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.

 

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

 

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

I have to disagree with his suggestions.:whistling2:

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

 

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. :thumbsup:

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!

 

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

 

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/So...iterature/SolidStart_I-Joists_LPI_20Plus,_32Plus_and_42Plus_Technical_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

 

Jen,

You've got it! :thumbsup: 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

 

Jen,

You've got it! :thumbsup: 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!.

 

...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.:yes: The second layer DOES NOT get fastened to the floor joists.:no:

 

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

 

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

 

Thanks jaz,

I reread the ditra page it shows a pic on page 12 of this link. http://www.schluter.com/media/brochures/DitraHandbook-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

 

Jen,

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

Jaz

 

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

 

Jen,

I didn't realize we were neighbors.:thumbsup: You must have added a signature line?

Latex Portland cement mortar in this case is Latex or polymer modified thinset mortar. (A118.4 or A118.11). This and the unmodified thinset are the adhesives. You install the mat according to whatever directions the manufacturer wants. Of course the mat does not cover the entire floor areas. So, your choice on how to proceed. You can shim-coat the rest with modified to get a flat surface, or maybe use a SLC (self leveling cement) product to get it flat. You can also use a patching cement for this. Be sure the leveling products will work over plywood, you'll probably need a primer. The floor has to be flat, it does not have to be level.

Since after the above you will now have a cementitious substrate, Schluter wants you to use an unmodified under Ditra. You then install your tiles over Ditra with a good quality unmodified thinset. (A118.1)

Are you shopping at HD, Lowes, or a real tile store? There are many modified thinsets available, (A118.4 or A118.11), As for a good unmodified, I always use Ditra Set, but it's not so easy for you to find. I believe HD no longer carries any unmod. Lowes carries "MegaBond". It's made by Laticrete for them. It's the only one they carry that can be used either with water for A118.1, or mixed with their latex to make it a modified.

Whatever you do, if you're shopping at the big box, do not pay any attention to what they tell you. Although it could happen....most of the time those nice people have no clue. :furious:

Jaz