Installing Hardiebacker cement board tile backer for a tile floor in a bath room remodel project.

Tile Floor Installation:

Installing HardieBacker® Tile Backer Board On A Bathroom Floor

In This Article:

Sheets of HardiBacker cement tile-backer are cut to fit, then fastened with thinset mortar and special screws.

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Skill Level: 3 (Intermediate) Time Taken: About 5 hours for 60 sq. ft.

By , Editor


Ceramic or stone floor tile is typically applied over a sturdy, continuous cement surface. Years ago, a tile layer would build up a thick bed of mortar, perhaps 1 to 2 inches thick, before laying floor tiles.

Today there are cement-based tile backerboard products that take the place of a mortarbed. These tile backer materials are cement-based sheets that are mortared to the floor and fastened with nails or screws. Durock® and Wonderboard® are two products that are made from cement and glass fibers. HardieBacker® from James Hardie Corporation is made from cement and wood fibers, similar to fiber-cement siding. HardiBacker is easier to cut and fasten than the cement-and-glass materials.

I used 1/4" thick HardieBacker, which is available at Lowe's and Home Depot. Half-inch thick HardiBacker is also available, which is called HardieBacker 500.

The bath room subfloor in this 110-year-old house was replaced during remodeling. The old subfloor was heavy OSB, which had begun to rot because of a small leak around the toilet.

We replaced the subfloor with pressure-treated 3/4" plywood in the areas that could get wet, and regular plywood elsewhere.

New plywood sub-floor in a bathroom remodeling project.


Laying out cuts in Hardibacker cement board tile backer. I laid the first sheet of HardieBacker on the floor and used a straightedge to mark the cut line.


This is a scoring tool for fiber-cement backerboard. There are 2 sharp tungsten-carbide cutting teeth on the end. Fiber cement scoring tool with twin carbide cutting edges.


Cutting Hardibacker with scoring tool and ruler. I just held the straight-edge in place and ran the scoring tool along the edge, pressing firmly.

I made about 5 or 6 passes with the scoring tool, which cut about halfway through the cement board.

After the first two passes, I didn't need the straight-edge because the groove was deep enough to guide the tool.


Then I simply lifted the edge of the HardieBacker board, and the material snapped along the scored line.

This is different from scoring and snapping drywall, where you fold the board away from the scored side. With HardieBacker, you can fold the board towards the scored side... as long as the scored line is deep enough.

Scoring and snapping Hardibacker fiber cement board.

For a smoother edge, it's possible to score both sides before snapping the board. Just make sure the score lines are aligned perfectly.


Laying out a hole in Hardiebacker, drilling small hole with carbide drill bit.

Cutting The Hole For The
Toilet Drain Flange:

I laid out the location of the circle (7˝ inches diameter, in my case), based on the location of the centerline from the edges of the panel.

To cut the hole, I first drilled several small holes with a 3/8" carbide-tipped masonry drill bit.


Then I used a jig saw to cut along the circle.

I was going to use a tungsten carbide jig saw blade, but it would not fit in this newer Bosch jig saw. Bosch has their own style of jig saw blades, with a sort of "T" shape at the end.

So I had to use an ordinary wood-cutting jig saw blade, figuring the blade would be destroyed after just a few cuts in this cement board. Actually, the teeth didn't get dull as fast as I expected.

Cutting large hole in Hardibacker with a jig saw.



The instructions for cutting HardieBacker say to do any power tool cutting outdoors to keep dust out of the home. Obviously I did not follow those instructions, but I did have a large fan in the window nearby to extract dust.

The instructions warn about breathing the dust from cutting HardieBacker, which contains silica. If too much silica is breathed in, it can cause silicosis, a serious lung disease.

I strongly urge you to follow the instructions: wear an N95 dust mask, keep the room well ventilated if cutting with power tools, and use a HEPA-filter vacuum up any dust or shavings instead of sweeping.


Sheets of Hardiebacker set in place before fastening. The first two pieces of HardiBacker set in place after being cut.

I cut all the pieces of HardiBacker and made sure they fit properly before fastening them to the subfloor.


Also, before fastening the HardieBacker, I pre-drilled some shallow dimples (countersinks) for the screw heads.

I used a 3/8" carbide-tipped masonry drill for this.

Drilling shallow dimples in Hardibacker for screw heads.


Rock-On brand of cement board screws. These special screws are sold for fastening cement tile-backer panels.

Note the serrations on the underside of the screw head... these are supposed to dig a counter-sunk hole for the head, but it doesn't really work perfectly, which is why I drilled my own countersink holes earlier.


At this point, all of the HardieBacker panels have been cut and fitted, and the countersink holes have been drilled.

I spaced these hole 8 inches apart, wherever there were shallow marks molded into the backerboard.

Hardibacker sheets laid out on bath room floor, before fastening.


Mixing thinset mortar with heavy-duty drill and paddle-type mixing tool. I mixed some thinset mortar in an empty 5-gallon drywall bucket.

I used a paddle-type mixer with my heavy-duty 1/2" drill... although a "cage-type" mixer may be faster.

Mixing mortar requires a serious, powerful drill... preferably a drill with a low top speed.


I used a clean garden trowel to scoop out the thinset mortar from the bucket... Applying gobs of thinset mortar to plywood subfloor for fastening Hardibacker tile backerboard.


Spreading thinset mortar with smooth edge of tile-layers trowel. ...then I used the smooth edge of a notched trowel to spread the thinset mortar around.


After the mortar was completely spread over the target area, I used the notched side of the trowel to "comb" the mortar. "Combing" the thinset mortar with notched trowel.

The instructions say to use a 1/4" square-notched trowel to comb the mortar, but I used a 3/16" V-notched trowel instead. Why? Because I needed to keep the overall thickness of the tile floor to a minimum, to avoid a problem where the bathroom tile meets the hardwood floor at the doorway.

Thinset mortar applied to plywood sub-floor. I only applied thinset mortar to the area beneath the first piece of HardiBacker... otherwise I'd step in the mortar.


I laid the first piece of HardieBacker in the mortar. First sheet of Hardiebacker tile backer board laid in thinset mortar.


Fastening Hardibacker cement board with special screws. Then I used an impact driver to drive those special cement-board screws through the HardieBacker into the plywood subfloor.


Drill-Driver Vs. Impact Driver:

I started driving screws with an ordinary drill-driver, because both of the batteries for my impact driver had become discharged. I found that my 14.4 volt drill-driver would leave the screw head slightly above the surface, in spite of the countersunk holes I pre-drilled.

After an hour of recharging, I tried setting screws with the impact driver. I was able to drive the screw heads below the board surface with no problem. It seems that the hammering action makes a big difference in helping those serrations under the head do their cutting action.

Maybe a more powerful drill would sink the screws deeper, but I think this is a good example of the benefits of an impact driver: a drill-driver just uses a simple motor to turn the screw, while an impact driver uses a rotating hammer to pound the screw through the material with minimal effort and no kickback.


After the first piece of HardieBacker (against the windows), I laid the next large piece towards the door.

Then I installed a couple of narrow strips: at the door way, and on the left-hand side next to the wall.

Sheets of Hardiebacker fastened to floor in bathroom.


Final sheets of Hardibacker fastened to bathroom subfloor. Then I installed two more pieces of HardieBacker in this L-shaped bath room to complete the project.
The dark patch is where I filled in this gap with mortar, because my Hardibacker panels shifted slightly from the intended position.

The instructions say to stagger the joints and avoid a "4-way corner", where 3 or 4 panels meet at one point, as they almost do here. This wasn't practical in my situation... unless I cut that first panel narrower and made a wider final strip at the doorway. We'll see if any cracks appear at this point.

Staggered gaps between rows of Hardiebacker panels, mortar used to fill gaps.


Fiberglass mesh tape applied over seams between panels of Hardibacker tile backerboard. Note that fiberglass mesh tape (red arrow) must be applied over the joints between panels of HardiBacker.

Normally this can be done right after installing the backerboard panels, or right before laying the floor tile. In my case, I had to install floor warming cables before doing the tile, so initially I applied the mesh tape only where it would be covered by the metal anchor straps.

I figured the mesh would be peeled up from walking on it, so I waited as long as possible before placing tape on all joints.

More Info:

Tools Used:
  • Cordless Drill/Driver
  • Cordless Impact Driver
  • Jig Saw
  • Fiber-Cement Scoring Tool, Carbide Tipped
  • 4-Foot Ruler or Straightedge
  • Tape Measure
  • Trowel, 3/16" V-notched (1/4" Square notched normally recommended)
  • Garden Trowel
  • Clean 5-Gallon Plastic Bucket
  • Heavy-Duty Drill
  • Paddle-Type Mixing Tool
Materials Used:
  • HardieBacker, 3'x5' Sheets
  • Rock-On Screws for Cement Board
  • Thinset Mortar (About 25 pounds for 60 square feet of backer board)
  • Fiberglass Mesh Tape
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Written September 22, 2010