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.