National Gypsum's PermaBase cementitious backer unit is a suitable underlayment for interior applications including shower and tub enclosures, garden/whirlpool tubs, countertops, and backsplashes.

The industry has come a long way since gypsum wallboard was introduced, especially since water resistant (WR) gypsum board was the primary choice of backing for bathrooms and shower walls. The gypsum boards, while marginally successful, had failures from moisture attacks from behind due to water leaks.

A succession of more water-resistant and waterproof backerboards now gives the industry more faith in long lasting installations of ceramic tile and stone in wet areas. Upon its introduction, the backer board we now know as a Cementitious Backer Unit (CBU) almost immediately became the backer unit of choice.

Another improvement over WR gypsum board was accomplished by the introduction of Coated Glass Mat Water-Resistant Gypsum board. This product complies with ASTM C-1178 and appears in the 2003-2004 Tile Council of America (TCA) Handbook in detail W 245-03 and as a new detail for floors in detail F 151-03.

The limitations in the floor detail relate to the fact that it is for indoor use only and for use with 8-inch-square or larger tile. This detail also points out under the "limitations" section that it should not be used in areas such as shower floors. For wall installations, organic adhesive Type I may be used. Floors are limited to thinset mortar applications.

These products first appeared in the 1993 Tile Council of America Handbook under Special Products. Georgia Pacific's DensShield is an example of Coated Glass Mat Water-Resistant Gypsum on backer board.

WonderBoard, used here in a plywood installation, is a cementitious backer unit manufactured by Custom Building Products.
In 1995, the TCA Handbook added Fiber Cement Underlayments. This product is a dispersed fiber reinforced cement backerboard and underlayment. The use for these products may be found in various TCA details. For walls, it appears in detail W 244-03. A thinset mortar installation is required. The floor detail is F 144-03. Again, thinset mortar installations should be applied. Two examples of this type of product are Hardibacker, made by James Hardie Building Products, and Custom Building Products' RhinoBoard. These products may be used in wet areas.

Another example of backerboard found in the TCA Handbook is Cementitious-Coated Foam Backer Board, such as Custom's EasyBoard, Bonsal's Propanel and Wedi USA's Wedi Board. These products are polystyrene that is covered with fiberglass mesh and cement coated. These also may be utilized in wet areas, and can be installed over studs and subfloors. The W 246-03 wall detail in the TCA Handbook allows ceramic tile installations using thinset mortars, either dry-set or latex-modified thinsets.

The CBU was the workhorse of the industry. This is a cement-based product, as the name implies, stabilized by fiberglass mesh. It is totally waterproof, and ceramic tile can be installed with dry-set mortar, latex-modified mortar, epoxies and/or organic adhesives (Type I only).

It's worth noting that, if the CBU has been stored outdoors and installed "wet," organic adhesives may not function properly. The CBU must be dry. There are currently a number of producers of this type of cement board. Among the available products are Custom's WonderBoard, Fin Pan Inc./T. Clear Corp.'s Util-A-Crete and ProTEC, USG's Durock, and National Gypsum Co.'s Permabase.

The recessed pattern found on fiber-cement 1/4-inch Hardibacker EZ Grid tile backerboard from James Hardie Building Products is designed for easier measuring, layout, scoring, and fastening.
Another entry into the market is USG's Fiberock, which is classified as a Fiber-Reinforced Gypsum Panel Backerboard/Underlayment. This product is designed for dry areas only. This is found in TCA detail for floors F 170-03 as a new detail.

And finally, we all know sheetrock, which is a gypsum wallboard panel. The WR-grade gypsum wallboard type may be used for bath tub walls, per detail B 413-03. The wall detail for this product is W 243-03, and it applies to dry areas only. For some reason unknown to me, organic adhesives have been omitted as a bond coat in the detail. Typically, organic adhesive is a very large part of the installation process in dry areas.

As with any products used in ceramic tile installations, problems can occur. Some of the following items are noteworthy. You should be aware of the possible pitfalls associated with each situation.

  • Tile bond failure when organic adhesive is used over "damp/wet" CBUs.

  • Grout turning yellow. Possible reasons include use of organic adhesive, and use of water that contains too much sulfur.

  • Rust-colored stains in the grout. This is probably caused by fasteners subject to corrosions. The TCA advises use of non-corrosive and non-oxidizing fasteners.

  • Tile and/or grout cracking may be caused by improper seaming of backerboard joints, inadequate fastener spacing and/or deflection of backerboards.

If backerboard manufacturers' instructions and TCA guidelines are followed, most -- if not all -- of these problems likely would not occur. Take care to do the job according to the book. That will go a long way to preventing installation-related headaches after the job is complete.