Proper surface preparation for residential installations of ceramic tile or stone is probably the most important phase of the work, yet often the most overlooked. Even when it is recognized, not enough time or resources are allocated to ensure that it is done correctly. Let’s discuss what is required of surfaces to receive ceramic tile and stone.
First, all substrates must be structurally sound and should be free of any bond-breaking or bond-inhibiting materials. These include sealers, waxes, curing compounds, form release agents, paint, dirt, grease, oils, and similar contaminants.
There are also several things we need to be aware of with concrete. One condition can be concrete laitance from over-troweling. This is the weak cement water slurry brought to the surface during the concrete finishing. Trying to adhere to this surface or any other contaminated surface may result in partial, if not complete, loss of bond.
The other extreme is a very dense and shiny concrete finish. Some concrete floors are finished to the point where the surface is almost glass like. The adhesive mortars for tile and stone can have a hard time adhering to this type of surface. In addition, some additives used in the concrete mixture can also present bonding issues. If any additives are used, it may be best to consult with the manufacturer to determine if there will be any potential bonding issues.
Any bond-breaking or bond-inhibiting materials must be completely removed prior to the installation. One of the best ways to remove these contaminants is by mechanical abrasion. Usually, the contaminants have penetrated into the substrate’s pores and have blocked the adhesives’ ability to create a mechanical bond or attachment to the surface. It is not a good idea to use chemicals or acid to strip and remove contaminants. These chemicals or acids can also get into the substrate and will be virtually impossible to remove thoroughly. Some other potential bond breakers can include ice on the surface or a frozen surface.
Some other items that need attention are the cure times for new concrete. It may be a good idea to determine the moisture vapor emission rate if the finish material and the installation system is sensitive to this condition.
At this point we have a clean surface. Are we ready for tile? Not yet. We now need to make sure that the surface is rigid enough to accept the tile or stone finish. The Ceramic Tile Handbook published by the Tile Council of America explains that it is the responsibility of the project architect or engineer to determine if the substrate meets the proper deflection criteria. The substrate needs to meet the maximum allowable surface deflection for the finish and the installation system that will be installed. Keep in mind that deflection is the potential movement that the installation can experience when subjected to use.
An interesting fact is that 70-percent of all ceramic tile installations and 78-percent of all stone installations do not meet appropriate deflection ratings. How can we figure the amount of movement my tile floor will experience? Usually, this should already have been done by the time you get on a project by the architects or engineers. However, renovation projects may again present the biggest challenge in this area. The following are a few formulas that you can use as a rule of thumb.
If installing a stone floor, then we need to meet the L/720 requirement. This can be explained as L/720 or 1” (2.5 cm) of deflection over a 60-ft. (18 m) span (where L = Length of span; and 720 = Number of inches in 60 feet).
For example, if floor joist length is 288” (24 feet, or 7.32m), then the substrate can only experience 288”/720” = .4” (1 cm) of movement.
It is important to note that these deflection standards are the minimum requirements for the surfaces. The potential added weight of any appliances, stone countertops or even the intended use of the area needs to be taken into consideration. For example, a room that will be used to entertain guests will require a more stringent deflection rating than an area subjected to less stress.
Another area of confusion exists in the L/360 designation. It is important to note that the L/360 deflection criteria required by the ceramic tile industry and the L/720 deflection required by the stone industry are subjected to a 300-lb. concentrated load. This is much different than the L/360 building code requirement deflection rating which is usually distributed uniformly over the entire area. The concentrated load will require a much stiffer surface.
The surface to be tiled should also be within the manufacturers required temperature range. The surface to be tiled needs to be smooth enough to receive the tile or stone. Industry standards require the surface to be true and level with a maximum variation in the substrate not to exceed 1/4” in 10-ft. Some installations may even require a more stringent tolerance for in-surface plane irregularity. How about when a pitch is required towards drains or scuppers? As a rule of thumb, the industry requires 1/4” per foot of slope for maximum water evacuation.
To adjust irregularities in the substrates, high strength patching, leveling or screeding mortars can be used to bring the substrate into compliance.
Non-structural shrinkage or spider web type cracks can be treated with crack isolation or anti-fracture membrane to help reduce the potential for the transmission of those cracks through the finished surface.
Requirements for Mortar Bed
- Well compacted
- Appropriately sloped
- Unbonded mortar bed; minimum thickness and wire-reinforced (1-1/4” min./31 mm
- Bonded beds; no minimum thickness required. Slurry bond coat required
- Waterproofing membrane (if necessary)
- Interior only
- No chemical treatments (e.g. marine grade or pressure-treated plywood)
- Refer to industry standards for design criteria
- Waterproof (if necessary)
- Expansion joints
- Deflection guidelines
Do not install tile or stone directly to:
- OSB (Oriented Strand Board)
- Particle Board (any variety)
- Marine Grade or Fire Retardant Plywood
- Hardwood Flooring
- Foam Board
We all have a vested interest in installing permanent trouble-free tile and stone installations. This helps to promote greater consumption of ceramic tile and stone. As this consumption grows, all of us in the industry benefit. Therefore, let's make sure a successful tile installation starts with good surface preparation.