Laying cement tile backerboard. Tile Job Preparations:

Installing WonderBoard® Cement Backer Board For A Tile Floor

In This Article:

Cement backer board is installed over a wood subfloor, and the joints are taped.

Related Articles:
Skill Level: 3 (Moderate) Time Taken: 4 Hours

By , Editor

At this point, I had finished installing the 1/4" thick WonderBoard cement tile backer in the smaller bathroom. This was the first time I had installed cement tile backer using screws and mortar to adhere it.


For each piece of WonderBoard I made a sketch on a file card.


I did most of the cutting outside. I used a 4-foot drywall T-square to layout long lines.


The first tool I used for cutting WonderBoard was a jig saw with a carbide blade. This made a moderate amount of dust.


The carbide blade does not really have teeth. Instead, it has a rough coating of tungsten carbide, which is extremely hard.


The "thin-set" tile mortar, and the acrylic admixture, which improves the flexibility of the mortar, among other properties.

This large mixing paddle is very useful for mixing mortar.


I simply added the acrylic admixture instead of water...

... and let my large drill motor do the work.

Mixing thinset mortar for tile and cement backer board.

This mixing paddle, about $17 at Home Depot, is an essential tool. I also use it for mixing drywall compound.

I used a 1/4" square-notched trowel for applying the thin-set mortar to the sub-floor. Notched tile for laying tile.


I scooped up some mortar and used the flat side of the trowel to smear it into the cracks and grooves.


Then I "combed" the mortar with the notched side of the trowel. Combing mortar before installing cement tilebacker.


I applied only enough mortar to cover as much floor as one 3' x 5' sheet of WonderBoard.


I earlier cut the sheet to fit. (I tested the fit before I spread the mortar.) Then I just laid the sheet on the mortar.


I walked on the WonderBoard to press it into the mortar.


These screws are made for installing cement backer board. They have a special corrosion-resistant coating.

Notice the ridges under the screw head? They dig a hole in the cement board for the screw head, so the head does not stick up.

Special screws for fastening cement tile backer board.


I drove in the "Rock-On" screws every 6 to 8 inches, into the floor joists where possible.


I drove the screws until their heads were buried at or below the surface of the WonderBoard.


Obstructions like these toilet pipes can be a thorn in the side. I covered the toilet drain with duct tape, so I wouldn't have to scrape mortar off the PVC later. Note that the toilet flange has not been installed yet. It was put in after the floor tile was completed. (View that article).


To cut a hole in WonderBoard, I marked the location and drilled a 3/8" starter hole.

Then I cut out the hole with the jig saw.

Cutting a hole in Wonderboard with a jig saw.


I staggered the joints from one row to the next, so the seams would not line up.


The hole for the toilet supply line is far bigger than it needs to be, but that will be hidden by the tile.

I did not cut out the hole for the drain until later.


Halfway through this job I purchased an inexpensive 4-1/2" angle grinder ($20 on sale) and a not-so-inexpensive diamond masonry blade (about $40).


Cutting a square hole in cement board with a diamond blade. So for the remaining holes I used the diamond blade. This worked fery well, but made a lot of dust and kicked out lots of fragments of cement. Wearing safety glasses is necessary!


For some of the holes, I simply laid the WonderBoard right over top and later went back and cut them out. Here I drilled small holes up through the corners to mark their locations above. (I didn't want to use the diamond wheel over my head and get covered with dust.)


Then I went upstairs and marked the outline of the hole and cut it with the diamond blade.


Applying mortar over joints in Wonderboard tile backer. When all the WonderBoard was installed I applied fiberglass mesh tape to the seams and a coating of thin-set mortar.


Once the cement tile backer was installed, I used a chalk line to make a base line for laying the tiles. 

It was necessary to layout the base lines for the tiles before installing the floor warming cables, because the cables will prevent me from making a mark on the floor.

Next Step: Wiring For Tile Warming


Tools Used:

  • Cordless Drill/Driver
  • Jig Saw & Carbide Blade
  • Angle Grinder & Diamond Blade
  • Basic Carpentry Tools
  • Drywall T-Square
  • Framing Square

Materials Used:

  • Cement Backer Board, ¼" Thick
  • Thinset Mortar
  • Rock-On Screws


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Copyright © 2001, 2005

Written February 21, 2001
Revised January 11, 2005