Floor Deflection Cracked MY Kitchen Tile

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Old 12-30-09, 06:07 AM
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Floor Deflection Cracked MY Kitchen Tile

My house is about 13 years old. Put 12" tile in the kitchen a while back and after a few months the grout started cracking and tile started breaking. The floor under the tile is: 3/4" plywood subfloor, 1/2" OSB, and 1/4" wonder board.

Problem is too much deflection in the floor. Upon inspection in the basement, the joists (engineered) are 12" set 24" OC. Not a typo...24" OC. The thing I don't know is is the deflection in the joist or is it between the joists?

Thanks,
Greg
 
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Old 12-30-09, 07:09 AM
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Joists

How long are the joists? (between support beams,foundation walls)
 
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Old 12-31-09, 05:07 AM
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The shortest span is about 12'. Some are longer.

Greg
 
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Old 12-31-09, 06:04 AM
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How much longer? That is what Ken is looking for. Too much span and you will have deflection at 24" oc.
 
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Old 12-31-09, 07:38 AM
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Joists

There should be bracing between the joists in the center of the spans.

Incorrect installation of the sub floor can be a problem. If the joints are not staggered, then there will be more deflection where the joints are located. This cannot be verified unless you tear out the floor. Just trying to mention some possible causes.

24 in. O.C. is not good. The greatest I have seen is 19.2 in. when floor trusses were used. No tile was used in these applications and strong backs were installed in the trusses.
 
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Old 12-31-09, 07:57 AM
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If these are i-joists you need to stick with the manufacturer's instructions, which can sometimes mean that you should not install blocking or cross bracing or it could void their warranty. They should have brand names on them, and you can either find the info on the web or contact a dealer directly.

I think you should identify the brand of joist you have and determine whether or not blocking can be installed, and follow the method they suggest. Apparently nails placed midspan can weaken the entire joist. (I think they worry about multiple nails being shot, and the possibility of splitting the engineered wood)

Wirepuller mentioned strongbacks, which if placed across the bottom of the joists, helps alleviate twisting without nailing into the side grain of the engineered wood.
 
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Old 12-31-09, 11:09 AM
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Problem is too much deflection in the floor. Upon inspection in the basement, the joists (engineered) are 12" set 24" OC. Not a typo...24" OC. The thing I don't know is is the deflection in the joist or is it between the joists?
Unless things have changed recently, cement board can be used on 16" and 19.2" centers, but not 24". To answer your question, its likely that your issue is deflection between the joists. In most cases that I,ve seen, tji's are speced to a deflection of L480 which is good for ceramic tile, but not for natural stone. What kinda tile do you have?
 
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Old 12-31-09, 12:12 PM
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Read the label on the sub-floor and see the span rating: http://www.trioforest.com/pdf/Load-Span_Tables.pdf
OSB stiffness, creep, etc.: Georgia-Pacific Plywood and OSB Performance Comparison A poor filler for strength, and being 7/16", not that much help unless glued and screwed: The design of renovations - Google Books

You may no know the OSB orientation for strength calculations: APA Blog for Professional Associates: Proper Orientation of Plywood Underlayment in Floor Systems
Now you have the strength, deflection you need but also have to isolate the tile from the floor: Ditra Floors | Tile Your World
The wonder board, with thinset installed underneath, is just a surface for flat, it doesn't isolate the deflection problems from the span/strength of the joists/plywood. http://www.ilevel.com/literature/TJ-4000.pdf

Be safe, Gary
 
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