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 knowledgeable, willing or able to install a mortar bed system, have made mortar installations 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 the traditional 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. Sometimes, the answer may not be a board at all, but a membrane product. Membrane systems are used every day in lieu of traditional board type products with increasing regularity. That will be the source of a future article inTILE Magazineso we will not attempt to discuss the values and usage of membranes as underlayments for the purpose of this article.
Cementitious Backer Unit: Some of the better-known examples of this type of product are Durock, Permabase, Util-A-Crete, and Wonderboard. They are defined as a cement substrate designed for use with ceramic tile in wet or dry areas. Most are available in various lengths, widths, and thickness. This material can be applied over studs and subflooring. Ceramic tile can be bonded to it with modified and unmodified thinset, organic adhesives and epoxies. There are specific use recommendations numerous installation details contained in the Tile Council of America handbook. Industry specifications for interior installation and material specifications are contained in ANSI A108.11 and ANSI A118.9 or ASTM C-1325. For exterior applications, consult manufacturer’s written instructions.
Coated Glass Mat Water-Resistant Gypsum Backer board: The better-known example of this type of product is DensShield. It is designed for use on floors, walls, and ceilings in wet or dry areas. This material is applied both vertically and horizontally, directly to wood or metal studs or a suitable subfloor. Ceramic tile can be bonded with modified, unmodified, or organic adhesive. Follow the manufacturer’s instructions for installation and joint treatment. This is critical where waterproofing applications are desired.
Fiber-Cement Underlayments: Some specific examples of this type of product are made by Hardibacker, and CertainTeed. This panel is a dispersed fiber-reinforced cement backer board and underlayment designed for use with ceramic tile in wet or dry areas. Available in various lengths, widths, and thicknesses, this material can be applied vertically over studs and over code-compliant subflooring. Ceramic tile can be bonded to it with modified or unmodified thinset and organic adhesives. General interior installation and material specifications are contained in ANSI A108.11 and ASTM C-1288. Consult the manufacturer’s written literature for more specific application details.
Cementitious-Coated Foam Backer board: Some specific examples of the type of product are Easy Board and Wedi. This particular product category has a wide range of products. Some are constructed from extruded polystyrene and coated with fiberglass mesh and cementitious coating and offer waterproofing properties. They are designed as a substrate for ceramic tile in wet or dry areas on floors or walls. They are available in various lengths and thickness. Extruded material can be applied over studs and sub flooring and bonded with modified or unmodified thinset or epoxies. Other types of foam boards are expanded polystyrene with a cement coating which does not provide waterproof properties and is limited to wall only applications. As there are no current standards for these products, manufacturer’s instructions should be strictly followed. There are several details for this product in the TCA handbook.
Fiber-Reinforced Water-Resistant Gypsum Backer Board/Underlayment: is backer board/underlayment conforming to ASTM C1278. Some specific examples of the type of product are USG Fiberock and CertainTeed ProRoc. Designed for use on floors, walls, and ceilings in dry or limited water exposure areas, this board is applied directly to wood or metal wall studs or over wood subfloors. Ceramic tile can be bonded to it with latex/polymer modified portland cement mortar, organic adhesive, or epoxy by following the backer board manufacturer’s instructions.
When it comes to backer board installation, all panels share some basics but many other recommendations are panel specific. Manufactures offer warranties based on their specific instructions, not industry recommendations. Industry recommendations only provide for a basic overview. There can be no opinions what works if you wish a warranty to apply. If you desire to apply your own installation recommendations, you do so at your own risk. Let’s explore some basic concerns and recommendations that nearly all backer board products share.
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, hence the term bedding. This not a leveling coat! A level coat would mean when a level is placed on a floor, the bubble is in the middle. Many end users mistakenly assume that the floor will be level often after hearing misused terminology. When you speak of tile floors, the preferred term is flat, not level, there is no level requirement for ceramic tile floors. Thinset is the product of choice for bedding the panel by all manufacturers with few exceptions. A common but misguided practice is to glue the panel down with a tube of construction adhesive. This is not a bedding coat and a poor choice for a bonding coat. Coat means covered with a thin layer. Long-term: this coat for bedding the panel is important to the performance of the installation. The benefit of full support is not realized in the short-term. There will always be inconsistencies in wood floor construction resulting in a less than flat surface. While these irregularities may be minor, they can and will result in cracked grout joints, which may cause bonding loss or possibly cracked tile.
A basic house design typically provides for a dead weight (weight of the structure) of 10 pounds. So the basic support issue is the structure needs some additional rigidity such as thicker subfloor panels and/or bigger joist, and shorter spans to accommodate the added weight. This may be accommodated to a degree by using a material to bond as well as bed the backer underlayment 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.
The down side to this remedial measure is backer boards were not designed nor intended to be structural panels and provide very little structural value, but bonding the panel can offer provide minimal assurance in what could otherwise be a marginal installation. The quality of thinset 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 of the tile installation by not subjecting the finished floor to normal stresses of the structure.
Manufacturers are 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” head diameter. Galvanized roofing nails work 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 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 tightly 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 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. Taping the backer board joints has never been a fun job but to achieve product performance, a necessary evil. Alkali resistant tape is recommended because cement is alkaline and may cause standard fiberglass drywall tape to separate and degrade the reinforcing value.
It is hard to cover all the nuances and do’s and don’ts of backer board in an article. Each product carries specific recommendations several pages or greater in length. We receive a fair amount of calls and emails on backer board issues and this article attempts to cover the issues or questions most often received. The question of what is the best backer board cannot be answered based solely on product performance. Rather it must be based on what the purchaser or specifier is trying to achieve. One also should have the ability to distinguish between relevant and valid performance needs rather than marketed performance values. Paying $25 a sheet for a waterproofing foam board makes perfect sense in some instances. Then again, paying $10 for a gypsum panel with a waterproofing surface makes sense in others. Just have to sort out the overall goal of the application.