking of using vinyl (which looks and feels fancy these days), but then my wife read that it is super toxic and we abandoned that idea.
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Unread 08-04-2020, 11:27 PM   #8
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The industry calls for a minimum of 3/8", but in reality, it is nearly impossible to find 3/8" ply that will lay flat, so typically, nominal 1/2" stuff is called for. That needs to be installed properly, and there's an article in the 'liberry' that describes how to do it and the specs on the plywood that is required.

It's really tough to stiffen the floor from below with I-joists unless you're willing and able to fit in new ones in between the existing ones. Well, that may not require I-joists, depending on the unsupported spans, but one reason to use I-joists is that it is easier to have longer clear spans than with solid wood joists.

Adding joists would improve both the 'along the joist' and in between the joists stiffness.

If you can feel motion in the existing floor, it probably is not suitable for tile as is. One thing that helps to stiffen the floor is if they used construction adhesive on top of the joists prior to installing the subflooring...that ties things together better than just nails or screws. Too late for that effect, if it was omitted. But, a typical I-joist installation would meet the minimum requirements for tile IF the subflooring was adequate. The only way to verify that is to contact the manufacturer. That type of support being engineered, can't be directly related to the span charts that you can find for solid wood timbers.
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Unread 08-05-2020, 02:22 PM   #9
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I am actually now looking into adding joists in between my I-joists. I checked the basement and doesn't both ends of the joists have access and doesn't seem like it will be a big problem to add some in between.

The span of the I-joists is 12 ft with one end sitting on the concrete basement wall and the other end sitting on a metal beam supported by metal poles and then the reverse setup goes to the other wall of the basement. My kitchen is sitting exactly between the wall and the metal beam.

I googled and doesn't seem that I can get I-joists anywhere by myself, I guess those are only in supply houses for builders. Though my big store around caries this item - "2 in. x 10 in. x 12 ft. #2 and Better Prime Doug Fir Lumber". The actual hight of this is 9.5, which is exactly the hight of my I-joists (and i-joist flange width is a bit less than 2").

Am I on the right track here or going completely off?
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Unread 08-05-2020, 02:30 PM   #10
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If you can run the new joists across beams supporting each end, yes, adding them will significantly increase the rigidity of the floor. You'll want to use some construction adhesive, and then screw the subflooring to them before that starts to set up so you don't have any gaps.
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Unread 08-05-2020, 03:07 PM   #11
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That makes sense, thank you! Is that lumber I noted in my previous post sufficient to be used as additional beam? I feel I am getting into a builder grade territory which I am not even close to. I am trying to fix what should've been done properly during construction. Maybe I am going too far with it.
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Unread 08-05-2020, 04:22 PM   #12
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Viktor, it frequently helps if you'll add that geographic location to your User Profile so it appears in each post to aid in answering some types of questions.

The lumber you described would be suitable for added joists, not a beam of any kind. I think that's what you meant, but I can't be sure.

Probably a whole lot easier to add a second layer of plywood rather than extra joists, but that's up to you. The added joists would help both the joist structure and the subflooring, while the plywood would help only the subflooring. But the subfloor is the only part that actually requires any improvement.

My opinion; worth price charged.
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Unread 08-05-2020, 04:36 PM   #13
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Quote:
Originally Posted by CX
Probably a whole lot easier to add a second layer of plywood rather than extra joists
Waaaay easier. That's the route I'd take, no question about it.

I'm guessing there are no plumbing, electrical, or HVAC obstructions running through the joists? If there are, then you don't have much choice.
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Unread 08-05-2020, 05:22 PM   #14
Shevaman
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Sorry guys, added my profile info. I am in NJ, USA.

My joists are surprisingly open, with no obstructions, nothing. One of the end is open (metal beam) and I can put additional joists (not beams, sorry) through that end. I just counted I can put 10 extra joists, one in between each of 24" OC. Which essentially will change it to 12" OC if I am correct. I would glue it with PL Premium and screw plywood into it. 2x10x12 is $20 at HD, which makes this "reinforcement project" worth just $200-$300.

If that is the case, then looks like I can use 1/4" backer board and any tile, cause this should meet almost any tile requirements as I understand.

But I may be oversimplifying it a lot and may still end up putting an extra layer of plywood instead.
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Unread 08-05-2020, 05:44 PM   #15
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Done correctly that would meet the subflooring requirements for any ceramic tile installation.

My opinion; worth price charged.
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