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Old 10-22-2013, 08:35 PM   #1
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Floor deflection and tile?


I have a floor in an older house that we want to tile. It has 2x8 joists.. the older style rough cut ones that are thicker then your standard 2x8. On top of that there is diagonal 1x6's then on top of that appears to be 22/32 plywood.

I know the deflection of is probably somewhere between L/302 and L/402 but not completely sure.

I assume you'd still put down some type of cement board to stiffen the floor up even more or Ditra?

What do you guys think?

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Old 10-22-2013, 09:20 PM   #2
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Floor deflection and tile?


Cement board does not stiffen the floor. Neither does Ditra. Cement board is useful as a substrate for tile, Ditra is useful as a debonding membrane. You need to take actual measurements of the joists, then you can compute the stiffness by making a reasonable assumption about the modulus of elasticity of the joists. John Bridge website has a stiffness calculator.

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Old 10-22-2013, 09:32 PM   #3
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Floor deflection and tile?


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Originally Posted by BlueBSH View Post
I have a floor in an older house that we want to tile. It has 2x8 joists.. the older style rough cut ones that are thicker then your standard 2x8. On top of that there is diagonal 1x6's then on top of that appears to be 22/32 plywood.

I know the deflection of is probably somewhere between L/302 and L/402 but not completely sure.

I assume you'd still put down some type of cement board to stiffen the floor up even more or Ditra?

What do you guys think?
Cement board doesn't do much for stiffening up the floor. Its purpose is to provide a consistent, moisture-friendly surface for the thinset/tile. If your floor isn't rock solid and you're worried about grout cracking, then Ditra is the way to go if you can afford it. If you have a bouncy floor, neither product will save you. The only solution is to stiffen up the floor. Depending on how much give you have in the floor, building up the subfloor may work or it may too fall short. Knowing the joist sizes is only part of the equation. Spans and spacing are equally important.

So, my answer to your question is: No idea. Impossible to tell over the internet. Ultimately, it comes down to a judgment call on how stiff your floor is. If you give it the bounce test and you feel no give, then you may be fine with either cement board or Ditra. If you feel some give, then you may need some professional help to figure out what measures to take to ensure a crack free tiled floor.
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Old 10-22-2013, 09:36 PM   #4
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Floor deflection and tile?


Sorry Daniel. I promise I didn't read your post and regurgitate.
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Old 10-22-2013, 11:41 PM   #5
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Floor deflection and tile?


I cannot add anything to what Daniel and cortell said other than if concerned about deflection could always sister some joists to the existing.

Good luck!
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