How to Finish a Cement Board Joint

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Cement board is a type of backer board that is installed under ceramic tile. Cement board is water and mold resistant. It is used in areas of the house with high moisture and helps protect the sheathing from water damage. Cement board comes in 3-by-5 foot sections and installs similarly to drywall. It is screwed or nailed into the sheathing underneath. The seams between the cement board must be finished after installation. Finishing the joints in cement board is critical to ensuring they are properly sealed.

Things You'll Need

  • Tape measure
  • Caulk gun
  • Silicone sealant
  • Utility knife
  • Rag
  • Measure the distance between the cement board at each joint. Run a tape measure from the edge of one piece to the edge of the piece butted up against it and verify that there is a gap of 1/8 inch between each piece.

  • Place a tube of silicone sealant into a caulk gun. Cut off the nozzle located at the top of the tube. Slice a small piece off the top with a utility knife at an angle. Line up the tip of the nozzle with the first seam between the cement board beginning at any corner of the room.

  • Push down on the trigger and release the silicone sealant. Run a bead along the seam between the two sheets of cement board. Follow along the seams with the silicone slightly touching the gap between the pieces.

  • Move on to the next piece of cement board and run a bead of sealant in that seam. Continue around the room, installing sealant at each seam until they have all been covered. Wipe off any excess sealant with a clean rag.

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