Subfloor for bathroom ceramic tile

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Old 02-26-16, 07:23 PM
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Subfloor for bathroom ceramic tile

Hey all, I have a question that I know gets discussed a lot around here--but I am not finding the answer I am looking for.

I see the advice provided regularly that subfloors for tile should be 3/4" + 1/4" then CBU/Ditra + mortar + tile. However, Hardibacker's installation instructions recommend only 5/8" minimum for ceramic tile.

I am having a hard time reconciling why the advice is to add nearly an inch onto this? Is this out of an abundance of caution, or is there some tile code that recommends this and Hardiebacker is just trying to sell more product? Or is this recommendation to allow for natural stone and not ceramic tile?

For what it's worth, my bathroom floor joists are 1 5/8" thick, 9" tall, and between 14-16" on center. The unsupported span of the joists is probably about 10.5' but I have no way to verify for sure--this is a very close approximation.

Am i not able to lay 3/4" subfloor over the joists, meeting HB's installation requirement, and sleeping in comfort?
 
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Old 02-28-16, 08:01 PM
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Hi,

I don't know of any knowledgeable person that would recommend adding " underlayment over any subfloor in a ceramic tile installation. Do you recall where you saw that so someone might comment on it.

I am aware that Hardie (and some others), say ⅝" subfloor grade ply @ 16" oc should be good enough. I don't agree, but it might work, most of the time. They also say that the subfloor must meet min. deflection of L360. both for the joists and the subfloor. So, if the grout starts to crack, guess what?

From experience I can tell you a " properly installed subfloor over a joist system that meets L360 will work well for ceramic tiles. Anything thinner scares me. If you have the height and wanna add an additional layer of underlayment, go no thinner than ⅜", then the cbu or membrane such as Ditra.

Natural stone requires a system where both the joists and subfloor meet L720, so no, that spec isn't good for that either.

2x10's of most commonly used species and grade that span 10'6" @ 16" in good shape should be fine.

Are we to assume your subfloor is ⅝"?

Jaz
 
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Old 03-01-16, 04:15 PM
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Thanks for your response Jaz. I could have sworn I read that advice on this forum a couple of times, but maybe I am incorrect?

The subfloor is ripped out so I am putting down a 3/4" plywood. I was hoping to avoid having to put the 1/4" underlayment on top that I thought I had read to be the proper way to install on this forum, so it looks like I am good without.

So 3/4" plywood, thinset, 1/4" hardi, then thinset and tile.

Thanks for helping me out.
 
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Old 03-01-16, 04:26 PM
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This was one such thread, where the original poster suggested he will be using 3/4" + 1/4" ply for the subfloor/underlay and nobody seemed to correct or question him.

http://www.doityourself.com/forum/wa...specifics.html
 
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Old 03-01-16, 04:31 PM
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czizzi also recommend 1 1/4" subfloor before hardi/ditra in this thread

http://www.doityourself.com/forum/fr...ml#post2381531
 
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Old 03-02-16, 07:44 AM
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You're misreading the numbers - no one is saying to use a 1/4" layer but if you use 3/4" and 1/2" you end up with 1 1/4" total. 1/4" plywood adds no real stiffness to a floor and therefore a layer that thin has no place in a tile subfloor.
 
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Old 03-02-16, 01:56 PM
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Wow. I managed to garble my numbers in multiple posts.

When I said 1/4" I meant to say 1/2". On top of the 3/4", that would give a 1 1/4" subfloor/underlayment. I don't know how I managed to translate 1/4" to 1/2" multiple times, so my apologies.

So it is the 3/4" + 1/2" that has been recommended in multiplle posts. My question then is: where does this advice originate? Is a 1 1/4" floor, meeting the deflection criteria, really necessary? Or is 3/4" adequate?
 
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Old 03-02-16, 02:16 PM
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There is joist deflection which is what the L/360 talks about in most recommendations. There is also deflection between the joists in the subfloor itself. Adding thickness eliminates the secondary source of deflection and movement.
 
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