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Q.Should an underlayment be used with ceramic tile over a plywood subfloor? If so, what material would you recommend?

A.When bonding ceramic tiles with an adhesive, you should use an underlayment to absorb movement in the subfloor.

Otherwise, the movements, caused by the expansion and contraction of the wood subfloor with changes in moisture content, can crack the inflexible grout between the tiles. A reinforced cement-board, such as Wonderboard or Durock, works well as an underlayment. It is nailed to the subfloor and the tile is attached with adhesive over it. The old method of notching the floor joists to drop the subfloor and setting the tile in a thick mortar bed on the subfloor works well, but is seldom used today.