How to Lay Out and Set a Tile Floor

Why this matters

Two things decide whether a tile floor gets a callback: what the customer sees standing in the doorway, and what is under the tile where nobody looks. The first is layout, a sliver cut at the door or a pattern that runs off square across the room. The second is mortar coverage, and a tile set on uncollapsed ridges with hollow corners cracks under the first dropped pan or the first fridge roll-out. Both are decided in the first hour of the set, and neither can be fixed afterward without tearing out tile.

This card owns the shop's coverage rule. Substrate flatness, movement joints and mortar choice belong to the sibling cards named in References.

Before you mix anything

Start state. The substrate is already accepted: flat to the tolerance in 'Floor Flatness Tolerances by Floor Type', the right assembly per 'Tile Assemblies Over Wood and Concrete', clean, and dry. Have on hand: the tile (from at least two boxes, opened), the thinset the tile maker and the mortar maker both list for that tile size, the trowel the mortar maker's chart names, a margin trowel, a mixing paddle at the speed the bag states, a chalk line, a tape, a level or straightedge of at least 4 ft, spacers or a leveling-clip system, and a wet saw.

Measure the tile, not the box. Nominal and actual sizes differ, and the layout arithmetic runs on actual size plus joint. Check two tiles from different boxes for size and for warp across the diagonal. Shade lots should match on every box.

Silica. Cutting porcelain and stone releases respirable crystalline silica. Cut on a wet saw with its water feeding the blade, which is the control 29 CFR 1926.1153 Table 1 lists for stationary masonry saws, and keep dry grinding out of the house. Any dry cut or grind indoors goes through 'Respirable Crystalline Silica and the Construction Standard' before anyone starts it.

Step 1: Find the two layout options on each axis

Every room has two candidate starts per axis: a joint on the centerline, or a tile centered on the centerline. One of them gives larger cuts at the walls. Do the arithmetic for both on both axes before snapping a single line.

Why. Starting a full tile against the most visible wall puts whatever is left over at the far wall, often a strip under 2 in that chips on the saw and pops loose in a year. Balancing from the center gives equal cuts at opposite walls, and picking the better of the two center options keeps them at half a tile or more, the shop's default floor for a perimeter cut.

Subtract the perimeter gap. The cut tile stops short of the wall by the perimeter movement joint, width per 'Movement Joints in Tile Floors'. A cut computed to the wall is too long by that gap.

Step 2: Snap two square reference lines

Snap line A on one axis and line B on the other, each on a joint line from Step 1, not on a tile center. Prove them square with a 3-4-5 triangle scaled up to the room: 6 ft along one line, 8 ft along the other, and the diagonal between those marks reads 10 ft exactly.

Why scale it. A 1/16 in error on a 3-4-5 is twice the angular error of the same 1/16 in on a 6-8-10, and over a 13 ft run it shows as a joint drifting off a wall. Measure the room's diagonals too: if they differ, the walls are out of square, and the lines stay square to each other while the taper goes to the wall that hides it best.

Step 3: Dry-lay one row along each line

Lay tile with spacers, no mortar, along A and along B, wall to wall, and measure the real end cuts. This catches a wall with a bow in it or a door casing that will see a sliver, while moving the line costs a snapped string.

Step 4: Mix to the maker's water and slake time

Put the measured water in the bucket first, then the powder, and mix at the paddle speed the bag gives. Let it slake (stand so the polymers and cement wet out) for the time printed on the bag, commonly in the range of 5 to 10 minutes, then remix briefly. Do not add water after that.

Why. Extra water makes the mortar easy to comb and weak and shrinky when cured, and it shortens open time on a warm slab. Retempering a stiffening batch with water does the same thing to a mortar that has already started to set. Mix only what can be used inside the pot life the bag states, and throw out the rest.

Step 5: Key in, then comb straight ridges

Key a thin layer into the substrate with the flat side of the trowel, pressing hard, then add mortar and comb with the notched side at a steady angle, ridges straight and all in one direction. Comb only as much area as you can cover before the surface skins; touch it, and if mortar does not transfer to a fingertip, scrape it up and re-comb. 'Trowel Notches, Open Time and Adhesive Transfer' covers notch selection and how open time moves with heat and airflow.

Why straight, not swirled. Swirled ridges trap air in closed pockets the tile cannot push out. Straight ridges let air escape at the ends as the tile is pressed across them, so they collapse into a solid bed.

Step 6: Back-butter, set, move, beat in

For large-format tile (any side 15 in or longer), flat-trowel a thin skim on the back of every tile to fill its pattern and wet its whole face. Set the tile, slide it about 1/4 in back and forth perpendicular to the ridges, and bed it with hand pressure or a rubber mallet and beating block.

Why move it across the ridges. Pressing straight down flattens the tops of the ridges and leaves the valleys between them open. Sliding across them folds each ridge into its neighbor's valley.

Step 7: Lift a tile and look

Pull a freshly set tile with a margin trowel and read the back. Coverage is judged as the share of the back in full contact with mortar:

A tile with 85 percent coverage and one bare corner fails both rows: the unsupported corner is where the crack starts. These are, in substance, the ANSI A108.5 coverage figures (the natural stone row follows the TCNA Handbook's stone guidance), which bind the installer where the contract or the tile maker's warranty references the ANSI A108 standards, as many warranties do.

Lift the first three tiles of the set, then one tile about every twenty, and one after every fresh batch of mortar. The shop default is written down as a count because "check periodically" turns into never by mid-afternoon.

Step 8: Control lippage as you go

Lippage is the height difference between adjacent tile edges. Hold a straightedge across every new joint before the mortar grabs, and tap or lift-and-rebutter until it reads flush. A leveling-clip system holds edges together while the bed cures; it does not fix a substrate that is out of flat, and wedges driven against a hump will snap a tile later. ANSI A108.02 sets allowable lippage in substance at 1/32 in for joints from 1/16 in up to just under 1/4 in, and 1/16 in for joints 1/4 in and wider, each plus the tile's own allowable warpage; check the current edition. Rectangular tile with a side over 18 in is laid at an offset of no more than one third of its length unless its maker says otherwise, because a center-crowned plank set at half offset puts its high middle beside its neighbor's low end.

Worked example: an 18 in porcelain kitchen

A kitchen-dining room measures 134 in east to west and 163 in north to south. The tile is an 18 in square porcelain that measures true to size, set with a 3/16 in joint, so one tile plus one joint is 18-3/16 in. Movement joints card: 1/4 in perimeter joint at every wall.

East-west, 134 in, half is 67 in.

• Joint on the centerline: half a joint (3/32 in) plus 3 tiles and 2 joints reaches 54-15/32 in; add the next joint and it is 54-21/32 in. 67 minus 54-21/32 leaves 12-11/32 in; minus the 1/4 in gap, the cut is 12-3/32 in.
• Tile on the centerline: the center tile reaches 9 in, then a joint and 3 tiles with joints reaches 63-9/16 in, another joint 63-3/4 in. 67 minus 63-3/4 leaves 3-1/4 in; minus the gap, a 3 in cut.
• Joint on center wins: 12-3/32 in at both walls, two-thirds of a tile.

North-south, 163 in, half is 81-1/2 in.

• Joint on the centerline: 4 tiles reach 72-27/32 in with the following joint, leaving 8-21/32 in; minus the gap, 8-13/32 in. Under half a tile.
• Tile on the centerline: 3 tiles past the center tile reach 63-3/4 in with the joint, leaving 17-3/4 in; minus the gap, 17-1/2 in.
• Tile on center wins: 17-1/2 in at both walls.

Close the arithmetic the other way. East-west: 6 full tiles (108 in) plus 2 cuts (24-3/16 in) plus 7 joints (1-5/16 in) plus 2 gaps (1/2 in) is 134 in. North-south: 7 full tiles (126 in) plus 2 cuts (35 in) plus 8 joints (1-1/2 in) plus 2 gaps (1/2 in) is 163 in. Both close.

              north wall, window
    +-----+-----------+-----------+-----+
    | C1  |  C2 row: 17-1/2 in    | C1  |
    +-----+-----------A-----------+-----+
    | C1  |  3 full   A  3 full   | C1  |
    |     |  4 full rows, the     |     |
    |     |  first one straddles  |     |
    |     |  the room midpoint    |     |
  B BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB B
    | C1  |  3 full rows          | C1  |
    +-----+-----------A-----------+-----+
    | C1  |  C2 row: 17-1/2 in    | C1  |
    +-----+-----------+-----------+-----+
              south wall, door

  A  = N-S joint line at mid-width, 67 in
       from each side wall
  B  = E-W joint line 9-3/32 in south of
       the N-S midpoint
  C1 = 12-3/32 in cut, west and east walls
  C2 = 17-1/2 in cut, north and south walls

Square. Lines A and B were checked at 6 ft, 8 ft and a 10 ft diagonal. The room's own diagonals differed by 3/4 in, and measuring from A to the east wall at both ends showed it running 3/8 in out over its 163 in. With A held square, the east cut tapers from 12-3/32 in to 11-23/32 in. The east wall is the base-cabinet run, so the taper sits at the shadowed toe kick rather than along the open west wall. Moving A to chase the wall would have pushed the whole grid off square to the north wall, where the window light runs straight down the joints.

Coverage. The mortar maker's chart called for a 1/2 by 1/2 in square notch at this size; ridges ran north-south. The first lifted tile read about 70 percent, ridges still standing under the middle and two corners bare. Two causes: the setter was back-buttering only every other tile, and was pressing straight down. With a skim on every back and a 1/4 in slide across the ridges, the second and third lifts read 90 percent or better with all four corners in mortar. A kitchen is an interior dry area, so the gate is 80 percent with corners supported, and it passed on the second lift, not the first.

What changes the answer

• A long plank tile. The layout arithmetic runs separately for each offset row, and at a one-third offset there are three different end cuts per wall instead of one, each of which has to clear the half-tile floor.
• A door is the focal point. If the room is entered through one opening and the far wall is behind an island, it can be right to start a full tile at the threshold and accept a smaller cut where nobody stands.

How to check you got it right

Before grout, with the mortar cured: tap every tile with the handle of a margin trowel, listening for a hollow note, and pull any hollow tile while replacing it is still clean. Joints sit straight to the A and B strings. Every perimeter cut clears the half-tile floor, and the perimeter gap is open and free of mortar all the way round. Then stand in each doorway, in normal room light: the pattern reads square to the room and the cuts even on opposite walls.

References

• ANSI A108.5 (setting with dry-set or latex-portland cement mortar) and ANSI A108.02 (general requirements, including lippage), as referenced by your contract and the tile maker's warranty; check the current edition
• TCNA Handbook for Ceramic, Glass, and Stone Tile Installation, current edition
• 29 CFR 1926.1153, respirable crystalline silica in construction, Table 1
• See related: Floor Flatness Tolerances by Floor Type; Movement Joints in Tile Floors; Trowel Notches, Open Time and Adhesive Transfer; Tile Assemblies Over Wood and Concrete
• See related: Mortar + Grout Selection Reference; Respirable Crystalline Silica and the Construction Standard