Am remodeling, and installing tile floor. Joists are 2x8 16 o.c., subfloor is 5/8 plywood perpendicular to joists, for underlayment should I use 3/8 plywood+1/2 in. backerboard or 5/8 ply+1/4 in. b-board? (also, should underlayment be laid same direction as subfloor? (with offset seams) thanks, Ray

  • What kind of tile? Aug 30, 2018 at 3:44
  • Porcelain, 6x36.
    – Ray
    Aug 31, 2018 at 4:01
  • First thing to check out is this: johnbridge.com/vbulletin/deflecto.pl Aug 31, 2018 at 4:08
  • Came up just short of specs for tile, wonder if blocking would work?
    – Ray
    Aug 31, 2018 at 14:18
  • 1
    Good luck. Normally porcelain wouldn't be an issue but since yours is 36" long, definitely worth spending a bit more time ensuring it won't snap in half! Aug 31, 2018 at 14:28

1 Answer 1


When I did this in my old house I had a 3/4" subfloor and used 1/4" backer board (with 1/4" notch thinset between the subfloor and backer). I don't think that direction matters given how many fasteners you'll use on that backer board. Don't neglect to tape the joints either.

That floor never moved after adding tile on top.

  • Ok thanks, sounds like its just getting the same base 2 different ways, and either way it should be adequate. Thanks for reply
    – Ray
    Aug 31, 2018 at 4:04

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