Hardibacker Board Installation on a Kitchen Floor

Subfloor Requirements

• The kitchen subfloor and framing must meet specific requirements. Joist spacing must not exceed 24 inches on center. The subfloor plywood must equal 5/8-inch thick or more and it must be exterior grade. The subfloor construction design must not allow a deflection of greater than 360/L for tile and 720/L for a natural stone installation. Most building codes require designs to exceed a 360/L deflection factor. The deflection factor defines how much a floor will sag under a given amount of weight. Higher numbers mean less deflection.

Hardibacker Thickness

• In some cases, using 1/2-inch thick sheets is practical since it eliminates the need to add an additional layer of underlayment. For example, a 3/4-inch thick hardwood strip floor installed over 1/4-inch thick underlayment adjoins the kitchen floor. After laying the mortar bed, the 1/4-inch Hardibacker, a second mortar bed and 1/4-inch thick tile, the total thickness of the kitchen floor is no more than 3/4 inches thick, while the adjoining floor is 1/4 inch higher. Using 1/2 inch board makes the two floors level.

Installation

• A layer of thinset mortar is placed on the directly on the subfloor or overlying underlayment with a notched trowel. The Hardibacker is laid on top of the mortar and embedded into it by applying pressure to ensure complete adhesion to the subfloor. Cement board screws placed on a 8-by-8-inch grid to fasten the Hardibacker to the subfloor in addition to the mortar. The joints between adjoining panels should not align with subfloor joints, nor should more than two corners of any pieces meet at the same joint.

Before Tiling

• Prior to or during tiling, the joints between boards are filled with the same mortar used for tiling. The joints are covered with fiberglass mesh tape and pressed smooth with the edge of a trowel or a wide putty knife. The mortar bed for the tile is added over the top of the boards and the joints with a notched trowel and the tiles embedded in the second layer of mortar.