Tape and mud cement board seams so the floor underlayment is flat, stable, and ready for tile without cracks.

Start by cleaning the joints and ensuring all boards sit flush and screw heads are set slightly below the surface. Mix a dry set thinset mortar to a peanut butter consistency using only clean water and a margin trowel or mixing paddle as recommended by the manufacturer. Apply a thin layer of thinset into the joint with the flat of a putty knife to fill any gaps and to provide a bedding for the tape. Embed alkali resistant fiberglass mesh tape centered over the seam while the thinset is still wet, pressing it in with the putty knife until it is fully saturated and there are no high ridges. Immediately add a second thin layer over the tape and feather the edges out about three to four inches on each side to blend with the board surface, keeping the overall build low to avoid telegraphing through tile. Allow this to cure completely overnight, then lightly sand any high spots with a sanding block and remove dust before priming or applying a tile bonding mortar.

Common problems include tape flotation where the mesh lifts and creates a bump, which you can prevent by firmly pressing the tape into fresh thinset and wiping away excess before it sets. Another issue is voids or underfilled joints that later show as cracks in tile grout lines, so always pack the joint with thinset first and check with a straight edge that the seam is level before proceeding. A third mistake is using paper tape or non alkali resistant tapes that degrade on cement board, so always choose alkali resistant fiberglass mesh tape and follow the thinset manufacturer cure times.

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