Substrate Deflection

Most contractors understand that a bouncy floor will result in cracked tile. The allowable deflection (or "sag") for a tile floor system is generally limited to L/360. What many contractors fail to understand is that the L/360 tolerance also applies to the floor areas located between individual joists. Before you start looking for tables that list the deflection of plywood substrates, it would pay to review the Handbook for Ceramic Tile Installation (Tile Council of America, P.O. Box 1787, Clemson, SC 29633; 864/646-8453). According to the Handbook, there is no approved method for installing tiles over joists spaced more than 16 inches on-center. The prudent approach when dealing with wider joist spacing is to install additional joists between the existing ones. If you install tile on floor systems with joist spacing that exceeds 16 inches on-center, be prepared to shoulder the liability, or get assurance from all involved manufacturers that the system you intend to use will perform adequately (see "Durable Substrates for Thinset Tile," 8/96).

Getting Adhesives to Stick

Whether you’re using an organic mastic adhesive or a Portland-cement-based thinset adhesive, there is an important sentence found in the instructions that most builders ignore. It usually reads something like this: "The approved substrate should be free from dust, dirt, oil, and grease." Bedding tile in a dirty substrate will result in a weakened bond (and in extreme cases, a nonexistent bond), and it sets the stage for cracking when the inadequate bond fails to hold the tiles in place. Before you reach for the adhesive, all floors should be scraped and thoroughly vacuumed, using a heavy-duty shop vac. Concrete floors should be wet mopped (Figure 3), and plywood substrates damp mopped, changing the mop water as it gets dirty.

Figure 3. Dirty substrates create poor bonding conditions. Concrete substrates should be wet mopped and plywood substrates damp mopped, with the mop water changed regularly.

If you fail to change the mop water, you’ll be creating a homogeneous bond-breaking layer that contributes to, rather then prevents, bonding problems. I’ve had to mop floors as many as four times before the water came up clean. In most cases, these intense cleaning chores can be avoided by placing a protective covering over the substrate as soon as possible. For a few days’ protection, red rosin paper may be all that’s needed. When long-term protection is needed, I often put down a layer of polyethylene, followed by rosin paper topped with protective tarps.

The Advantage of Additives

Many tilesetters argue that there is no need to add an acrylic latex additive to thinset mortar. Some parts of their argument may have merit, but the overriding reason to use an additive is that it eliminates the need for damp curing (a seven-day affair involving wet tarps and repeated trips to the job site). Some brands of thinset mortar have the acrylic latex additive mixed in at the factory (all you do is add water). If you’re using a brand of thinset mortar that is not latex modified, you’ll need to substitute a liquid latex additive for a portion of the mixing water. Mixing thinset mortar is another area where contractors often stumble. A new batch of thinset should never be mixed in with a previous mix, and mixing buckets should be emptied and sponged out between mixes. This prevents remnants of a previous mix from "heating up" the fresher batch, causing an accelerated setting time that weakens the bond.

Match the Trowel to the Tile

Notched trowels are used to spread tile adhesive, and while there are no hard-and-fast rules when choosing notch configurations, there are some guidelines. For standard wall and floor tile up to 6 inches square, a 1/4x1/4-inch notch configuration is recommended (Figure 4).

Figure 4. Notched trowels come in many configurations. Shown here, from left, are a 1/4x1/4-inch notch, for tiles up to 6 inches square, a 1/4x3/8-inch notch, for tiles up to 12 inches square, and a 3/4x9/16-inch U-notch, used for irregular handmade tiles.

Figure 5. After applying the setting adhesive with the straight edge and bottom of the trowel, the tilesetter combs the setting bed with the trowel held at a 45-degree angle.

One last note on trowel technique: Before laying down the full setting bed, a thin layer of adhesive should be worked into the substrate using the straight edge of the trowel, with ample pressure to work the adhesive into cracks and voids.

Grouting Tips

If you’ve ever wondered whether you should use a latex additive when mixing grout, stop wondering: A latex additive should always be used. Latex increases bonding strength and water resistance, improves color retention, produces denser grout joints, increases stain resistance, and eliminates the need to damp cure. About the only downside to using a latex additive is that when it’s used with an absorptive tile (cement-bodied tile, for example), the additive makes it more difficult to clean the residual grout haze. You may need to apply a grout-release product to prevent the grout from getting a "bite" on the surface of the tile. Always follow the recommended powder-to-liquid ratios when mixing grout. The latex additives create a mix that is much more sensitive to variations in water content, and tilesetters must be careful when adjusting the "feel" of the mix. Too much liquid results in a weak grout that is more porous when it dries, while a mix that is too dry will have trouble bonding to the sides of the tile and may not cure properly, resulting in powdery joints. Grout containers carry instructions recommending that freshly mixed grout be allowed to "slake" for 10 to 15 minutes before remixing and applying. Slaking provides time for the powdered portion of the mix to absorb and react with the newly introduced liquid. Impatient tilesetters who bypass this step will be working with inconsistent grout, and face the prospect of explaining to a customer why spot defects have occurred in the grout joints. Colored grouts raise the level of difficulty by introducing a color consistency issue. Maintaining the same color throughout numerous mixes can be challenging. The first line of defense against mismatched batches is to empty all the bags of powdered mix into a common container and thoroughly mix them together while dry. This will defuse any color variations on the manufacturer’s side. The next step is to carefully measure out the liquid-to-powder ratio and scrupulously follow the same measuring method every time a batch is mixed. Finally, make sure that all the peripheral conditions are the same for each mix. If mixing by hand, count the number of strokes. If you’re using a paddle mixer attached to a drill, note the run time of the drill. If the first mix is slaked in the shade, make sure all remaining mixes are slaked under the same conditions. Spread the same amount of grout over the tile each time and be careful not to let any errant water drip on the grout joints (as from a wet sponge, for example. The edge of the rubber grout trowel should be used to remove excess grout, not a wet sponge.) The color consistency may look fine as you’re working, but water-induced color variations can show up a week or more later.

Premature Traffic

No matter how careful you are with a tile installation, premature traffic can instantly create problems, often causing the grout to crack and come loose. Many mortar manufacturers offer an accelerated setting system that will match a required "in use" deadline. Generally, the quicker you need to get on the floor, the more expensive the setting material. Before making any promises to your customer, check with your supplier to be certain the product will perform to your customer’s expectations. Remember, the penalty for inferior tile work may no longer be death, but it can kill your reputation.