black Marble 12x24 subway tile shower wall

What Happens During Installation

You are absolutely right. Walls in a shower are rarely perfectly plumb (vertically straight), square, or flat. When you install large-format rectangular tiles like 12x24, these imperfections become highly visible.

The main issues are:

1. Lippage: If the wall has dips or bulges, the edges of the tiles will not sit flush with each other. This creates a "lippage" effect where one tile is higher than its neighbor, which is a tripping hazard for hands, feels unprofessional, and catches light awkwardly.

2. Crooked Grout Lines: As you try to follow an unplumb wall, your vertical grout lines will begin to slant or drift, destroying the clean, linear look of the subway pattern.

3. Awkward, Sliver Cuts: By the time you tile to the adjacent wall or corner, the lack of squareness will force you to cut very narrow, tapered pieces of tile, which looks like an obvious mistake.

What We Need To Do When Installing

The goal is to create a perfectly flat and vertical plane before the first tile is set. Here is the essential process:

1. The Mandatory First Step: Wall Preparation

• Check for Plumb and Flat: Use a long (6-8 foot) spirit level or a straightedge across the entire shower wall area. Identify all the high spots, low spots, and how far out of plumb the walls are.

• Fix the Substrate: This is the most critical part. You cannot fix a wavy wall with extra thinset.

♢For significant irregularities: The walls must be floated with a plaster mix or a cementitious leveling compound to create a new, perfectly flat and plumb surface.

♢For standard backer board (e.g., cement board): Ensure it is installed as flat as possible. Shim the studs behind it during installation to eliminate any inward bowing.

2. Careful Layout Planning

• Find Your Starting Point: Do not start tiling from a corner. Find the most visible, center line of the back wall and establish a perfectly plumb reference line using a laser level or a chalk line weighted with a plumb bob.

• Dry-Lay the Tiles: Place tiles (with spacers) along the floor and up the reference line. This shows you how the tiles will fall at the corners and edges. Adjust your starting point to avoid sliver cuts (less than 2-3 inches wide).

3. Installation Technique for Large Format Tiles

• Use the Right Mortar: Use a premium, polymer-modified (fortified) thinset mortar with a 1/2" x 1/2" square notch trowel. This ensures maximum coverage and prevents hollow spots behind the large, heavy tiles.

• Use a Leveling System: This is highly recommended. Clip-based leveling systems (like LASH or Spin-Doctor) pull tiles flush with each other as the mortar sets, virtually eliminating lippage and creating a perfectly flat surface.

• Back-Butter: "Back-butter" each tile by skimming a thin layer of mortar onto the back of the tile before setting it. This ensures 100% mortar coverage, which is crucial in a wet area like a shower.

• Check Constantly: Use your level constantly to ensure each tile is flat and your rows are staying perfectly level and plumb.

4. Addressing the Final Gaps

• Despite all preparation, the gap between the last tile and the adjacent wall will likely be uneven. This is normal.

• This gap will be covered by the shower fixture trim plate and, ultimately, sealed with a color-matched silicone caulk. The caulk will bridge the uneven gap, creating a clean, waterproof seal.