Thinset: It is Not Just Sand and Cement Anymore
Ceramic tiles installed on a floor have to bear a considerable amount of weight and traffic. For this reason, each tile should be thoroughly bedded in the material used to bond the tile to the substrate. If there are any gaps in the bond coat layer, the tile may crack. Even support is also very important to equalize building stresses. Gaps in the bond coat layer may also be susceptible to water penetration. In a freezing climate, expanding ice crystals formed by this moisture can force the tile off the substrate or damage the tile.
Installers have numerous bond coat choices. One of these, ceramic tile mastic, which is sometimes called organic mastic, is used to bond floor and wall tiles. Organic mastics can be put to good use on low or no impact tile installations that have very flat substrates and dry conditions; but for most wet installations, like bathroom floors, tub areas, showers, laundry rooms and entry halls, organic mastics should be used with caution. Mastic needs to be applied in a thin layer, which rules out its use with few exceptions in installations where large tiles are used, or on uneven substrates. The industry standard is smooth, flat and true to within 1/4-inch in ten feet, and 1/16-inch in 12 inches. Organic mastic should never be used as a filler on either a concrete or a wood substrate to compensate for substrate irregularities. Few mastics are approved for use with porcelain tile, especially in floor applications. While there are many new and vastly improved mastics available, thinset mortars will cure harder and perform better in most applications where high and long performance is desired.
For convenience, some manufacturers add a dry latex to the sand and cement so that only water has to be added at the job-site. Other kinds of thinset may feature both a dry and a liquid latex. A very different variety of thinset, called 100 percent solids epoxy thinset, is mixed with epoxy resin and hardener. Most manufacturers offer a wide variety of thinset mortars suited for a wide range of applications. With all the available products, to select the performance level you need, consult the manufacturer’s printed literature. If you need a 12-hour cure time for a porcelain tile over a suspended floor system, you will no doubt find one to suit your purpose. If you have specific questions, contact the technical service representative for the product you are considering. This way, you are assured of getting the maximum performance at the lowest price per square foot.
With problem substrates, like a cracked concrete slab, a membrane should be installed between the tiles and the substrate to prevent the crack from telegraphing through to the tiles. A separate membrane takes time to install and its use can increase costs, but recently, thinset mortars that can perform simultaneously as both a bond coat and membrane have come onto the market. Although not currently available from every mortar manufacturer, this kind of product is the result of intense competition that points to more dual-purpose products in the future.
Because it is a liquid-activated powder, thinset mortar should be stored off the floor at normal room temperature and humidity. After use, partial sacks should be kept closed and used within a reasonable amount of time—a week or two—after that, it should be discarded. Thinsets have useful shelf lives typically of 6 months to a year. If you cannot read the date on the bag, consult the manufacturer for the date code location.
Before thinset mortar can be applied, the substrate must be thoroughly cleaned of any construction debris, drywall compound, paint residue, paint overspray, grease, oil, wax, dust or any other foreign substance that will reduce the bond between the tiles and the substrate. Dust left over from sweeping, in particular, should be removed with a vacuum or wet mop.
Thinset mortar can be mixed with power equipment or by hand, and it is always applied with notched trowels. Before using the notched side, it is important to “key” or “burn” in a layer with the flat side of the trowel. If this step is ommited it will reduce the performnce and particularly the bond strength of the product. The size trowel required for each installation is determined by the size of the tile, its backing surface, the nature of the substrate, ambient temperatures, the absorbency of the tiles and other factors. Notch recommendations are just that, recommendations. Each brand has general notch sizing guidelines on the bag, but it is up to the tile installer to make the final determination. The industry standard is to provide a minimum 80 percent evenly distributed coverage on the back of each tile in a dry area. In wet areas, the minimum coverage should be 95 percent.
Thinset mortar needs to harden and cure before being subject to foot or cart traffic. 24 hours means 24 hours, not the next morning. It also assumes you have a 65-70 degree temperature and 45-50 percent relative humidity. Use of membranes can also be a factor. If there is no porosity in the substrate, the drying time will be extended. If time is an issue, select an appropriate thinset for the installation time frame allowed. Remember, regardless of the thinset mortar you select, the very best results are obtained by following the manufacturer’s instructions to the letter. This includes slake time, water ratio, mixing speed, and proper curing time. Things have come a long way since the days of sand and cement. If you have not read your favorite setting material manufactur’s product literature in the last year, surprise; it’s changed. You have more to choose from.