The most popular brand is James Hardi backer board (HardieBacker), followed closely by USG’s DUROCK and Wonderboard. But regardless of the brand, the two primary reasons for using this wall and floor construction material is for strength, primarily on the floor, and to provide a moisture barrier for wall cavities and plywood subfloors.
How to Install HardieBacker Cement Board : James Hardie
Particle board is never an acceptable subfloor for this project. If that’s what the home has, take it out and install a plywood subfloor.
When installing floor tile, obviously the fiber cement board must go down first. The subfloor should be as flat as possible, without high spots or cavities. In this situation, “flat” is not the same as “level”. Floors must have a bedding coat applied before laying the boards down.
This is meant to provide adequate and evenly distributed support, but should not be thought of as a floor leveling compound. Thinset is the best bedding coat to use, and it is advantageous to add an acrylic admixture to it, substituting it for water. This adds flexibility to the mixture.
The boards should be screwed down with special screws designed for the purpose. Leave a 1/8” gap between all joints.
Sealing the Board Joints
The joints need to be sealed on both the floor and on walls. Some sources recommend using fiberglass mesh drywall tape, but this is a bad move. It’s better to spend the extra money for cement board tape. It will resist the alkali in the cement while drywall tape will not.
After applying the tape, skim the joint with thinset. The technique is the same as taping and floating drywall.
Preparing Tub and Shower Surround Walls
Here is a gray area. Some manufacturers recommend a vapor barrier between the wall studs and the boards; some do not. Some local building codes address this; some do not. The best bet is to follow the code first, then the recommendation. And finally, it never hurts.
On base of walls and at tub edges, do not allow the board to touch the lower surface. Leave a 1/4” gap that can be caulked later. Leave a 1/8” gap between all joints.
Cutting Fiber Cement Board
The manufacturers instructions will give their recommendations. HardieBacker board recommends scoring with a carbide blade knife and snapping. For circular holes, they recommend scoring and whacking it with a hammer.
Other manufacturers allow using grinders and jigsaws with carbide blades. Regardless of the method used, it’s a good idea to use a respirator to avoid inhaling silica dust. Also, use safety glasses to guard against chips.