Prep for Tile on Floors and Walls
It’s what’s underneath that counts. When it comes to hard surface flooring of any type, substrate preparation is critical to a successful installation. When it comes to tile, not only do we have floors to worry about but also wall installations that require preparation.
The tile industry has done a good job of placing responsibility on the owner, general contractor or project design professional. The section on substrate requirements in the 2018 Tile Council of North America (TCNA) Handbook for Ceramic, Glass, and Stone Tile Installation states: “The owner should communicate in writing to the project design professional and general contractor the intended uses of the tile installation, including in-service loads or information to allow a project design professional to calculate such.”
Here is where tile contractors are given some help. The Handbook notes, “The tile contractor shall not be responsible for problems resulting from any structural subfloor installation not compliant with applicable building codes, unless structural subfloor was designed and installed by tile contractor, nor for problems from overloading. As tile is a finish applied to and relying upon the underlying structure, an inadequate substructure can cause a tile failure. In many cases, problems in the substructure may not be obvious, and the tile contractor cannot be expected to discover such and tile contractor shall not be responsible for designing flooring assembly, unless specifically engaged to do so in writing. Tile contractor cannot determine possibility of an overloaded condition.”
Keep in mind though that if there are concerns with the subfloor it’s the professional’s responsibility to address concerns with the involved parties. A few items often overlooked on installations that end up failing are natural stone over wood substrates, substrate tolerances for large tile (which we seem to be seeing more and more of), stress cracks/joints in concrete, spot bonding, and tile over gypsum-based pours with encapsulated hydronic tubing.
There are flatness requirements for both floors and walls. The TCNA Handbook states that for tiles with all edges shorter than 15”, maximum allowable variation is 1/4” in 10’ from the required plane, with no more than 1/16” variation in 12” when measured from the high points in the surface. For tiles with at least one edge 15” in length or longer, maximum allowable variation is 1/8” in 10’ from the required plane, with no more than 1/16” variation in 24” when measured from the high points in the surface. For modular substrate units, such as plywood panels or adjacent concrete masonry units, adjacent edges cannot exceed 1/32” difference in height.
The finish flatness requirement for stone tile installations is 1/8” in 10’ from the required plane, according to the Marble Institute of America (MIA).
The Handbook also lists substrate tolerances for Natural Stone Tile Installations Over Wood Substrates: “Two layers of structural wood panels are required on floors to receive stone tile when backer board will be used as the tile substrate. The MIA prohibits installation of stone tile over single-layer wood floor systems under backer board because of the discontinuity of the system at seams between the subfloor panels. If an unbonded mortar bed will be installed as the tile substrate, a single layer of wood subflooring is permitted.”
How many times have we seen a backer board over a single-layer subfloor installation with stone? Far too many. Do we see successful installations with this type of system? Yes. Do we also see failures? Yes. It’s a roll of the dice. If you play the odds and are a winner 100% of the time, then I recommend you buy lottery tickets.
With so many large tile installations, the need for a thin-bed method to support these products has led to the creation of Large and Heavy Tile (LHT) mortar. TCNA substrate requirements for this state: “As tile size increases, the negative effect of substrate irregularities is compounded. If specifying a thin-bed method, project specifications should include a separate specification and requirement (such as pourable underlayment) to bring the substrate into compliance if the substrate does not meet the required flatness tolerance.
“Alternatively, when specifying tile with any edge longer than 15” consider specifying a recessed installation substrate and a mortar bed (thick-set) method to produce a tile substrate that meets the more stringent flatness requirements for large format tiles. There is no medium bed installation method that can be used to flatten the substrate while installing the tile, as mortar (including LHT mortar) is not intended for truing or leveling substrates or the work of others.”
Let’s be honest. How often do we see substrates this flat, especially with plank flooring with edges more than 15” in length? That’s why we always communicate the need for floor preparation with our clients. If you don’t, and there is no money bid in for floor prep, what happens? Thin-set mortar becomes the all-in-one product that levels and bonds. I’ve lost track of how many times we go into a floor that has issues, and when it comes to the destructive testing we see that mortar was used to level the tile.
Lippage systems are designed to maintain minimal lippage of tile and stone but do not compensate for a substrate that does not meet flatness requirements. As my friend Phil Green, the inventor of the Back Butter Buddy, always says: “Flat is required and level is desired.” Photos 1 and 2 depict a shower wall out of flatness of 1/2” in 4’, which leads to the tile contractor having to redo the tub surround walls due to flatness and squareness concerns from the framing.
Spot-bonding of floor and wall tile is an installation practice that unfortunately is used to compensate for undulating floors and walls that do not meet flatness requirements. We see so many failures due to this type of installation and the costs for these types of failures get expensive. We see too much of the five-spot or spot bond method being used with thin-set mortars. Sure, they look good for the first few months but after a little time these improper installations start to expose themselves.
So why, then, is the spot bond method used? The TCNA Handbook, Spot Bonding W215-18 Interior Walls Over Masonry or Concrete—Spot Bonding Epoxy, covers the proper method for spot bonding using epoxy and not thin-set mortar. Photo 3 shows a spot bonded shower that failed and is being completely redone.
When it comes to addressing stress fractures or joints in concrete substrates, there are several products available. ANSI A108.17 and A118.12 cover crack isolation membranes. These can include liquid applied, roll on, cleavage/uncoupling membranes or one of the crack isolation thin-set mortars. Whatever you’re using, make sure it meets the performance requirements for the installation. These products do not have warranties for any vertical movement at cracks or joints; only horizontal in-plane movement. Photo 4 shows a crack suppressing membrane installed over control joints. Photo 5 depicts concrete break out for plumbing, with potential vertical movement that no manufacturer will warrant.
One other concern is tile over a gypsum-poured substrate. These are typically in multi-family units or over radiant heat systems. The concerns with a gypsum pour are bonding capability and compressive strength. If the gypsum powders up easily there will be bond issues. If the gypsum scratches easily with a screwdriver tip or knife that’s another red flag.
TCNA RH122-18 addresses the proper procedure for a poured gypsum underlayment encapsulating hydronic tubing. Minimum compressive strength required for a gypsum-based pour is 2000 psi. If the surface scratches easily, chances are it is not hitting the 2000 psi mark. Additionally, over radiant, there needs to be a minimum thickness of 3/4” gypsum poured above the hydronic tubing. For powdery or lower psi substrates, primers/densifiers are used to create a surface to bond to.
Follow the manufacturer’s recommendation for application. Some manufacturers require a dilution ratio with one to four applications followed by a more concentrated ratio for final application. In Photo 6, depicting a gypsum pour with screeds, tile was installed direct bond over a soft gypsum substrate with no primer.
These installation guidelines are here to help protect tile contractors. However, if installing outside of the recommended guidelines, your job could end up costing you a lot more than just the initial profit.