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Movement joints

Going deeper into TCNA EJ171

All construction materials move – at different rates from other materials – and in some cases can expand and contract very quickly. This is why movement joints in tile work are “essential and required” in the Tile Council of North America (TCNA) Handbook for Ceramic Tile, Glass and Stone Tile Installation’s Movement Joint Guidelines section EJ171. Please don’t read this article until you familiarize yourself with the aforementioned Handbook section. The article assumes you know this section, as you must if you install tile, and should if you’re in the industry. 

If you install sealant, you must also know ASTM C1193 Standard Guide for Use of Joint Sealants, which explains proper movement joint construction. In this article, we’ll go a bit deeper and real-world.

The design professional is supposed to show the specific locations and details of movement joints in tile work and specify the sealant application in the sealant section of the specifications. Often though, the tile section specification asks for shop drawings from the tile contractor showing locations of movement joints, which the tile contractor will then install. These drawing must be in compliance with EJ171. Typically, an interior concrete slab on-grade will have a saw-cut grid; this is your layout. If these saw-cuts are too far apart to satisfy EJ171, include “generic” movement joints between them: EJ171F.

EJ171 Generic Movement Joint

Relocating saw-cut joints

Either honor all saw-cut joints and bring them through the tile work, or install a crack-isolation membrane approved by the manufacturer to “relocate” them to the next nearest grout joint per TCNA F-125 (preferred Full Coverage to spread and minimize stress). 

There is no table, chart or code that dictates how far you can “relocate” a saw-cut joint. When ANSI A118.12 was written, the Materials and Methods Standards Association (MMSA) Committee – chaired by the late Harvey Powell – envisioned a 12″x12″ on grid, missing a joint by an inch or two. We never dreamed of an 18″x36″ or 24″x48″ running bond or 8″x48″ random plank pattern. The generally-accepted industry rule of thumb remains the same: with an approved crack-isolation membrane, saw-cut control joints (assumed to be a “controlled crack” with no vertical displacement) can be “relocated” to a soft/movement joint in the next nearest grout joint per F125 Full Coverage in the TCNA Handbook. But note that the farther you relocate, the more you are asking of the membrane, and you increase the risk of potential failure. Full coverage by membrane is best to spread the stress over larger areas rather than partial coverage, where much of the assembly is hard-bonded to the concrete slab with no stress absorbing resiliency within the system. 

When setting the tile, rake out all thinset in the designated movement joint. Thinset must not be in this joint or you defeat the purpose as you’re not breaking/stopping, or dividing the tile assembly into separate sections to allow freedom of movement. This is most critical at perimeters and in-field interruptions like columns, wing walls, elevator chases, escalator bases, planters and the like. Rake out the thinset during installation when it’s wet and workable. It is not practical to cut or dig it out once the
thinset cures. 

For above-ground concrete slabs, there may be no saw-cut joints, so look for logical places to suggest them. They can be placed over a supporting beam where natural bending stresses will occur, or off the corners of openings to the floor below, or at cold joints or “L” shapes – and no more than 12′ apart. 

Post-tension concrete slabs, once the cables are tensioned, are under extreme stress and more sensitive to building movement and live and dead loads than concrete poured on a pan. Consider movement joints more frequent than 12′ on center, especially if the tile is larger. Always use a crack-isolation membrane to ensure the tile assembly can accommodate anticipated life-cycle movement. 

Waterproofing and crack-isolation membranes come in many forms.

Waterproofing and/or crack-isolation membranes come in various forms: liquids, fabric-reinforced liquids, trowel-applied compounds, bonded sheets and peel-and-stick sheets. There will always be good, better, and best performance. Consult your manufacturer of choice for guidance on membrane selection for “relocating” saw-cut control joints.

This mockup of saw-cut relocation per F125 shows a diagonal pattern on the left and a running bond pattern on the right.

Exterior concrete slabs on-ground will also have a grid of saw-cut control joints. EJ171 requires movement joints every 8′-12′ for exterior work, more frequently for larger, dark tile in environments with extreme/rapid thermal expansion/contraction cycling. If the saw-cuts are not frequent enough, generic joints between them should be in compliance with EJ171. 

When applying a waterproofing membrane over the backer board, it must be fabric-reinforced at movement joints, and it must dip into and back out of these joints to allow movement and continuous waterproofing.

It is wiser and safer to honor saw-cut joints in exterior work and not relocate them. Technically, it is often possible and acceptable to a crack-isolation membrane manufacturer, but you are asking a lot from the assembly. 

Cement backer board over wood or metal studs must have movement joints in the backer board, and joints must be honored in the adhered tile: 3/8″ wide on an 8′ grid and 1/2″ wide on a 12′ grid. This requires double studding (EJ171K) or other framing to allow proper backer board edge attachment. You cannot “toe-nail” two sheets of backer board into a single stud with such a wide gap. And when applying a waterproofing membrane over the backer board, it must be fabric-reinforced at these joints, and it must dip into and back out of these joints to allow movement and continuous waterproofing. 

The calculation that EJ171 provides allows one to calculate the expected movement of the project. The example table in the TCNA Handbook for Dry Interior Generic Movement Joints Not Exposed to Direct Sunlight gives various movement joint widths and frequencies depending on a tile’s particular coefficient of linear thermal expansion (CLTE) or α, expressed in inches per inch per degree Celsius (in./in.°C). In the example calculation, figuring a 50°F change in temperature and ɑ 7×106, movement joints every 20′ would be 3/16″ wide. 

Exterior movement joint minimum widths start at 3/8″ for joints 8′ on center and minimum 1/2″ for joints 12′ on center. They increase 1/16″ for each 15°F tile surface temperature changes greater than 100°F between the summer and winter. In a scenario typical in the Southeast, based on a 12′ on center joint spacing, figuring a 115°F change in temperature (20°F in the dead of winter and 135°F in the heat of summer), the width requirement per EJ171 becomes 9/16″.

Create proper joints in your tile assembly to accommodate movement, or failure is certain.

Conclusion 

Tile assemblies will expand and contract throughout their life-cycles. Create proper joints in your tile assembly to accommodate movement, or failure is certain. Make sure that what the design professional specifies and/or what you propose is EJ171-compliant. Always confirm with the manufacturer of the membrane and system components for suitability of applications and warranty terms before installation.  


For additional information on movement joints see the latest CUSTOM Top Five Tips for Installing Movement Joints video on YouTube: https://bit.ly/3vwlCZA.

Chip O'Rear
Southeast Architectural Representative at Custom Building Products | + posts

As Southeast Architectural Representative for Custom Building Products, Chip O’Rear assists design professionals in specifying proper tile work. Educational presentations, plan and spec reviews, job site problem resolution and failure investigation are his areas of expertise. Earning the privilege of being selected and specified is his mission. 

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