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How to Install Cement Backerboard around a Tub

The ideal underlayment for ceramic tile is cement backerboard, which comes in 1/4- and 1/2-inch-thick panels measuring 3 x 5 feet and 4 x 8 feet. The dense, gray material is very heavy, so use the smaller size for replacing tub walls.

The panels are nasty to handle because of their sharp edges, so wear heavy leather gloves. Protect the surface of the tub with a heavy dropcloth.

Be prepared for a noxious odor when you cut cement backerboard. No kidding. Wear a respirator when cutting this or any other cement-based product.

Installing cement backerboard is not on the top-ten list of fun projects because the material is difficult to handle. But working with the panels is a short-term stint compared with the long-lasting and durable surface they provide. Before you get started, have the following tools and materials handy:

  • Carbide-tipped hole saw (or a jigsaw with a carbide blade)

  • Carbide-tipped scoring knife

  • Carpenter's level (4-foot length)

  • Caulk gun

  • Cement backerboard

  • Electric drill/driver

  • Fiberglass tape

  • Galvanized backboard screws

  • Silicone caulk

  • Thinset mortar

  • Trowel

  • Wedges or shims

There's really room for only one person to stand in the tub while installing the panels, but it's useful to have a helper who measures and cuts and then hoists the panels into the tub area for installation.

  1. Measure the length and width of the middle tub wall.

    Plan the layout of the backerboard panels to use as many full-size panels as possible. You should install the cement backerboard only as high as the top of the tile or wall panels.

  2. Score the face of the panels with a carbide-tipped scoring knife, and then hit the panel behind the score line to snap the panel.

    image0.jpg
  3. Cut through the fiberglass mesh on the back of the panel to separate the pieces.

  4. Make a cardboard pattern of the location of the valve, spout, and possibly the shower head.

  5. Transfer these locations to the cement backerboard, using the cardboard as a template.

  6. Use a carbide-tipped hole saw or a jigsaw with a carbide blade to cut the openings.

  7. Before attaching the backerboard, use the straight edge of a level to make sure that stud faces are all on the same plane.

    If they’re uneven, plane down any high spots.

  8. Install blocking for future grab bars while the walls are open.

  9. Carefully lift the backerboard panel into the tub area and set it on several shims placed about a foot apart to maintain a 1/4-inch gap between the board and tub.

    image1.jpg
  10. Secure the panels to the wall framing with galvanized backerboard screws.

    The fasteners should be flush with the surface.

  11. Continue measuring and cutting panels, aligning and fitting them together with a 1/8-inch gap between panels for expansion and contraction.

  12. Measure and cut to length fiberglass tape to seal the seams of the panels.

  13. Reinforce the seams by applying thinset mortar with a trowel.

    image2.jpg

    Apply a silicone caulk around the perimeter where the backerboard meets the surface of the tub.

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