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Unread 07-04-2014, 02:22 PM   #1
rad
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Engineered Floor Joists

Hi -

I am in the process of tiling my kitchen floor that has engineered floor joists. The existing 3/4 plywood subfloor is failing, I think due to moisture exposure when the house was built, so I am replacing it with 3/4 Advantech. And since I need to tie that into the existing plywood I will be using 3/8 exterior grade plywood as an underlayment. Also, I am not happy about it, but I will be taking all of the cabinets out to replace more of the subfloor, and will leave a 12in perimeter of original plywood that is on the outside walls.

My first question is that I have engineered I-beams that are 14" deep and span 16-17ft. I ordered a tile package from the builder which doubled up all of the I-joists so I have 2 I-joists every 19 inches except for 1 bay that is 24". I am not sure how to check the L360 since these are engineered and doubled up. There is an island in the middle of the floor with a granite countertop.

Will this structure be sufficient ?

Now for the subfloor, as I indicated I will be using 3/4 Advantech with 3/8 exterior grade plywood. As a tile backer should I use concrete board, Ditra or Ditra XL ? I am a little old school with the concrete backer, and it seems that there are several failure points in the installation of Ditra, especially the potential for pockets under the mat from a less than perfectly flat subfloor. I totally understand the use of Ditra on a concrete floor, but just concerned on am imperfect plywood floor

Many thanks !!
Rich
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Unread 07-04-2014, 02:58 PM   #2
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Welcome, Rich.

Your builder was apparently not your friend.

If your engineered joists were not sufficient to provide the necessary L/360 deflection for a ceramic tile installation, the preferred method of upgrade would have been to reduce the joist spacing if it was necessary not to increase the depth.

Only the manufacturer of the engineered joists can tell you what you ended up with there. You can get the manufacturer's name, and frequently a phone number, from one of the joists and call them if you are in doubt.

I would personally recommend you use a minimum of nominal half-inch plywood for your second layer of subflooring. And it's an exterior glue plywood you are looking for, not an exterior grade. The plywood must have no face of grade lower than C and much have an exposure rating of Exterior or Exposure 1.

There is no reason not to use a tiling membrane such as Ditra instead of CBU over a wood framed floor. Many pros much prefer such membranes due to the ease of transport and installation even though the material cost is somewhat higher. Properly installed, the reputable membranes are no more problematic than the CBUs.

My opinion; worth price charged.
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Unread 07-04-2014, 05:41 PM   #3
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Quote:
Originally Posted by CX
the preferred method of upgrade would have been to reduce the joist spacing if it was necessary not to increase the depth.
To clarify - are you saying that taller joists would be the first choice, but if that wasn't possible then reducing the joist spacing is the next choice? And that using double joists on the original spacing schedule (as I understand the OP) is somehow less good?

If I got all that right, my question would be why? To me the doubled joists seems about the same as a single joist that is taller (assuming they both result in the same deflection number)..and if anything the reduced spacing would be the preferred option since it reduces the deflection of the subflooring between the joists. What am I missing?
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Unread 07-04-2014, 05:53 PM   #4
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Technically two joists have the same strength whether together or spaced evenly across the given distance ( 24" OC double joist = two 12" OC joists) .

However, as Steve said, you lose the opportunity to significantly improve between joist stiffness which would be very helpful on this floor given the wider 19" - 24" spacing.

I believe the wide spacing is why CX recommended a double layer of plywood.
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Unread 07-04-2014, 10:22 PM   #5
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Engineered floor joists

Thanks folks -

I actually did get a hold of the manufacturer after my post and he indicated that the configuration would support a ceramic \ porcelain tile, just not a heavy natural stone like 1/2 granite\slate.

And if I use an extremely stiff subfloor like a 3/4 Advantech I would be fine since it is so much stiffer than standard plywood.

The manufacture did indicate that providing bridging between the I-joists under the island would help to stiffen the joists. I was willing to glue and nail a 3/4 Advantech to the side of the joist but he said that wasn't necessary.

Huber Corp also said I would be able to use a single 3/4 Advantech with my joist spacing but that it would require Ditra, but said the additional 3/8 was a good idea.

Still thinking about the comment on 2 joists side by side. Agree that reducing from 19 to 16 would require less rigid plywood, but from a load perspective, 2 joists will distribute more load across their span to double the number of "pinned" points on the supporting walls\girders. Same idea for headers, double\triple beams etc

So 1/2" over the Advantech would offer improvements over the 3/8 ?

Thanks for the good ideas and conversation !
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Unread 07-04-2014, 11:47 PM   #6
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Quote:
Originally Posted by Rich
So 1/2" over the Advantech would offer improvements over the 3/8 ?
I think what you will hear from those who know is that the 3/8" tends to not be as flat and or is harder to make flat by screwing it to the subfloor. There is a strength difference, too, but I get the sense that it is secondary to the installation issue.

As far as Advantech being so much stronger than standard plywood - do you ave any numbers for that? I used Advantech on my bathroom remodel (they have good marketing, I guess) but I have found that the pros seem to shrug their shoulders at Advantech. If it were significantly better than plywood, I'd think they would be on board.
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Unread 07-05-2014, 06:36 AM   #7
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Quote:
Originally Posted by Rich
And if I use an extremely stiff subfloor like a 3/4 Advantech I would be fine since it is so much stiffer than standard plywood.
1 - not sure they said Ditra versus concrete backer board. Both offer good decoupling characteristics which you must have but neither will help with the between joist deflection.

2 - I'd skip the Advantech if you're planning to use Ditra. The waxy smooth surface may affect bonding of the the thinset. Plytanium or other plywood will work just as well.

Quote:
Originally Posted by Rich
but from a load perspective, 2 joists will distribute more load across their span to double the number of "pinned" points on the supporting walls\girders. Same idea for headers, double\triple beams etc
Floor loading is calculated differently than beam loading.

Beam loading is typically calculated using a single point load in the center of the beam, so more material joined together as a single beam is better.

Floor loading is calculated based on a square foot load (40 lb live load / 10 lb dead load) distributed over the
entire floor including the spaces between the joists.

Numerically, the single joists evenly spaced will give you exactly the same rating as double joist spaced at twice the distance. However you will have less deflection between joists with the closer spaced singles resulting in a flatter over all floor under load.

Sort of like - - - - - - - versus ~~~~~

Quote:
Originally Posted by Rich
So 1/2" over the Advantech would offer improvements over the 3/8 ?
What Steve said. The 1/2 plywood (not sheathing!) will be flatter and stronger.

Last edited by PC7060; 07-05-2014 at 07:26 AM.
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Unread 07-05-2014, 06:56 AM   #8
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Hi

The Huber Advantech has several great qualities, nearly perfectly flat increased fastener holding, strength in both direction, increased stiffness etc.

You can find the performance values at Huber.com, flooring, performance. The bending stiffness is 1250 to 770 for plywood/OSB and the stiffness is 383,800 to 330K for plywood. I know that numbers can be deceiving, but to me the flatness and strength across both axis is a big improvement. And when I spoke to the joist manufacturer they even commented that was a good choice.

Now the potential bad is increased cost and weight -

Agree on the 1/2 ply, and will prob end up using it, but just trying to manage the floor height with the other floors, exterior doors etc. I already need to pull a the cabinets and a set of stairs due to a min increase of 1 1/8

Any feedback on removing the subfloor and leaving 10-12in perimeter of the original subfloor to tie into with the underlayment

I did read a few postings about folks gluing multiple layers of plywood together but that is not recommended. The second layer of underlayment should not be glued and not fastened to the joists. It should only be fastened to the field
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Unread 07-05-2014, 07:02 AM   #9
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Ditra versus Concrete Backer Board

I love concrete backer board, but it not a decoupling membrane, it is screwed and should be set in thin set.

I am concerned about ditra since I haven't used it, but what Advantech said is that the floor structure will have several different materials all of which expand differently, joists\I-beams, Advantech, plywood and concrete backer board, and it that expansion at different rates that will crack the floor.

And I believe that Ditra is the only material approved for going over 3/4 subfloor
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Unread 07-05-2014, 07:15 AM   #10
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I like to use 3/4" T&G Sturdifloor. It's plywood and holds up very well. I install Ditra over it all the time.

You could use just about any of the tile backers available with 3/4" plywood or OSB. If you use Ditra directly onto OSB, I would give the OSB a light sanding to give the thinset something to grab onto.

I agree that an additional layer of plywood would be in your best interest given your joist spacing.
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Unread 07-05-2014, 07:22 AM   #11
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Quote:
Originally Posted by Rich
I love concrete backer board, but it not a decoupling membrane, it is screwed and should be set in thin set.
I'll defer to JB on this one. Take a look at this article, excerpt provided below.

Quote:
Originally Posted by John Bridge
• Cement Backer Boards

Cement backer boards, such as PermaBase, Wonder Board, Durock, Hardi-backer, and others are used to “uncouple” a tile installation from the subfloor below. Before they are fastened, CBUs, as they are called, are bedded in thin set mortar, which is the usual adhesive used in setting floor tiles. The panels are then nailed or screwed to the subfloor following manufacturers’ specific directions. CBUs do NOT improve the stiffness or structural value of the floor.
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Unread 07-05-2014, 07:55 AM   #12
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Decoupling

Great thanks for that article !

Interesting, I was thinking that ditra\noble actually allows for some horizontal movement which is not the case with a concrete board screwed down.
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Unread 07-05-2014, 08:08 AM   #13
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Rich, that may or may not be entirely true, and we won't ever know until the industry comes up with a test for "uncoupling," which we currently do not have.
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Unread 07-05-2014, 08:17 AM   #14
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Gotcha thanks !
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Unread 07-07-2014, 11:41 AM   #15
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Change in course

Ok, I have decided not to pull up my 3/4 plywood since I would probably destroy my joists

So I was thinking 5/8 or 3/4 plywood over my existing 3/4 plywood. if there was some cupping in the 3/4 in plywood would that transfer to 3/4 underlayment or will a combined 1 3/8 -1 1/2 be strong enough ?

And then a 1/4 concrete backer overtop -

Thanks,
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