Movement Joints in Tile Floors

Why this matters

A tile floor with good coverage, a flat substrate and no movement joints can still fail, and when it does it fails loudly: a row of tile lifts into a tent along the middle of a sunroom on the first hot afternoon, or a crack runs straight across a kitchen exactly where the slab was saw-cut. Both are callbacks the crew caused by grouting a joint that should have been soft. Movement joints are cheap to put in at layout and nearly impossible to add after grout cures, so the decision belongs at the start of the job, on the layout drawing, not at the grout bucket.

What a movement joint is, and what it is not

A movement joint is a gap through the tile and the setting bed, down to the substrate, filled with a flexible sealant instead of grout. Tile, mortar and grout are rigid and nearly incompressible. The things under and around them are not: a concrete slab shrinks for months after placement and moves with temperature, a wood subfloor grows and shrinks with humidity, and tile in direct sun heats and expands faster than the slab below it. A grout line cannot absorb any of that. A sealant joint can, because it stretches and compresses within its rated movement capability.

Three things are commonly confused with a movement joint:

• A grout joint filled with caulk. Siliconized acrylic "sanded caulk" sold to match grout colors has little movement capability. It looks right on the day of install and tears or hardens within a season. EJ171 calls for sealant meeting ASTM C920, and on a floor that means one classified for traffic use (Use T in the C920 classification).
• A crack isolation membrane. It bridges small in-plane cracks under the tile. It does not replace the joint pattern, and it does not cover a structural expansion joint.
• The expansion space at the walls. On a floating floor that gap is the whole movement strategy. On tile the perimeter joint is one part of a pattern that also runs through the field.

The joint pattern, as TCNA EJ171 commonly states it

The TCNA Handbook carries the movement joint guideline EJ171. It binds nobody on its own: it reaches a job through a project specification, a tile or setting-material maker's warranty terms, or a contract that names the Handbook, and a residential crew should treat it as the standard a dispute will be judged against. Check the current Handbook edition; the figures below are as EJ171 is commonly cited.

Perimeter joints go wherever tile meets a restraining surface: walls, columns, cabinet bases, a curb, a step, the frame of a floor drain or a fixed island. They are usually hidden under base or shoe.

Field joints go through the tile field at a spacing that depends on exposure:

Joints over joints in the substrate. Every control joint, construction (cold) joint and expansion joint in the slab gets a joint in the tile directly over it. A structural expansion joint is honored at its full width, usually with a prefabricated movement profile, because the building is designed to move there.

Changes of plane. Where floor tile turns up a tiled riser, a curb or a wall, the inside corner is a movement joint, not a grout joint.

Width and depth. EJ171 sets minimum widths that grow with exposure, and the sealant maker sets the depth-to-width ratio and whether the sealant goes over a backer rod or a bond-breaker tape. The point of the backer is that the sealant bonds to the two tile edges only. Sealant bonded to the bottom of the joint as well (three-sided adhesion) cannot stretch and tears. Read both documents for the joint you are building rather than carrying one width from job to job.

Worked example: a great room, a kitchen and a sunroom on one slab

A 1990s house, slab on grade. The customer wants one porcelain plank tile through a 34 ft by 18 ft great room and kitchen and a sunroom off the south wall that is 14 ft wide, 10 ft deep and one 6 in step down. The slab has one saw-cut control joint running the 18 ft width, 15 ft from the west wall, and an island sits in the kitchen on the slab.

Great room and kitchen. Interior, not in direct sun, so the 20 to 25 ft spacing governs. Honoring the saw cut divides the 34 ft length into 15 ft (west of the cut) and 19 ft (east of it); 15 plus 19 is 34. Both runs are under 20 ft and the 18 ft width is under 20 ft, so the saw-cut joint is the only field joint the room needs. The layout drawing has to place a grout line exactly over the saw cut, which is why this is decided before the first tile is set: if the plank layout puts the middle of a tile over the cut, the joint either gets cut through the tile (ugly, and the tile edge chips) or the layout shifts.

Sunroom. The glass makes it an interior space in direct sun, so the 8 to 12 ft band governs. The 14 ft width is over 12 ft, so one field joint runs across the 10 ft depth and splits the width into 7 ft and 7 ft. The 10 ft depth is inside 12 ft, so no joint runs the other way.

Perimeter and plane changes. Every wall in both rooms, the island base, and both edges of the tiled step riser: the top edge where floor tile meets the riser tile and the bottom where riser tile meets sunroom floor tile.

 perimeter joint at every wall, under the base
 +----------------+-----------------------------+
 |  GREAT ROOM    |  KITCHEN      +--------+    |
 |  15 ft         |  19 ft        | ISLAND |    |
 |                |               +--------+    |
 |                |   joint at the island base  |
 |                |                             |
 |                | <- field joint over the     |
 |                |    slab saw cut             |
 +---==========---+-----------------------------+
 |   6 in step    |  joints at top and bottom
 |       :        |  of the tiled riser
 |  7 ft : 7 ft   |
 |       :        |  SUNROOM 14 ft x 10 ft,
 +-------+--------+  field joint at 7 ft

What goes in the joints. A traffic-grade C920 sealant in a color matched to the grout, over a closed-cell backer rod sized to the joint, tooled flush or slightly concave. The sunroom joint gets the same sealant; direct sun is the reason for the spacing, not for a different product, unless the sealant maker says otherwise for UV exposure.

What the customer is told. Before tile goes down: "There will be a soft line across the kitchen and one across the sunroom. They look like grout, they feel slightly softer, and they are what keeps the tile from cracking where the slab moves." Customers who discover a soft joint later assume it is a defect and pick at it.

How the example changes

An uncoupling membrane is going down. A membrane does not delete the EJ171 pattern. Its maker states how movement joints are handled over the membrane and over slab control joints, and the warranty depends on following it. Structural expansion joints come through regardless.

The slab has a random shrinkage crack in the kitchen. A crack that is not a designed joint is usually treated with a crack isolation membrane per that maker's limits on crack width and vertical offset, not with a field joint. A crack with vertical offset or one that is still opening is a substrate question first. See related: Subfloor Assessment + Repair Reference.

Radiant heat in the slab. Heated floors move more. The heating system maker and the setting-material maker may call for tighter spacing than the unheated interior band, and their instructions govern.

A wood subfloor. Plywood moves with humidity, so the perimeter joint is doing more work than on a slab. Tile over wood is its own assembly. See related: Tile Assemblies Over Wood and Concrete.

Natural stone. Stone generally follows the same EJ171 pattern; the setting material and sealant makers may restrict sealant chemistry to avoid staining the stone, so check before buying the sealant.

Reading the failure in the field

Tenting. Tiles lifting in a ridge, often with a hollow sound either side, is compressive stress with nowhere to go: no perimeter joints, grouted perimeter joints, or field spacing too wide for the exposure. A sunroom tented in the first hot week is the textbook case. The diagnosis of loose tile and tenting is its own subject; see related: Loose Tile + Floor Movement Diagnosis Reference.

A straight crack. A crack that runs in a straight line across the field, through tile and grout alike, usually mirrors something below: a saw cut or cold joint that was tiled over without a joint. Find it with the slab drawing or by measuring from the walls to where the control joints show in the garage or an unfinished area. The worked case is in The Tile Floor That Cracked in a Straight Line.

A sealant joint that has torn or gone hard. Look at the bottom of the joint. Sealant bonded to the slab with no backer (three-sided adhesion), a non-C920 caulk, or a joint narrower than the movement it has to take are the usual causes. Cut it out, install backer, and re-seal; the joint location was right.

How to verify the pattern before grout

• The layout drawing shows every joint: perimeter, field, over every substrate joint, and at every plane change, with the measured distance from a reference wall.
• Each field joint location on the drawing has been checked against the spacing table for that room's exposure, with the runs written down (15 ft and 19 ft; 7 ft and 7 ft) and none over the band.
• Before setting, chalk lines on the substrate mark the slab joints so the tile layout can land a joint on them.
• After setting and before grout, every joint location is clean down to the substrate or membrane: no mortar bridging, no spacers left in, no tile edge crossing it. A joint with mortar packed in the bottom is a grout joint in disguise.
• After grout, joints are dry and clean, backer is seated at the depth the sealant maker states, and the sealant is tooled with no voids.

Finished work is judged standing, in normal light. A well-matched sealant joint should read as a grout line from standing height.

References

• TCNA Handbook for Ceramic, Glass, and Stone Tile Installation, EJ171 Movement Joint Guidelines (current edition)
• ASTM C920, Standard Specification for Elastomeric Joint Sealants
• Sealant and setting-material manufacturers' installation instructions and warranty terms
• See related: Loose Tile + Floor Movement Diagnosis Reference; How to Lay Out and Set a Tile Floor; How to Grout a Tile Floor and Seal the Movement Joints