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Discussion Starter · #1 ·
I want to install slate tile in a hallway and a foyer (entrance to a raised ranch). I came across the L360 and L720 stiffness (or deflection) grades. However, most of the information I have found relates to room installation where the length of the joists is large, making it necessary to have a very thick subfloor (two layers of plywood plus concreteboard) to provide the necessary stiffness.

In my case, the areas are quite small with short unsupported joist lengths. My hallway is 3'6" x 23' and has supports on both sides, meaning that the joist length is only 3.5 ft. Similarly, the landing has joist lengths of 5 ft. The joists are 2x8 at 16" on center, not sure of the material. When I input this information into the deflection calculator, I get L values in the thousands. Am I interpreting this correctly?

If this is correct, could I get away with a single 3/4" plywood subfloor with a 1/4" concreteboard on top? The thing is, I would like to have my (3/8" thick) slate tile flush with 3/4" hardwood floor, so 1/4" concreteboard + 1/8" thinset + 3/8" slate tile = 3/4".

Thanks for any help.
655240
 

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retired framer
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Have you had your eyes on the joists under the hall, often there is just one bearing wall in the middle of the house?
 

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Discussion Starter · #3 ·
That's a good point. I was able to see two of the joists on the left end of the hallway through a light fixture that I removed but wasn't able to see further back. I have a scope camera that I can use to check out the rest.

In any case, your comment has made me realize that both wall end early in the bottom floor. I suppose there is still some type of beam along the middle wall that bears the load on that side of the room. But for the second wall there is likely no joist, support for the joist, making the deflection much larger.
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retired framer
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That's a good point. I was able to see two of the joists on the left end of the hallway through a light fixture that I removed but wasn't able to see further back. I have a scope camera that I can use to check out the rest.

In any case, your comment has made me realize that both wall end early in the bottom floor. I suppose there is still some type of beam along the middle wall that bears the load on that side of the room. But for the second wall there is likely no joist, support for the joist, making the deflection much larger.
Just guessing but I looks like this to me
Red lines would bearing walls and blue lines would be beams below the main floor.

1623899498925.png
 

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Discussion Starter · #5 ·
It makes sense. I'm gonna try to verify this with an endoscope camera I have.

If you guess is accurate, I suppose the single plywood subfloor plus concreteboard would not be sufficient to install the slate tile, correct? Since the position of the hallway is so close to the support beam, I wonder how much deflection there would be over a 3.5 ft length.
 

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retired framer
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It makes sense. I'm gonna try to verify this with an endoscope camera I have.

If you guess is accurate, I suppose the single plywood subfloor plus concreteboard would not be sufficient to install the slate tile, correct? Since the position of the hallway is so close to the support beam, I wonder how much deflection there would be over a 3.5 ft length.
I will let others help figure that out, I just didn't believe you have 4 ft joists.
 

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First - concrete backerboard is worthless - yes worthless. Well almost, it beats laying it directly on plywood or osb. It does nothing for strength or stiffening the area. Either go with a product like Schluter Ditra or at least a crack resistant membrane - no they don't strengthen or stiffen but help with other issues like expansion contraction...
If you need to, but can't install another layer of plywood above the existing, you can also do it underneath, sister joists, and the list goes on to help stop deflection issues. Also make sure you leave appropriate gap by drywall / framing & follow the directions on all products
Scientific test (yes tongue firmly in cheek) get the largest person you can to jump up & down on the floor - if it doesn't move while you are looking at it, you should be good
 

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Yeah, technically cement board isn't worthless, but you should assume it is for this purpose.

Generally speaking, short joist spans are great and will be fine, any details I'm too lazy to look into notwithstanding. I would skip the cement board and use Ditra. If you need extra height that you normally would have gotten with the cement board, you can use Ditra XL. Normal Ditra adds about 1/8".

As for the jump test SLS mentioned, I've found it's easier to actually feel it in your own feet than watch someone else do it, but that can work too.
 
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