Use grout in the joints between flat tiles on the same surface, and use a flexible sealant (caulk) at shower inside corners and other changes of plane, such as the wall-to-floor joint and the line where tile meets a tub. Grout is rigid, and MAPEI notes that standard grout in those joints will eventually crack, so a flexible joint goes there instead.
Where each material belongs
Grout: the flat joints
Grout fills the spaces between neighbouring tiles on the same surface. According to the Ceramic Tile Foundation, grout joints offer no movement-absorbing qualities, which is why joints that need to move are filled with a resilient sealant instead.
Flexible sealant: the changes of plane
MAPEI lists the joint spaces between floor and wall, countertop and wall, tub and wall, and the corners of a shower as movement joints that need something flexible. The TCNA Handbook guidance reproduced by Stone World also calls for interior movement joints at all inside corners. In a typical shower that means:
- Vertical inside corners where two walls meet.
- The joint where the wall tile meets the floor tile or shower base.
- The joint where tile meets a tub.
Why grout cracks at corners
A shower corner is a joint between two planes, and MAPEI treats it as a movement joint. Grout packed into it is the situation MAPEI says will eventually crack. ANSI A108.02 section 4.4.3, as cited by Stone World, says movement joints are kept free and clear of setting and grouting materials. In practice that means leaving the corner joint open while grouting the field tile, then filling it with sealant once the tile and grout are dry.
What the tile standards say
The tile-industry references for these joints are the ANSI A108 specifications and TCNA detail EJ171. The points below come from the guidance quoted in the sources linked in this article:
- Sealant type: ANSI A108.01 section 3.7.4.1.1 lists silicone, urethane and polysulfide as suitable sealants, as quoted by Stone World. The Ceramic Tile Foundation points out that the list does not include acrylic latex or siliconized latex caulking.
- Joint width: for an interior change of plane, the TCNA guidance says the joint is the same as the grout joint but never less than 1/8 in (about 3 mm).
- Backing: where a back-up strip is used, the sealant must not bond to it, according to the Ceramic Tile Foundation.
- Sealant standard: the sealant should meet ASTM C920 or whatever the project specification indicates, according to the Ceramic Tile Foundation. ANSI also says to follow the sealant manufacturer's recommendations.
- Timing and dryness: sealant goes in after the tilework and grout are dry, and the joint should be free of grout or mortar and dry.
These are tile-industry installation standards, not building code provisions, and this article does not cite any Ontario Building Code articles. Editions of ANSI A108 and the TCNA Handbook are revised periodically, so confirm the current edition and the instructions for your specific products. Confirm any code-related questions with your municipal building department.
Choosing the sealant
Silicone, urethane and polysulfide are the only sealant families named in the ANSI A108.01 list quoted above. As an example of a bathroom-oriented product, MAPEI describes its Mapesil T Plus as a 100% silicone sealant that is mould- and mildew-resistant and usable for joint widths up to 1 in (2.5 cm).
Manufacturer recommendations can differ from the ANSI list. The same MAPEI page describes its Keracaulk S and Keracaulk U as siliconized acrylic caulks that suit shower and bathroom corners. That is MAPEI's own product position. Siliconized acrylic is not on the ANSI A108.01 list of silicone, urethane and polysulfide, and the Ceramic Tile Foundation says siliconized latex caulking is not on it either. Do not assume a Keracaulk-type caulk satisfies an EJ171 movement joint. If a project specifies one, ask the installer and the manufacturer which product is acceptable for that joint.
- Keep corner joints open and free of grout when grouting so the sealant has a proper joint to sit in.
- Install the sealant after the tile and grout are dry, and follow the sealant manufacturer's instructions.
Grout and movement joints
According to LATICRETE, its epoxy grouts do not need periodic sealing. Other grout products vary, so check the manufacturer's data sheet for sealing, cure times and care instructions. Because ANSI A108.02 section 4.4.3 says movement joints are kept free of grouting materials, a corner or change of plane is still left open for flexible sealant whichever grout is used in the field tile.
Common mistakes
- Grouting a corner, then caulking over it. ANSI A108.02 section 4.4.3 says movement joints are kept free of grouting materials, so the sealant needs an open joint.
- Using acrylic latex or siliconized latex caulk where an EJ171 movement joint is specified. The Ceramic Tile Foundation notes these are not on the ANSI sealant list.
- Applying sealant over a joint that still holds grout. The joint should be free of grout or mortar and dry.
When to redo caulk
Replace sealant that has failed, and treat cracked grout in a corner as a sign that the joint needs flexible sealant instead. If the corner was grouted, the grout has to come out so the joint is free of grout, and the joint needs to be dry before new sealant goes in. Then follow the sealant manufacturer's instructions for application.
If you are planning a shower renovation, the bathroom renovations page is a place to start.
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