Tile Subfloor underlayment


I plan on installing 13x13 porcelain tile in my entranceway/hallway, bathroom, and kitchen. I have a question about the subfloor and underlayment.
The floor joists are 16" OC 2x10 joist (Douglas Fir) with a span of 12' and 13'. I look good on the deflection with L/458 and L/558.
My current floor is 23/32" plywood, then 1/4" Luan, and then vinyl flooring. I plan to tear out the vinyl flooring and Luan. I have been told that I should not put Hardibacker on Luan since it has a greater tendency to warp.
After I pull out the Luan I have about 1/2" to use for building the underlayment since I will run into problems with height restrictions namely two exterior doors that can't be cut down.
How should I build my underlayment on top of the 23/32" plywood? I want to have a layer of hardibacker and it looks like 1/4" is actually stronger than 1/2" ? Will 1/4" hardibacker on top of the 23/32" plywood be enough support? Could I do this and then add blocking to the joist under the floor to increase the support of the floor?
Thanks for the help.
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snipped-for-privacy@gmail.com wrote:

You could also use 1/2" cement board. I used 5/16" cement board on top of 1/2" plywood and haven't had a problem yet (1 year) YMMV.
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Have you considered using "Ditra" instead of a more traditional underlayment? Check with your tile supply shop.
This is a fantastic product. I've used it under tiles, heated and unheated, and eliminated the need for thicker substrates. Even in applications where there are some movement problems with foundation or the like, I've never so much as had a crack in the grout when I've properly applied this product. I've used it under 18"x18" slate, right down to mosaic tile. Always works well.
It takes a fraction of the time to put down (all you need is some thinset and a knife), and doesn't generate the noise and mess you get from plywood.
Here's the website:
http://www.schluter.com/english/products/2002/sectionf/ditra/601-index.html
It was written up last year in Fine Homebuilding, I can get the issue date if you need it.
good luck!
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I was also thinking about Ditra but haven't decided between the Ditra or Hardibacker. Would I still have to add the layer of 3/8 plywood on top of the 23/32" subfloor if I go with Ditra?
I just need to figure out if I have to add the 3/8 plywood to make the floor stronger before I add the Ditra or 1/4 Hardibacker?
I am a little confused because some are saying I need 1-1/4" subfloor before I lay hardibacker or ditra. Then on Hardie's website they say " For all floors: Use minimum 5/8" exterior grade plywood or 23/32" exterior grade OSB, complying with local building codes and ANSI A108.11. Joists' spacing not to exceed a maximum of 19.2" on center. "
So according to hardie's site my 23/32" subfloor is enough and I can lay 1/4" on top.
Does anyone know if I would need to add the 3/8 plywood? If I don't add it and run into problems after the installation would I be able to block my joist under the floor to improve the strength?
Thanks -mark
maurice wrote:

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I don't think you'd have any trouble using Ditra over the 23/32" subfloor. According to their technical data, you can use them over 19/32" tongue / groove subfloor, with 1/8" spacing between the boards.
I've put Ditra directly over the tongue/groove subfloor with no problems. Some differential movement is okay under this product. Read their installation manual to get more information:
http://www.schluter.com/english/products/2002/pdf/ditra_handbook.pdf
Make sure you follow their directions regarding the type of thinset to use - you should use modified thinset to put the Ditra down, but unmodified to lay the tile.
If you're using really small tiles, Ditra's not as good an option - I would still use it, but I'd opt for a thicker substrate.
What's really nice about it is its ease of use - you can prepare 1000 square feet for tile in a few hours, and start tiling immediately if you want.
I don't know much about hardibacker...
good luck!
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