Backer/Concrete Board - How thick?

For a bathroom project, I plan to use backer board on walls and floor. Backer board used on walls will go directly against studs and for the floor it will lay on top of 3/4" ply. My local warehouse store carries backerboard only in 5/16" and 1/2" sizes. Due to size restrictions and other issues, I'd prefer to use 5/16" for everything. Can anyone see a reason why this would be a problem?
A DIY book I was reading said the floor should be at least 1 1/8" thick using backerboard no less than 3/8" (over ply). I'll be 1 1/16" total thick with only 5/16" backer board. 1/16" isn't anything to worry about, right?
Thanks for your opinion.
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1/2" on the walls and 1/4" over proper subfloor on the floor. You decide if the 1/16" is important or not. I find that following or exceeding the minimum recommendations usually gives the best long term results.
Colbyt
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If I could find 1/4" for the floor I might use it, but so far nothing local can be found.
It's so tight I won't know until I try it, but I think I can get away with 1/2" on the walls. I guess that will offer better support considering there'll be nothing behind but the studs. Thanks for the reply.
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I believe the floor thickness of 1 1/8 refers to the total thickness of the subfloor (excluding the baker board). What is between the plywood and joist? The concern to how much flex the floor has. If the 3/8 on the floor is solid when you jump on you probably okay.
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There is nothing between the plywood and the joist. In my case the DIY book I have (Ceramic Tile by Creative Homeowner sold at my local Big Box store) it's clear the reference is for the total thickness of subfloor and underlayment. Whether or not the book is right is open for debate, but your jump test seems like a good idea either way. Again and for the record, I won't be using 3/8, I'll be using 5/16 but I think your point is the same. Thanks for your thoughts.
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Try this site for your question: www.johnbridge.com
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You'd better double check what your book says. I'm pretty sure the NAtional Standards for ceramic tile require 1 1/8" thickness of subfloor (i.e. plywood) to prevent any movement. Even 1/2" Hardibacker doesn't provide any significant structural support, so including it in the 1 1/8" could lead to cracked tiles down the road. Since you have space restrictions you could make the floor stronger by adding 2x6 supports perpendicular between the joists. And do apply thinset under the backer board for added stability.
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I have checked my book. It not only states 1 1/8" total thickness but even shows it in an illustration. Perhpas I'm confusing terms and that's getting me into trouble. My understanding is that the SUBFLOOR and the UNDERLAYMENT together must be at least 1 1/8". I thought the backerboard counted as underlayment. If it does not then that's where I'm wrong. Can anybody clear this up for me?
Here's Home time's definition of the 1 1/8" which again seems to suggest 1 1/8" total including the backerboard which they (further down) describe as underlayment. http://www.hometime.com/Howto/projects/ctile/tile_4.htm
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