By Gregory Mowat, Forensic Tile Consultants
Backer boards come in different sizes, thicknesses, weights and materials for different end uses. Manufacturers continue to expand their backer board product lines for these end-use requirements. Typically backer boards are chosen for installation horizontally or vertically under ceramic tile, glass tile stone tile and specialty tile in lieu of wire-reinforced mortar beds. Installations of backer boards must follow the respective backer board manufacturer’s installation requirements explicitly.
Backer boards do not contain any wood or organic materials, are resistant to rot and mold, and are non-combustible. They resist swelling and warping, and may be waterproof depending on the manufacturer and end use. Edge treatment of backer boards also vary between manufacturers.
Backer boards may be used in an exterior adhered veneer application where recommended and warranted by the backer board manufacturer and installed as required by the backer board manufacturer’s installation instructions. Ceramic tile, glass tile, stone tile or specialty tile can be directly bonded to the backer board, following recommended installation with the respective backer board manufacturer. When backer board is specified in contract documents, it should follow installation requirements. Backer boards installed by other contractors should be inspected to verify proper installation.
Backer boards listed in the Backer Board Selection Guide on page 20 of the 2021 Tile Council of North America Handbook for Ceramic, Glass and Stone Tile Installation (hereafter referred as the TCNA Handbook) include: Cement Backer Board; Coated Glass Mat Water-Resistant Gypsum Backer Board; Glass Mat Water-Resistant Gypsum Backer Board; Fiber Cement Backer Board; Fiber-Reinforced Water-Resistant Gypsum Backer Board; and Cementitious Coated Extruded Foam. While Foam Core Backer Boards are not listed in the 2021 TCNA Handbook, I will discuss them in this article.
Cement Backer Board
The cement backer unit (CBU) is a nailable/screwable backer board or underlayment panel which is composed of stable Portland cement, aggregate and reinforcement with a significant ability to remain unaffected by prolonged exposure to moisture. It is manufactured with Portland cement, fiberglass mesh, sand and typically lightweight concrete.
USG Durock, PermaBASE (provided exclusively by National Gypsum Co.) and Custom Building Products’ WonderBoard are manufactured cement backer boards that meet ASTM C1325 and ANSI A118.9 requirements. ANSI A108.11 Interior Installation of Cementitious Backer Units is the guideline for installation.
Cement backer board has had previous names including: Concrete Backer Board; Glass Fiber Mesh-Reinforced Concrete Backer Board; CBU Cementitious Backer Unit; Glass Fiber Reinforced; and Glass Mesh Mortar Units.
Coated Glass Mat Water-Resistant Gypsum Backer Board
A backer board conforming to ASTM C1178. Designed for use on floors, walls and ceilings in wet or dry areas. The Coated Glass Mat Water-Resistant Gypsum Backer Board is applied directly to wood or metal studs or over wood subfloors. Ceramic tile, glass tile, and/or stone tile can be directly bonded to the Coated Glass Mat Water-Resistant Gypsum Backer Board with latex polymer modified Portland cement mortar or epoxy by following the backer board manufacturer’s instructions.
installation at seams.
Georgia-Pacific Building Products’ DensShield Tile Backer is a mold-resistant tile backer board with fiberglass-mat facings and a unique acrylic coating that blocks moisture from entering the board. Fasteners should be spaced 6” (152 mm) on center for walls and ceilings. Drive fasteners flush with coated surface. Do not countersink. Caulk corners and joints prior to addition of 2” (51 mm) fiberglass mesh tape in skim coat of tile setting material. Seal penetrations and abutments to dissimilar materials.
For steam room application an ANSI A118.10 waterproof membrane may be directly bonded to DensShield. DensShield is not to be installed over a waterproof membrane. The back side of DensShield must be breathable.
backer board seam.
Glass Mat Water-Resistant Gypsum Backer Board (ASTM C1658)
DensArmor Plus Interior Panels from Georgia-Pacific feature fiberglass mats on the surface of traditional gypsum board and are ideal for areas prone to high humidity areas including basements, residential bathrooms and commercial installations. May include fire rating for exterior walls or corridors. Dimensionally stable and moisture resistant.
Fiber Cement Backer Board (ASTM C1288)
Hardie Backer fiber cement backer board is a tile underlayment made to use on walls, with floors and countertops. Works as a tile backerboard and as an underlayment for tile, vinyl and wood flooring. The 0.42″ cement board is made of 90% Portland cement and sand, resists damage from moisture and provides excellent tile adhesion. Product formulation results in a combination of strength, uniform composition and performance.
installed backer board.
Fiber-Reinforced Water-Resistant Gypsum Backer Board (ASTM C1278)
USG Fiberock Brand Tile Backerboard and Underlayment panels are made from 97% recycled materials and engineered to meet water, mold and indentation resistance needs under and or behind tile, resilient flooring, carpeting, hardwood flooring and laminate flooring in new construction or remodeling. USG Fiberock Brand Tile Backerboard and Underlayment panels may be used in fire-rated wall assemblies U.L. Fire Resistance Directory under “Type FRX-G.”
Cementitious Coated Extruded Foam (ASTM C578)
Wedi Board or Wedi Building Panels include blue core of CFC- and HBCD-free material, made from waterproof extruded polystyrene foam, reinforced with fiberglass mesh on both sides. Wedi Board is coated with synthetic polymer resin mortar and embeds the mesh layer resulting in the panel’s strength and the ability to develop high adhesion strength to virtually any type of thin-bed mortar. The embedded mesh eliminates the risk of delamination of installation layers.
Foam Core Backer Boards
GoBoard is a waterproof, ultra-lightweight, tile backer board from Johns Manville. GoBoard is a Polyiso foam board engineered for durability. It’s easy to handle, cut and install, making it ideal to complete the tile project faster.
Laticrete Hydro Ban is a high-density, waterproof extruded polystyrene core and a reinforced, waterproof membrane on both sides to give triple protection from water and vapor intrusion. Hydro Ban is suitable for steam rooms. A preformed curb overlay for shower curbs/dams is available.
Schluter Kerdi Board is an extruded polystyrene foam panel with reinforcement material on both sides and fleece webbing for effective anchoring in thin-set mortar. With suitable preparation of the abutting seams and joint areas the Kerdi-Board may be used for bonded waterproofing assemblies. Kerdi Board is not intended for exterior applications. Prefabricated substrates are available for showers.
Wall Tile Movement Joints in Framed Wall Assemblies
Location and frequency of movement joints shall be determined by the design professional or engineer, including the design of the joint itself with framing, sheathing and backer board considerations. Consult the backer board manufacturer for specific recommendations. Design details must be specified before the wall is assembled as movement accommodations cannot be retroactively added.
board in shower.
Estimated Weights for Backer Board in Floor Installations
A weight of 4 lbs. per sq. ft. is assumed for 1/2” cement backer board and a weight of 2 lbs. per sq. ft. is assumed for 1/4” board, although some manufacturers list weights closer to 3 lbs. per sq. ft. for 1/2” cement backer board and 1.5 lbs. per sq. ft. for 1/4” board. Weights for other backer boards are listed in the product literature of the backer board manufacturer.
Backer board failures are common when the backer board is not installed following backer board manufacturer’s installation instructions explicitly. Failures are further magnified when the tile assembly is not installed to ANSI A108 installation requirements. Steam room failures are common when not following movement joint requirements at all changes in plane.
Hints for Success
Use non-corrosive resistant fasteners. Properly tape seams with alkali-resistant fiberglass tape and follow fastener spacing and patterns per the recommendation of backer board manufacturer. Organic adhesives are not recommended in areas continually exposed to moisture. Do not use drywall tape of drywall compound for taping backer board seams. Always work toward successful installations!
