By Richard Wall
For hundreds of years, tile installers made sure of their substrate's integrity by making it themselves with mortar. These days they're more likely to inherit a concrete slab as the substrate for their tile job, and it probably won't be in perfect condition. Installers now have to make more judgment calls preparing the substrates for their particular tile applications, and experts say they aren't always getting it right.
Backerboard, which is often cement board that is fiber-reinforced, glass-coated or otherwise treated, serves for wall and countertop tile applications where moisture can get to the substrate. Each type of backerboard has different qualities and capabilities, and installers should check with the manufacturer for proper substrate usage. For walls and areas where moisture is not a concern, gypsum board or similar products are acceptable substrates. When used with the proper techniques and products, these substrates are not a problem for most tile installers and even DIYers.
More on the floor
It's on the floor — concrete slab or wood subfloor — where installers are more likely to have substrate problems. With the increasing popularity of tile floors, installers are more often than not inheriting a concrete slab on the ground. (Applications on concrete upper floors are more complicated.)
The first thing an installer should do is to make sure the slab is clean and cured. Consider the following when installing on a concrete slab as substrate:
- A ground slab should be cured at least 28 days.
- The slab should be clean with open pores that immediately accept water. Many slabs won't, because they have a curing compound that adds strength. "But that compound is a bond-breaker for our adhesives," says Stephanie Samulski, an instructor for the Ceramic Tile Education Foundation. The installer should remove any curing compound or paint over-spray on the concrete slab/substrate. That can require expensive grinding equipment that many smaller companies don't have — and time that they may not choose to invest in the job.
- Large concrete slabs have movement or expansion joints in them that the installer has to respect in the tile layer with a grout joint directly above it, even if it doesn't fit in with the layout.
- When using larger tiles (18 x 18" and 24 x 24" are the trend coming out of Europe) on larger areas, Bart Bettiga, executive director of the National Tile Contractors Association, says, "The installer has to consider putting in a movement joint, such as you would see in a shopping mall or a commercial application. These movement joints in a home, on slab or wood substrate, proactively allow for movement of that tile." The movement joint, which is a soft compound instead of a grout joint, can prevent grout from cracking and the tile from separating, but it can be difficult to blend in with the grout.
- Treat cracks in the concrete slab/substrate with a crack-isolation membrane, a flexible material that allows the tile to move above that crack.
Wood bounces nicely; tile doesn't
Installers should make sure wooden subfloors are adequate as substrates. "You can have quite a bit of bounce to a wood floor and be safe as a floor, but for tile installations not to crack, you need rigidity," says Samulski. She points to three rigidity issues with wood subfloors as tile substrates:
1. Many "value engineered" homes use 19.2- or 24-inch on center joist spacing, which means too much bounce for tile. Stephanie says a sandwiching solution of two layers of wood is often required to stiffen up the floor to serve as a proper tile substrate. This is really a carpentry or engineering issue, but the installer needs to be on top of it.
2. Wood sub flooring panels are shipped very dry from the mill and will absorb more moisture and expand as they acclimate. Installing tile before acclimation is asking for trouble. Samulski says many times carpenters aren't leaving spaces between panels to allow for this, which can also cause tile to buckle.
3. Be cautious with OSB subfloors. While techniques accepted for exterior glued plywood might work on OSB, they are not tested for OSB. Check with the manufacturer of a backerboard product when using it with OSB.
Help in hand
"One trend with substrates is to install ceramic tile over poured self-leveling type floors," says Bettiga. "Poured gypsum floors are used quite often in high rises, condominiums, new construction and commercial buildings. And poured cement floors, called self-leveling cement underlayments, are poured over a wood substrate or a slab to give you a nice flat smooth surface. Both of those products are being used in new construction and remodeling in ceramic tile installations," says Bettiga. "It's a significant trend."
Samulski advises installers to pick up the 2007 TCA Handbook For Ceramic Tile Installation and theANSI Specifications for the Installation of Ceramic Tiles publications (go to www.tileusa.com). Another great source is the NTCA's Ask the Tile Man forum. Ask a question or check out the archives. You might learn something new.
Richard Wall writes for HGTVPro.com from St. Augustine, Fla.
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