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Unread 10-27-2004, 10:48 AM   #29
Tile Contractor
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Join Date: Aug 2003
Location: Seattle, WA
Posts: 4,332
All right CX you finally baited me into it.
There is a 2 layer subfloor requirement for all stone floors. When you add your final underlayment layer (CBU, membrane, or mud) that makes 3 layers total. The reasons you need two layers of subfloor are that stone is super sensitive to movement, & because deflection is not our only movement concern. There is also expansion/contraction. The overlapping 2 subfloor layers limit the effects of this by tying the whole floor together structurally. Imagine you found a subfloor material as thick & stiff as concrete. Let's just say you could put concrete up there. You still have a cold joint every 4x8 feet, and none of the underlayments let you just plow through & ignore cold joints. So a super stiff 1 1/4" panel is great for deflection, but it does nothing to tie the floor together structurally.
Tom please make sure your builder understands that cement board is not a structural panel. They will sag, crumble, & pull right through screws when subjected to structural stress because that's not what they were designed for. They're just an underlayment made to help isolate the tile from floor stress (not reduce the stress) and to provide a good glueing surface.

You need 2 layers of subfloor, 1 layer of underlayment for stone.
Tom Hulse
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