How to Install Cement-based Tile Backers like DUROCK™

by Jim Bessey

“If you want to succeed, you need great backers”–tile backer board, that is!

Tract home builders used to install ceramic tile directly on cheap lauan plywood. In fact, we’ve even seen one or two builders who glued floor tile directly to a house’s sub-floor decking. Either installation method will quickly void your tile warranty, and nearly as quickly produce shoddy results. For great, long-lasting tile floors, especially in bathrooms, always use a cement-based tile backer board.

image of backer board in progress

thin set mortar combed and ready for next backer board

You have three big-brand choices:

  • DUROCK™ cement board in ½” and ¼” thicknesses (and other brands, similar)
  • HardiBacker® 500 no-mesh cement board (.42″) and ¼” backer board
  • Schluter®-DITRA polyethylene grid membrane, for use with thin set mortar

All are fire-rated, mold and moisture-resistant, and offer specific advantages and disadvantages. Some tile installers swear by one or another (we like DUROCK’s Next Gen board); but the truth is your efforts at installation are more important than which type you choose. This is a truism you may have read before about siding, shingles, and paint–and we’ll continue to stress it when we talk about ceramic and porcelain tile projects.

You might be surprised to discover that the first two brands come in 3′ by 5′ sheets, (though both offer other sizes). Why 3′ by 5′? Generally, this size works perfectly with above-the-tub wall applications. Also because the sheets are heavy! Most bathrooms are small, and the smaller sheet size is much easier to work with than 4′ by 8′ panels.

DITRA is a different sort of backer system, described as a membrane. It’s orange, light-weight, available in two thicknesses, and sold in rolls. You can cut it easily with a utility knife. But you still need thin set mortar to make this product perform; so we like to think of it as a “cement-based backer” just the same. The Schluter® website offers excellent installation tips for this product.

Ditra- A different sort of backer

Installing tile backer board can be a dusty, messy job. Here’s a list of the sort of supplies you might need for this project:

  • Knee-pads or similar knee protection
  • Safety glasses, dust mask, work gloves
  • Buckets for thin set mortar and water clean-up (empty joint compound buckets are  fine)
  • Mixing paddle–check the paint department for less-expensive versions.
  • Electric or minimum 14-volt battery drill to run the paddle (and install screws)
  • Notched trowel, V-notched or ¼” square-notched. A cheap, throw-away trowel is fine
  • 4″ plastic “putty knife,” also sold as an economy drywall knife ($2, paint department)
  • Measuring tape, scoring knife, and T-square (or straight-edge)
  • Brand-specified screws or galvanized large-head nails (read website requirements)
  • Fiberglass mesh tape for cement board joints (varies by manufacturer)

Before you lay down any new tile backer, make the original sub-floor firm and solid. Repair any soft spots in the plywood. Walk around the room, checking for squeaks. A few well-placed screws can fix those. Don’t board-over ancient flaws.

Here’s the tricky part–layout issues.

It’s important to have a good idea about your eventual tile layout before you decide where to seam your backer boards. Never align grout lines and backer seams. Next, you also need to be careful not to line-up backer seams with plywood joints. While it’s not critical to place cement board perpendicular to the plywood sheets, you do want to be sure to offset all joints/seams by at least six inches.

Over the years, we’ve developed a simple system for installation: dry-lay all of your cement board pieces before you mix any thin set mortar. That way, once you start permanently installing the product, you can move right along and your mortar won’t dry up in the process. The rest is easy:

  1. Mix enough thin set mortar to do the entire job, if possible. (Check the bag for coverage)
  2. Use a 3/16″ V-notched trowel for best adhesion.
  3. One-by-one, remove your dry-laid pieces, butter then notch-comb the floor, then install that sheet.
  4. Spot-nail or screw, leaving about half the needed fasteners for last.
  5. Pay particular attention to keeping seams the same height in all directions.
  6. Walk all over the newly-laid cement board, letting your weight help settle it into the mortar.
  7. Finally, tape and bed-coat your seams with a very thin layer of mortar. Let that coat dry before you continue. If you have large edge-gaps (between board and walls), fill them with thin set, too.

You may find slightly different instructions on a given manufacturer’s website. If they differ significantly, always follow the maker’s requirements. On the other hand, don’t worry too much about a cement-board manufacturer’s warranty–nobody will replace your tile or the labor spent installing it. In general, cement board products don’t fail; installers do. Do the job right the first time, and your new floor will last a lifetime.

Helpful resources:

  1. Choosing the right thinset
  2. DUROCK®™   website:
  3. DUROCK® cement board next gen has a warranty for 30 years from the date of installation for interior application and a warranty of 10 years for exterior application. See complete warranty for details.
  4. Hardibacker®
  5. TEC™  thinset
  6. Schluter®-DITRA