Caulk vs Grout: Where Movement Joints Go and Which Sealant to Use

Use grout in the joints between tiles in the field of a surface, and use a flexible sealant (caulk) wherever the tile changes plane, meets another material or a wall, and at planned movement joints. TCNA EJ171 calls for these soft joints because rigid grout cannot absorb movement and will crack there.

Why tile assemblies move

A tile installation is a stack of materials (framing or concrete, backer or membrane, mortar, tile, grout) that respond differently to temperature, moisture and load. Concrete shrinks as it cures over months. Wood framing swells and shrinks with humidity. Tile and stone expand in heat and in sunlight. Porcelain and ceramic tiles also undergo slow, permanent moisture expansion over their life. Even a well-built floor deflects slightly under foot traffic.

Each of these movements is small, but tile and grout are stiff and brittle in tension. If the stresses have nowhere to go, they concentrate at the weakest point: usually an inside corner, a perimeter or a long uninterrupted run. The result is cracked grout, debonded tile or, in extreme cases, tenting, where tiles pop up into a ridge. A movement joint gives that stress a controlled place to go. For the full set of crack causes, see why grout cracks.

Where sealant goes and where grout goes

The inside corner of a shower is the classic mistake. It looks like any other grout joint, so it gets grouted, and within months a hairline crack appears from top to bottom. That crack is not a grout defect. It is the joint doing what EJ171 predicted.

EJ171 spacing and width guidelines

TCNA EJ171 is the industry's movement joint guideline, published in the TCNA Handbook. It is written for designers, who are responsible for specifying the joint locations, but installers and owners use it as the practical rulebook. Commonly cited provisions include:

Interiors near large sunny windows, rooms with radiant floor heating and any wet area fall into the higher-movement category, which is why showers need soft joints at every change of plane even though they are small. Exteriors get the tightest spacing because of large temperature swings and moisture cycling. See outdoor and pool grout for exterior grout selection.

EJ171 also says movement joints in tile must be placed over any joints in the substrate (control, cold, expansion and construction joints in concrete) and must be at least as wide as those substrate joints. Structural expansion joints are not to be bridged with tile; they come up through the assembly and need their own engineered detail.

Anatomy of a correct movement joint

A movement joint is more than caulk in a grout line. It needs three things to work:

• A clean break through the assembly. The joint must go through the tile and the mortar bed to the substrate (or to a membrane). Mortar or grout bridging the bottom of the joint ties the two sides together and defeats it.
• A bond breaker or backer rod. Sealant must stick only to the two sides of the joint, not the bottom. Three-sided adhesion means the sealant cannot stretch freely and tears. Use closed-cell or bicellular backer rod sized about 25 percent larger than the joint width (backer rod is covered by ASTM C1330), or bond breaker tape in shallow joints.
• The right sealant depth. Manufacturers commonly recommend a depth about half the joint width, within stated minimum and maximum depths (often no less than 1/4 inch deep for floor joints). A thin hourglass-shaped bead stretches better than a deep square plug.

Prefabricated movement joint profiles (metal or PVC with a flexible insert) are a common alternative in floors, especially in commercial work. They are set into the mortar as tile goes in and give a cleaner edge than field-applied sealant in high-traffic areas.

Choosing a sealant

EJ171 points to sealants meeting ASTM C920, the specification for elastomeric joint sealants. C920 classifies sealants by type (S single-component, M multi-component), grade (NS non-sag for walls, P pourable for floors), movement class (for example Class 25 means it can handle plus or minus 25 percent of joint width) and use (T traffic, NT non-traffic, plus others for substrate types). For a floor joint, look for Use T.

Two special cases are worth stressing. On marble, limestone, travertine and other porous stone, use a sealant labeled non-staining for stone (manufacturers often reference ASTM C1248 stain testing), because ordinary silicone can wick oils into the stone and leave a dark halo along the joint. See grout for natural stone. In showers, choose a mildew-resistant formula and accept that sealant is a maintenance item: plan on replacing it every few years, or sooner if it discolors or pulls away.

Installing sealant in a tile joint

1. Keep the joint clean during grouting. Tape the joint or fill it temporarily with backer rod so grout does not get in. Rake out any grout that does before it hardens.
2. Wait for the grout to cure. Sealant manufacturers typically ask for the adjacent grout to be cured and dry, often 24-72 hours or more, because residual moisture and alkalinity can affect adhesion.
3. Install backer rod or bond breaker. Push the rod in evenly with a blunt tool to set the sealant depth. Do not puncture closed-cell rod, which can outgas and leave bubbles.
4. Mask and prime if required. Tape both edges for a crisp line. Prime the joint faces when the sealant TDS calls for it on that substrate.
5. Gun and tool in one motion. Fill the joint fully, then tool with a spatula or tooling stick to press sealant against both sides and form a slightly concave surface. Remove tape immediately.
6. Protect while curing. Keep water off a shower sealant for the cure time on the label, often 24 hours, and keep traffic off floor joints as directed.

Fixing a cracked grouted corner

If an inside corner or tub edge was grouted and has cracked, adding caulk over the crack is a common but weak fix: the thin layer of sealant is bonded to cracked grout that keeps moving, and it soon splits. The durable repair is to remove the grout from the joint completely, ideally to the full tile thickness, clean out dust, then install sealant as described above. An oscillating tool with a carbide blade or a manual grout saw works well; see how to remove grout.

Recurring cracks across the field of a floor, rather than at corners, usually signal missing field movement joints, excessive deflection or a substrate crack telegraphing through. These need a diagnosis before regrouting. Our grout failure diagnostic chart and repair or replace grout guide help with that decision.

Making movement joints look good

Owners sometimes resist field movement joints because they expect them to look like a mistake. Done well, they are close to invisible. Place them on grout lines (never through the middle of a tile), align them with doorways, cabinet faces or transitions where a line already exists, and use a sealant color-matched to the grout. Sanded-finish sealants reproduce grout texture well in dry areas. On large floors, layout planning with the floor grout and joint plan together avoids last-minute compromises.