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Old 08-26-2008, 07:45 PM   #1
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Tile floor with I-Joists


Hi all,

I have Never done tile before but have been reading here for a while
I am trying to find out if my floor is Strong enough to put down ceramic tile. I have 10" I-joists they are 2 foot on center and span 13 feet 3/4" osb sub floor. If I cover this with 3/8 plywood,1/4 inch cement board then use ditra over that would this handle the tile.


thank you
Jen

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Old 08-26-2008, 10:08 PM   #2
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Tile floor with I-Joists


I don't have my span charts with me, but at 24" oc spanning 13', I imagine they're at the long end of the span chart. Lack of stiffness/excessive deflection in the floor might be an issue, which could lead to a tile job that won't hold up too well.

Your plan for adding plywood is a good one. That never hurts. 1/4" wonderboard set in thinset and screwed down will also work as a substrate.

Ditra is a great product, but I don't see any reason to use it over plywood when you can install cement board and plan to do so. On a concrete floor, Ditra is great. You just don't need it for this application.

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Old 08-26-2008, 10:21 PM   #3
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Tile floor with I-Joists


Hi Jen,

You'll have to contact the manufacturer of your I-joists to get their deflection value. I recommend you ask them to rate them at 50lbs. live and 20 lbs. dead load. The maximum deflection for ceramic or porcelain is L360, hopefully your number will be higher, which means less deflection. For example, joists rated at L480 are stiffer than L360. Natural stone requires L720.

I looked at my Trus Joist chart thinking that is what you have. Maybe it is, but they don't make a 10", the closest is 9.5" and there are several models. The manufacturer and exact model will be printed on the sides.

Having said that, I am sure your joists are strong enough at that span. The spacing, as I think you know is the weak part. You're on the right track adding more plywood, you need it. What you don't need or want is the 1/4" CBU. It is useless in this case, it adds no strength at all, or very little? Since you're going with Ditra, (which is good), you could add a thicker underlayment, say 1/2" or 5/8", the thicker the better.

Jaz
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Old 08-26-2008, 10:39 PM   #4
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Tile floor with I-Joists


I see the termite replied while I was writing my reply. Hi all

I have to disagree with his suggestions.

Mainly, you can NOT install Wonderboard on a subfloor system that is built 24" o.c. I hope you don't do jobs that way. Hardie has a method for that, but not Wonderboard or any other CBU maker that I can think of. Plus as I said, CBU'd do not add structural strength to a subfloor, plywood does.

Another reason I recommend the Ditra method is that it's recognized in the TCNA Handbood, method F-147.

I believe Ditra is superior to CBU's over plywood.

Jaz
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Old 08-26-2008, 11:00 PM   #5
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Tile floor with I-Joists


Yeah, I have to stand corrected on the 24" joist spacing and installation of CBU. Sorry about that. I spaced off I guess. Thanks for calling me on it JazMan.

Ditra's great, but CBU has been around for a long time and was THE method of doing this, and still is for a lot of people. I agree that with the 24" joist spacing you're much better off building up the thickness and stiffness of the subfloor with more plywood!
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Old 08-27-2008, 08:00 AM   #6
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Tile floor with I-Joists


Thanks for the info guys. Just let me make sure I have got this straight 5/8 plywood over top of the 3/4 then the ditra. No cement board. Should I glue down the plywood. Some of the stuff I have read says yes and some say no. If so what kind of glue also I bought a sun touch heated floor will this work with the ditra. I will have to get back to ya on the floor joist specifics all the paper work is at home and I am at work now.

thanks Jen
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Old 08-27-2008, 08:47 PM   #7
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The joists are lpi 20 9 1/2 inch looking at page 4 of the attached link I think I am okay

http://www.lpcorp.com/Literature/Sol...ical_Guide.pdf

L/480
40 psf Live Load
25 psf Dead Load
24" oc
13'-0" span

Of course I do not know what any of that means

Thanks Jen


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Old 08-27-2008, 10:25 PM   #8
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Jen,

You've got it! Install plywood with NO glue. Fasten new underlayment into the subfloor, not into the trusses. Use 1 1/2" underlayment screws, not cheap wallboard screws. Be sure to install the panels offset from the subfloor, but in the same direction. Leave 1/8" gap between panels and 1/4" at walls or other solid structures.

Do you have the Ditra yet? Do you know which types of thinset mortar to use? Have you selected the type of tiles?

Jaz
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Old 08-28-2008, 06:01 AM   #9
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Tile floor with I-Joists


Quote:
Originally Posted by JazMan View Post
Jen,

You've got it! Install plywood with NO glue. Fasten new underlayment into the subfloor, not into the trusses. Use 1 1/2" underlayment screws, not cheap wallboard screws. Be sure to install the panels offset from the subfloor, but in the same direction. Leave 1/8" gap between panels and 1/4" at walls or other solid structures.

Do you have the Ditra yet? Do you know which types of thinset mortar to use? Have you selected the type of tiles?

Jaz
I can see using 1 1/2" screws in the field, but why not 2 1/2" into the joists.??? your way the new subfloor is just secured to the subfloor and not the structure!.
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Old 08-28-2008, 06:55 AM   #10
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Tile floor with I-Joists


Quote:
...your way the new subfloor is just secured to the subfloor and not the structure!.
That's correct. That's the way it is supposed to be done. The second layer DOES NOT get fastened to the floor joists.
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Old 08-28-2008, 09:14 PM   #11
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Tile floor with I-Joists


Thanks for all the help everyone.
Jaz, I have the ditra and the 5/8 plywood. do I need to fill in the 1/8 gaps between the plywood also the thin set any suggestions on a brand. I don't want to cheap out and have problems. I do not Know anything about it other than the ditra web site says to use Latex p.c. mortar under the heat mat and unmodified thin set on everything does the latex just go directly under the mat and then cover everything with modified and put down the ditra anyway I start tomorrow wish me luck.

Jen
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Old 08-28-2008, 09:38 PM   #12
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Tile floor with I-Joists


Unless the thinset manufacturer says different, those 1/8" gaps will get filled when you install the Ditra. Modified thinset allows a very small amount of movement to compensate.

There are many brands of thinsets, they all meet certain standards. I have no idea what brands are available to you, don't even know where you're from.

You must use a modified thinset to bond anything to plywood. Once you install the heat mat, the entire plywood should therefore be covered with the modified thinset you used. If so, you will install the Ditra using unmodified thinset, and also use unmodified to install the tiles on to Ditra. Your last post got a bit confusing on this, and I think you miss understood something. Remember unmodified thinset will NOT stick to your plywood.

Jaz
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Old 08-29-2008, 11:30 AM   #13
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Tile floor with I-Joists


Thanks jaz,

I reread the ditra page it shows a pic on page 12 of this link. http://www.schluter.com/media/brochu...k-2008-ENG.pdf

It says to start with latex p.c. mortar.(does this = modifed thin set) Then put the heater down. Now cover that with unmodified thinset.(my heater does not cover the entire floor) Then Put down the ditra then unmodified thin set. Then tile. This is confusing because if there was no heater it would be latex p.c. mortar. Then ditra then unmodifed thin set then tile. The heater is just a bunch of wires I don't understand why I would need to switch to unmodified on top of the heater before the ditra. sorry for all the questions.

Thanks Jen
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Old 08-29-2008, 02:52 PM   #14
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Tile floor with I-Joists


Jen,

You switch to unmod after the entire floor is covered with modified thinset that you used to install the mat.

Jaz
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Old 08-29-2008, 06:23 PM   #15
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Jaz,
Is this correct then? latex p.c. mortar = modified thin set. So, I let the modified dry, then put the unmodified down? I am in Lapeer, MI if you have any brand suggestions.
Thanks for the help with this,
Jen

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