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Expansion Joints

Why are movement joints needed?

Recognizing that tile is a facade, movement joints are needed to eliminate stresses that can occur between the substrate and the tile due to differing amounts of expansion and contraction.

Where should movement joints be placed?

The TCNA Handbook recommends allowing for expansion and contraction in every tile installation. In small rooms, a gap at the perimeter of the room (often hidden by baseboard or shoe molding) is sufficient. For larger areas, the movement joints will be visible.

We cannot specify the exact location nor frequency of movement joints as there are many site related conditions that must be addressed; however, we do offer guidelines in Detail EJ171 in the TCNA Handbook. It is especially important to note for interior installations, movement joints are placed more frequently when moisture or direct sunlight is expected. For exterior installations, the range of temperature from summer highs to winter lows must be considered.

Why do rooms with more sunlight need more movement joints?

The intent of the guideline regarding sun exposure is to recognize that areas that get warm (or wet) may experience greater amounts of differential expansion. If the areas exposed to sunlight are warmer than surrounding areas, movement joints should be used more frequently. If the tile surface is not appreciably affected, no accommodation is needed in the joint spacing.

Only the area subjected to increased temperature needs to have movement joints more closely placed, not the entire floor if elsewhere the floor is an even temperature.

What other things should be considered when determining spacing for movement joints?

Many things can subject the tile layer to shear forces in addition to temperature and moisture. The following is a partial list:

  • Continued curing of the concrete substrate can put the tile in compression
  • Deflection and vibration of the substrate - particularly with suspended slabs
  • Seismic activity
  • Changes in the plane of the substrate
  • Location of weight-bearing columns
  • Type of tile or glass

From what is a movement joint made?

Movement joints are filled with material that allows for contraction and expansion. For floor applications, urethane, neoprene, or polysulfide are most often used in traffic areas and silicone sometimes where traffic is not a concern. Traffic areas require a sealant with a shore hardness of 35 or greater.

What causes tile to tent?

Tile heaved off the floor, or tented, and sometimes cracked is often a sign that movement joints were not used sufficiently. For tile over concrete, the curing of the concrete places the tile under compression.

Why do installations tent after a number of years?

How long it takes tile to tent is directly a function of at least three variables - the rate of concrete shrinkage, the shear strength of the thinset, and any expansive forces applied to the tile layer (for example, heat). When the tile is poorly bonded, the tile can tent very quickly. If there is a strong bond, often the grout will compress significantly before the tile will lose its bond. Of course, the type of tile is important as well. Thinset has a harder time bonding to porcelain than most other tile. At the other extreme, I have seen a saltillo installation where the tile did not tent but rather spalled as the thinset and grout were stronger than the tile.

When tile fails with a loud report, this certainly indicates that a good bond was present. Only when the shear force exceeds the strength of the bond, will the tile let go. Frequently, either the tile or the concrete will be without thinset residue - as if the thinset was not applied correctly originally. Usually, if the tile is tenting years after the installation, this was not the case. Had the thinset not been applied correctly originally, the tile would have tented long before. Rather, it is important to consider that the cleavage plane will usually occur at the thinset transition - either the bond to the concrete or the bond to the tile, depending on the relative permeability and exact composition of each.

Hence, it is common to see one surface or the other sheared clean of thinset. Even in "explosive" failures where the tile cracks and "jumps" off the floor, usually one surface is free of thinset. Clearly a good bond had been established.

With organic bonding agents and some of the polymer-modified thinsets, continued shear forces degrade the bond over time. So even when tile tents without an explosive report, the original installation may have had sufficient adhesive.

In summary, every installation should allow for movement. Properly designed installations, where expansion and contraction do not create shear forces, should have no problem for the tile to stay well adhered.