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Im going to tile my bathroom floor and the existing sub floor is 5/8 what thickness of cement board underlayment do you recommend?

Re: Underlayment

5/8 plywood is the bare minimum for tiling with ceramic or porcelain. If you are using a natural stone such as travertine, you'll need 1 1/4" minimum plywood.

1/4" backerboard (of any brand) will work just fine. So will Ditra.

Do your floor joists have the necessary deflection to allow for tiling? For ceramic you'll need a minimal deflection of L360 and for natural stone L720. If the floor doesn't meet the deflection requirements the tiles will pop off the floor after the grout crumbles away.

Re: Underlayment

locally using concrete backer board doesnt have very good results.. ive seen two floors get backer board as a substrate 2 of them failed as the board is too brittle.

for plywood though houstan is correct 1 - 1/4" is the minimum thickness of a subfloor for a tile floor to do its job. this can be achieved via two layers of 5/8", or 3/4 + 1/2. any movement in the subfloor and the tiles can crack

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