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I'm retiling the kitchen after a crawlspace water leak which led to professional moisture, mold, and tile removal. I have 3/4" plywood over 2x10 16 o.c. joist. The previous layers consisted of subfloor, 1/4" lauan, lino,1/4 cement board and tile which trapped in moisture and mold. Cleaned up subfloor and covered with Fosters 40-20. Floor is solid except for some sag between joist in a couple of places that dip no more than 1/8 ". My question is, I plan on installing 1/4" Lauan over the plywood subfloor and then the cement backer board. Should I use thin-set to fill in the void between plywoods or just screw it down tight on the subfloor and then fill in the voids between backer board and lauan with thin-set? Thanks for any help.
 

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luan has no place under tile. the minimum you should put down would be 3/8 plywood screwed to the subfloor but not the floor joists. can you add blocking between the joists in the places where the plywood sags between the floor joists? the 1/4 inch cement board is laid in a layer of thinset to fill the voids under it to make sure it is supported. it can be screwed or nailed. the cement board adds no structural strength to the floor, it only makes a suitable surface to adhere the tile to. also consider ditra instead of cement board for under the tile.
 

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I'm retiling the kitchen after a crawlspace water leak which led to professional moisture, mold, and tile removal. I have 3/4" plywood over 2x10 16 o.c. joist. The previous layers consisted of subfloor, 1/4" lauan, lino,1/4 cement board and tile which trapped in moisture and mold. Cleaned up subfloor and covered with Fosters 40-20. Floor is solid except for some sag between joist in a couple of places that dip no more than 1/8 ". My question is, I plan on installing 1/4" Lauan over the plywood subfloor and then the cement backer board. Should I use thin-set to fill in the void between plywoods or just screw it down tight on the subfloor and then fill in the voids between backer board and lauan with thin-set? Thanks for any help.



if you use cement board the floor is leveled after the cement board is laid, with ditra the floor is leveled before it is laid.
 

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if you use cement board the floor is leveled after the cement board is laid, with ditra the floor is leveled before it is laid.

Do you mind to elaborate? Are you saying the thinset under the cement board will leave the floor level, or are you saying that Self-leveler can go down over the cement board?
 

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If you use cement board for a backer----set the board into a bed of thinset (like a giant tile) and screw or nail the rock---if leveling or flattening is needed---this is done after the board is set and nailed--
 
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