Stone Tiling: Prep Before You Set

By Mike Daniels

The installation of natural-stone tile – as well as other tile types – is quickly becoming the most-popular surface in the flooring industry due to the durability, look, and hygienic advantages this hard surface offers.
However, as with all good things, comes some degree of problematic issues like cracks in tile and grout, as well as other issues that compromise what could be a beautiful, long-lasting installation. And, most of those problems can be traced to the substrate preparation – and how it affects the outcome of an installation.
The current market trend is for larger-format tiles installed over all kinds of substrates. As the size of tile gets larger, so does the risk of failure if the installer doesn’t pay close attention to surface preparation. If we understand the nature of the tile being installed and the substrates that the tile will be installed over, we can better understand how to properly prepare a substrate surface and select the correct setting material.

APPROPRIATE SURFACES
Ceramic tile and natural stone can be installed over a variety of substrates today. The substrates most likely to exhibit installation-related problems, due to lack of surface preparation, are concrete slab on grade and a variety of suspended types of floor substrates.
The “On Grade” substrate is almost always a concrete surface – but no two concrete slabs are alike. First, the concrete must be fully cured at least 28 days at 70° F (21° C), structurally sound and free of moisture problems, paint residue, oils, sealers, dirt or other foreign substances that inhibits a good bond. Concrete must be allowed to cure and develop strength; it often shrinks the most and has higher moisture content within the first 28 days of curing.
A more-accurate estimation is that concrete should be allowed to cure one day for every millimeter of thickness, which means that a 4” thick concrete slab could take as much as 101 days to cure by this rule. However, there’s a good track record indicating that – in most cases – 28 days is sufficient cure time.
Structural issues in an “On Grade” substrate could include freeze/rain damage; sub-grade support issues under the slab; punctured vapor barrier; and shrinkage cracking, to name a few. Freeze and rain damage will weaken the surface of the concrete, and will require scarification to remove all weakened material down to sound concrete, followed by patching or leveling with an appropriate cement-based patching material or self-leveling underlayment.
Options for the isolation of shrinkage cracks in a concrete slab includes membranes such as liquid roll-applied rubber, trowel-applied, peel-and-stick and sheet. These membranes are designed to distribute “in-plane” movement stresses of the substrate and prevent tile cracks.
However, these should not be used over structural control joints, or an area where “out of plane” or shearing movement could occur. For specific questions regarding a substrate, call one of the many technical-support lines offered by tile-adhesive manufactures, or trade organizations such as the National Tile Contractors Association (NTCA), Tile Council of America Inc. (TCA), Materials and Methods Standards Association (MMSA), or Marble Institute of America (MIA).
The reason for cleaning all foreign substances off a substrate is simple: Foreign substances are bond breakers.
One common bond breaker is a sealer or curing compound. If there’s suspicion about a possible sealer on the surface, a simple test can be conducted by dripping a couple of water droplets on a clean surface. The droplets should absorb easily into the concrete within a few minutes. If they bead up on the surface, it’s an indication of a sealer or curing compound.
For best bond results, the surface should be clean, dry, have open pore, and a texture like sandpaper – not glass.
Suspended substrates can be wood-, steel- or concrete-based. The extent of designs is quite numerous, but a few basic points remain important to a tile installation.
Deflection in a floor is a concern for installing natural stone; the industry will accept an L/720 for natural stone installations.
What does L/xxx mean? “L” is simply the distance of an unsupported span of the floor; 720 is the deflection criteria. Divide the L (span distance) by the criteria number (720); this resulting number indicates the maximum deflection, at the center of the span, that an installation can withstand when reaching its designed live and dead loads.
The reason for this limitation is that bending or deflection beyond those limits can cause problems such as grout cracks, tile cracks or bond failure of the tile to the surface. It’s best to consult an architect or engineer who can obtain this deflection information by analyzing the structure.
(Note: For the concrete-based suspended substrates, the same curing and surface-preparation issues apply as noted earlier in this article. The difference is that, in addition to those requirements, this surface will need to meet the deflection requirements of L/720.)
For wood or light-steel-frame residential construction, the basic requirements are 16” on center framing, with a minimum 1 1/4”-thick sub-floor and underlayment combination. The underlayment can be minimum 1/2″ thick APA Exterior Grade Group 1 CC/BC/AC plywood, or cement backer board with a thickness of 1/4″ or 1/2″.
There are some exceptions with wider support spacing, but the key to success is in the deflections not exceeding the number above both along the span of the framing as well as in-between the framing members. Recently, the TCA endorsed two new floor systems that will allow residential customers to break the old 16” on center framing rule.
One involves an un-coupling system, and the other a double layer of 3/4” plywood. These can be used over substrates that have the newer engineered truss and I-beam-shaped engineered wood beams.
Don’t install natural-stone tile over an existing wood-based underlayment unless absolutely certain that it’s a new exterior-grade underlayment. When installing underlayments, please follow the most current guidelines from the cement-backer board manufacturer, or from APA – The Engineered Wood Association for wood-based underlayment.

ADHESIVE SELECTION
The selection of the adhesive setting system is the next challenge with installation. There are many options with mortars, including dry-set, latex modified dry-set, fast-setting, epoxy, super-flexible, and full-contact. It’s best to select the adhesive setting system based on the tile to be installed.
As a rule, a mortar system with a liquid additive gives better overall performance in most applications. Because there there’s a wide selection of natural stone processed for tiles, contact the adhesive supplier in order to select the system that best meets the job’s needs.
For most tile installations over a concrete or approved plywood and cement backer board underlayment substrates, a one-step polymer-modified thinset adhesive will work very well. The one-step mortars are general purpose that offers good bond strength, the convenience of no liquid additive to carry and a reasonable price.
With larger-format tiles of 16” X 16” or greater, or with non-moisture-sensitive natural-stone tile, consider a full-contact or medium-bed mortar; the color isn’t important, unless it’s a translucent natural stone. These mortars are designed to allow for a thicker application of the adhesive, which will help compensate for substrate or tile variance and allow for a flat, finished surface free of lippage.
For heavy-duty exterior applications of natural-stone tile, consider a liquid additive and thinset system, or super-flexible latex modified thinset systems. These are designed to have tenacious bond, better water resistance and more flexibility; this compensates for expansion or contraction in the tile and substrate, and maintains bond integrity.
When a fast turnaround time is needed, or the tiles are moisture-sensitive, a fast-setting thinset system is recommended. These high-tech, cement-based products will set in two to three hours, allowing the floor to be open to foot traffic or grouting. This is especially helpful in exterior installations, airports or shopping-mall renovations where down time issues are critical.
The heaviest-duty applications – or ones using extremely moisture-sensitive natural stones such as green, black and dark red marble – should be installed with a 100-percent solids epoxy mortar system.
Remember that if you prepare your surface correctly, and select the best adhesive for your application, you can be assured of a long lasting durable tile installation for years to come.
Mike Daniels is manager of technical service for the United States for MAPEI Corp. in Deerfield Beach, Fla

This article first appeared in the November 2002 print edition of Stone Business. ©2002 Western Business Media Inc.