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tile and 24 oc joist


gculver's Avatar
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Join Date: Oct 2008
Posts: 1

10-05-08, 06:38 PM   #1  
tile and 24 oc joist

I am so confused on all that I have read online. I have 2x10 (9.25") wood joist on 24" centers that span 12.5' with 3/4 tung and groove plywood subfloor. I want to put porcelain tile in several locations on this floor. The area I intend to do first is a laundry room and adjoining first floor bathroom (toilet and sink). I have an unfinished basement, but in this area I can not access the joist because of a downstairs bath addition. I am very concerned about the waher/dryer vibration damaging the tile/grout over time. Want to do whatever I can, within reason to alleviate any future problems.

What is my weakest link here? Is it he joist deflection or the plywood deflection over 24" centers. I hear a lot of squeaking as I walk through the house.

In the areas that I can access, are joist cross braces helpful, I am having trouble find 24 oc braces. Can anyone recommend?

Donít get the tile formula L/360. Please define L. I assume L would have to take into account joist material, width, height, span, and centers.

What does adding a plywood subfloor on top of existing plywood do for me? What grade and thickness would you recommend?

What does CBU do for me? Worried about height build?

Does chicken wire in the thin set do anything for me?

Also, the existing floor has dips of ~.375 in various areas. Want to level using self leveling mortar. If I do this can I still add plywood floor on top of this, wonít the nails crack the mortar.

Help, please.

 
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OhioDraft's Avatar
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Join Date: Oct 2007
Posts: 157

10-06-08, 05:46 AM   #2  
What is my weakest link here? Is it he joist deflection or the plywood deflection over 24" centers.
yes. the industry standard for tile is L/720. 2x10's on 24" centers very likely won't meet that. to remedy you're probably going to have to add joists in between the existing ones or sister to them under the places where you want to add tile.

In the areas that I can access, are joist cross braces helpful
not helpful enough.

L is the length of the joist in inches. I.E. your joists are 12'6" long, holding them to the L/720 deflection limit would mean the joist would not be able to deflect (bend) more than (12.5*12)/720 = 150/720 = .21 inches. thats only around 3/16" and they are probably already there.

What does adding a plywood subfloor on top of existing plywood do for me?
it provides a little more rigidity as well as a smoother surface to apply the tile. hopefully someone with more experience will be able to answer your other questions.

 
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