The Lowdown on Backer Boards
It took many years to take hold but today, not even 40 years later, backer boards are used in approximately 1 billion square feet of the 3 billion square feet of tile sold in the United States. Direct bond using thinbed methods and backer board together most likely account for 85 to 90 percent of today's installed product. There can be no argument that a properly constructed mortar bed installation remains the Cadillac of all installations but today's structures along with the shortage of those willing or able to install a mortar bed system have made it increasingly rare. Backer boards are here to stay and the market continues to grow. That growth has brought on an array of products made not only of pure cement and aggregate but fiber cement, gypsum, and most recently foam along with a few other non mainstream hybrids such a resin composite panels. There are as many different recommendations for installation as there are products. An often asked question is what is the best backer board? Understanding and having worked with all of them, it is really a matter of preference for the most part and application in some instances. When it comes to installation, all panels share some basics but many other recommendations are panel specific. Manufacturers offer warranties based on their specific recommendations so there can be no opinions if a warranty is to apply. If you wish to apply your own installation recommendations you do so at your own risk. Let's take an opportunity to explore some basic concerns and recommendations that nearly all products share.
Bedding Coat: In floor applications all panels require a bedding coat. Recommendations may vary on what to use but universally everyone agrees the panels need to be fully supported. Thinset is the product of choice for all with a few allowing for mastic. A very common but misguided practice is to glue the panel down with a tube of construction adhesive. This is not a bedding coat, and it is a relatively poor choice for bonding. For the long term, bedding the panel is very important to the performance of the installation. The benefit of this is not realized in the short term. Think of trying to break a piece of stiff plastic or sheet metal; how long would that take with only a minute amount of movement? Yet we all know that sooner or later it will fatigue and either crack or break. A similar movement occurs in all wood floor systems and will most likely cause similar problems at some point in time.
Now for the most hotly debated item: should it be bedding or bonding? I have done both. In the perfect world a floor system would fully support the panel by providing not only support on the span of the floor joist but with adequate sheathing over the floor joist. All of this supporting structure would meet the L/360 deflection requirement. The span is rarely a problem as building code dictates it meet a minimum of L/360 under the anticipated load. However, building codes do not provide for deflection requirements on subfloor panels and the lumber industry does not rate panels or floor joist on point load (a refrigerator dolly or a couch leg with 4 people sitting on the couch). L/360 equates to a maximum movement of the unsupported area between the floor joists of .040 of an inch in the case of 16-inch centers. If tile floors were not part of the design criteria (think upgrade) then it is doubtful that spans were shortened and sheathing thickness increased. Tile and backer board generally weigh about 7-8 pounds. A basic house design typically provides for a dead weight (weight of the structure) of 10 pounds. So what basically happens at this point is the structure needs some additional rigidity to accommodate the added weight. That can be accomplished using a material to bond as well as bed the panel. Using a flexible mortar, you can both increase the rigidity of the system and allow for minor movement that is naturally occurring in all wood structures. Not all modified thinsets are created equal in this instance. Some manufactures require the use of an ANSI A118.11 EGP thinset specifically for warranty to apply. This is a very specific product and not all modified thinsets (ANSI A118.4) will meet the requirements. Backer boards were not designed nor intended to be structural panels and provide no structural value but bonding the panel can offer a little insurance in what could otherwise be a marginal installation. The quality of product used under the panel in this type of installation will have a direct correlation with long-term performance. When the structure is properly designed and both the sheathing and joist system fully support the floor system, bedding, not bonding the panel will increase the longevity by not subjecting the finished floor to normal stresses of the structure.
Fasteners: Manufacturers are very specific about what type of fasteners to use and where to put them. Once again, you should consult the manufacturer's literature for the product you are using but a few things have almost universal agreement. Fasteners should never penetrate the floor joist! The purpose of using an underlayment is to provide a surface not subjected to the stresses of the structure. All underlayment manufacturers recommend that the panels be offset from the subfloor below. Recommendations vary from 2 to 8 inches depending on the product. All fasteners should be corrosion resistant and have a minimum of a 3/8-inch head diameter. Galvanized roofing nails work very well in most installations. There is a caution though on the type of galvanized fasteners. Many of the nails readily available are known as EC roofing nails or electric coating. The quality of this plating can vary widely and in some backer boards, may be removed as it passes through the panel. In wet areas this can be problematic and result in a rust stain appearing in the grout or tile if the installation is exposed to moisture. This can be avoided by using HD or "Hot Dipped" fasteners. Some prefer to use a roofing nailer for speed and ease of installation. There are many pneumatic nailers out there that will do the job. In that instance, the caution is not only the plating (HD nails are available) but also the way the gun is used. Air nailers do not pull the sheets fully into the thinset. The nose of the gun must be firmly applied before you pull the trigger to firmly set the panel. When using screws, backer board screws are very specific. If you look closely you see what looks like a hat rim. This is an integral part of the holding power. The fastener must pull down tight without penetrating the surface. In most instances, penetrating below the surface of the product renders it ineffective. Occasionally, staple guns are used for fastening backer boards. History has shown this to be a very mixed performance method. Some swear by it, some swear at it. In a floor or wall application you are on your own if that is your favored method.
Subfloor Requirements: The tile industry has a position on minimum requirements that may or may not be shared by manufacturers of the product. Manufacturers offer warranties; the industry does not. That being said, industry recommendations are 5/8 CC or better tongue and groove panels for 1/2-inch backer board and 3/4-inch CC tongue and groove or better for 1/4-inch panels. These plywood panels must be rated for subfloor application and all must be properly gapped, fastened, and acclimated to service conditions prior to installation. This may sound like a perfect world recommendation but real world problems can result from excessively wet or dry installations. If the house has a crawlspace an effective vapor membrane must be in place. Lack of a membrane is a very common mode to failure in areas that use crawlspace construction. These are minimum guidelines and require adherence to recommendations for not only tile system installation but carpentry as well. Given the structure's ability to carry the load, a double layer floor system prior to backer board application certainly will only improve the support of the floor system but properly constructed single panel floor systems will perform adequate support for backer board panels.
Wet Area Wall Application: Most backer boards are rated for wet area applications. They (almost) all share a universal recommendation; do not set the panel on the tub or in the shower floor. The panels should be held a minimum of 1/4-inch above the tub or shower receptor and that area should be filled with a flexible sealant. The panels should be gapped at the wall and appropriate caulking material used as well. Most (not all) recommend a vapor membrane of 4 mil plastic, 15-pound roofing felt, or asphalt duplex paper be placed on the studs prior to panel installation. The purpose of the membrane is to stop a massive exodus of water or moisture vapor into the wall cavity. The exception to this is if the panel itself is fully waterproofed of which there are only a few on the market.
We could take any one of these subject areas and enter into the great debate in the installation community. However, one thing I have learned well since I have joined this side of the industry, is that there are positively reasons for every single recommendation. The industry view of things tends to be a little on the conservative side in nature to minimize risk to all parties. Tile Council of North America (TCA) Handbook and the American National Standards Institute (ANSI) Handbook recommendations are based on industry consensus in which all sides agreed on a specific item. Out of these come generic recommendations that, when followed, give an assured level of performance. The industry offers no warranties and there is no one to go to when a method fails. However, neither I nor anyone else I know has ever been on a job or taken a call for failure when all the requirements of a method were met. Manufacturers on the other hand make and sell products. They may be a little more inclined to take a certain amount of risk to accommodate an installation need. They all offer varying warranties from replacement of product to complete replacement of the installation material and labor when specific products are used and recommendations followed. One thing is certain; with sales of backer board at or approaching a billion square feet a year and nearly 40 years of experience with billions of feet already installed, they have a good idea of what works and what doesn't.