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Unread 04-22-2004, 05:51 PM   #1
rob 223
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Broken marble over ply joints.

I had to tear out 15 feet of marble today thank god for the air chisel . Anywho 3/4 ply 1/4 cb tgi's 16 oc. and when I tore it up, 3yrs after installing it some of the cbu came up and I saw that all the cracked tiles were over the ply joints. Now my q is other than floating mud how could this be avoided? Also do you think its from expansion/contraction or raised or lowered joist? I dont know if this offers any clues but the marble cracks were very hard to see, only at a certain angle with the light just rite it looked almost like there was a divit or ditch along the cracks not raised or seperated but lowered which makes me think maybe the joist sank or was lower than the others almost all the broke peices were along 1 joist and it wasnt a continual line, as the ply was staggered so were the cracks.

BTW this work was done with and for my boss and to builders spec and I know there should have been a better subfloor but no one wants height differences between floors hence the 1/4 cbu
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Unread 04-22-2004, 07:09 PM   #2
Davestone
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Did you put a straight edge over problem area to see if it sank? You said cbu came up,by itself? was it just nailed?no thinset or screws?was the cbu joint over the ply joint?
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Unread 04-22-2004, 07:15 PM   #3
rob 223
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The cbu was thinsetted and screwed and the cb joints were not over the ply joints. I didnt check with straight edge but I will tommorow.
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Unread 04-22-2004, 07:22 PM   #4
Davestone
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Strange,unless they'res a possibility the joist was weakened severely by plumbing or electrical notching enough to sink far enough to pull cbu with it. No water damage huh?
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Unread 04-22-2004, 07:26 PM   #5
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Or maybe that joist was already low severely enough that cbu bridged over it forming a void and eventually gave way?!
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Unread 04-23-2004, 04:28 AM   #6
David Taylor
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Was there a gap between sheets of plywood before you put the CBU on?

You're still a little thin on the subfloor at less than 1".
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Unread 04-23-2004, 05:39 AM   #7
bbcamp
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I think that stone needs double wood subfloors, regardless of the underlayment. Minimum subfloor thickness is 1-1/4"
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Unread 04-23-2004, 04:08 PM   #8
rob 223
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some of the joints in the plywood were gaped some were smacked tight, also some of the marble joints were to tight I think, when tearing out some of the tile the ones next to it chiped. When I was removing the grout I coulnt even get the knife blade between the tiles. In one area 3 peices became 7 even though I was being as carefull as possible the joints were very tight in this area and were full of thinset. Anyway I am so glad to be done with that.
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Unread 04-23-2004, 04:29 PM   #9
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I'm a mud man, of course, but I think double plywood would be the way to go, and certain folks are now thinking that the upper layer should not break over the joists at all. The butt joints should occur a third of the way across the joist space. Or is that a quarter of the way?
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Unread 04-23-2004, 04:46 PM   #10
David Taylor
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A quarter of the way, according to the good Doctor. He knows his stuff.
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