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I'm doing a bathroom with ceramic tile. The subfloor is 1/2" plywood, with joists 16" on center. Can I glue down 1/4" plywood to make it 3/4"? I will use 1/2" cement board with mortar after that. Or can I just lay the 1/2" cement board on the 1/2" plywood?

  • 1/2” + 1/4” is not the same as 3/4”, in terms of deflection of the floor – kponz Jan 8 at 2:31
  • So I need to rip up the subflorr and put 3/4 down – D Just Jan 8 at 2:50
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Yes, if you bond the layers of plywood together as a single structure using construction adhesive. Multiple thin layers that aren't bonded don't offer the same level of rigidity as a single, bonded layer. I'd use a v-notch trowel or a 1/8" bead every 3" (or closer) perpendicular to the joists.

If you were me, though, you'd drop in 3/4" plywood over your 1/2" and call it a day. It's more rigid than the cementboard, will perform perfectly well as a mortar substrate, and saves you a bunch of fitment and fastening of a third layer.

  • I would probably put down 3/4 plywood also as isherwood suggested the flooring needs to be 1-1/8" thick to prevent flexing cement board is great for laying tile but plywood would be better in this case since your floor is thin. + – Ed Beal Jan 8 at 17:06
  • Are you suggesting not to put the cement board at all? That would be an improper installation. – Quoc Vu Jan 24 at 5:51

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