Setting Floor Tile

Read the room before you open a box

Twenty minutes with a tape, a long straightedge and a pencil decides more about how a tiled floor looks than any bag of adhesive on the pallet. Measure both diagonals of the room. Check each wall against a stringline for bow. Sight along the longest uninterrupted run — usually the wall a person faces walking in through the door — and find how far from parallel it sits relative to its opposite. Older buildings routinely give up 10 to 20 mm across four metres. None of that is a defect mortar can correct; it is a quantity you have to spend somewhere.

Out-of-square never disappears. Distribute it, hide it, or concentrate it — those are the only three options, and the layout line chooses which one you get. Push the error to where the eye does not travel: under a toe kick, behind a door swing, along the wall a vanity sits against. Never let it run out along a threshold, a bath panel or the front edge of a cabinet run, because a joint tapering from 3 mm to 9 mm alongside a straight painted line reads as sloppy from the doorway and cannot be fixed afterwards.

Sketch the room on paper before any line goes down. Mark the door openings, the fixed obstructions, the joist direction if the deck is timber, and every measured deviation with its sign. That sketch becomes the argument you make to the client when the far corner runs off, the record of your setting out, and the reason the last cut against the end wall is a comfortable 60 mm rather than a fragile 8 mm sliver.

Striking the control line

A control line is not automatically a centreline. It is the line the finished floor will be judged against, and on most jobs it belongs to the dominant visual feature rather than to the arithmetic middle of the space. In a galley kitchen it runs parallel to the cabinet fronts. In a bathroom with a large shower, it aligns with the kerb so field joints continue into the tray without a jog. Down a hallway it follows the direction of travel, so plank joints do not wander visibly against a skirting.

Square the second line off the first with a 3-4-5 triangle at the largest multiple the room will take — 1.2 m, 1.6 m, 2 m, never 300 mm, since a small triangle multiplies its own error all the way down the run. Rotating lasers make this faster, not more accurate; check the laser against a tape before trusting it, because a self-levelling head knocked about in a van lies confidently. Snap in blue chalk on a floor that will be tiled the same day, and dust the line off rather than letting it sit under a light-coloured grout.

Both lines need to survive the job. Extend them past the field into the doorways, transfer them up onto the wall with a pencil tick at each end, and mark intermediate witness points every metre or so. The first bed of mortar buries everything on the floor. Setting out that exists only in chalk exists for about ten minutes, and re-establishing it mid-room from a tiled edge inherits whatever error is already there.

Dry lay outward, and walk the cuts to every wall

Loose-lay a full run of tile along each control line using the actual spacers, not the ones on the van. Then walk that grid outward to all four walls and read what lands there. This is the moment the geometry is still free: a shift of half a tile in either direction costs nothing now and is unrecoverable once the trowel is out. Look for cuts under about a third of a tile width, cuts that taper along their length, and cuts that fall awkwardly at a doorway where two floors meet.

Slide the grid until the perimeter cuts are as generous and as even as the room allows, then check what that shift did at the opposite wall — moving the field to fatten one edge always thins the other. Very narrow cuts are not just ugly; they break during cutting, sit proud because there is too little tile to bed properly, and pop loose under a door swing. On plank formats, keep the running offset to what the manufacturer states rather than defaulting to a half-bond, since long tiles with any bow in them produce their worst lippage at the centre of the adjacent tile.

A diagonal or 45-degree field changes the arithmetic entirely: every perimeter tile becomes a triangle, cutting time roughly doubles, and offcut yield falls, so order allowance goes up rather than down. Settle the pattern, the direction and the perimeter position first, then convert the finished layout into quantities — including the extra for cuts, breakage and a boxed-away attic stock from the same batch.

The substrate answers to the line

Large-format tile punishes flatness, not levelness. A floor can fall 15 mm to a gully and look perfect; the same floor with a 4 mm hollow under a 600 mm tile gives lippage at every joint crossing it. Check with a straightedge along and across the control lines first, since those runs carry the sightlines, then across the field on a grid. Permitted deviation tightens as tile length grows, and the exact allowance belongs to the standard in force for the job — BS 5385-3 Wall and floor tiling: Design and installation of internal and external ceramic and mosaic floor tilings in normal conditions, or the ANSI A108 series specifications for the installation of ceramic tile — so read the applicable clause rather than working to a remembered figure.

Stiffness comes before flatness. Timber decks intended for ceramic are normally assessed against a live-load deflection limit of L/360, tightened to L/720 for natural stone, with the governing limit set by the building code in the jurisdiction and by the tile system's own requirements; on a doubtful floor, ASTM C627 Standard Test Method for Evaluating Ceramic Floor Tile Installation Systems Using the Robinson-Type Floor Tester is what system ratings trace back to. Bounce that you can feel underfoot will crack grout long before it cracks tile.

Build-up sits on top of that assessment: cementitious backer units to ANSI A118.9, uncoupling or crack-isolation membranes to ANSI A118.12, bonded waterproofing to ANSI A118.10 in wet areas. Each layer adds height, and every one of those millimetres shows up at the threshold and against the door leaf, so total the stack before you commit — the backer board calculator will size sheets and fasteners once the deck is decided. Patching compound and levelling screed are located work, poured to the straightedge readings you took, not spread hopefully across the whole room.

Movement joints belong to the layout grid, not to the offcuts. Interior fields need soft joints at the perimeter and at changes of plane, plus intermediate joints on large or sun-exposed floors, following the movement-joint detailing in the TCNA Handbook for Ceramic, Glass, and Stone Tile Installation. Position them on a grout line of the finished pattern while the pattern is still on paper. A movement joint improvised later lands mid-tile, and the only way to honour it is to cut a tile you already set.

Bedding away from the line

Notch size is chosen to achieve contact, not to match a chart on the bucket. Trowel ridges must collapse into a bed with no voids, which on ribbed-back large formats means back-buttering as well as combing the floor, and combing in one direction so air can escape. Coverage requirements in the ANSI A108 series distinguish dry interior work from wet areas and exteriors, with the higher figure applying to showers, wet rooms and anything outside. Lift one tile early in the day and again after lunch to prove what you are actually getting.

Adhesive choice needs to match the tile and the substrate, not habit. The ISO 13007 classification for ceramic tile grouts and adhesives is the shorthand worth reading on the bag — the cement-based class, the extended open time flag, the slip resistance flag and the deformability class — with modified dry-set mortars covered in the United States by ANSI A118.4. Porcelain with near-zero absorption over a membrane is not a job for a basic unmodified mix chosen because it was on the shelf.

Skinning is the quiet killer. Comb only what you can cover before the mortar surface dulls, less in a warm dry room with the heating on, less again on an absorbent screed. Drag a finger across the ridges; if nothing transfers, scrape it out. Reworking a skinned bed costs one bucket. Leaving it costs the whole floor when hollow tiles turn up under warranty.

Work outward from the control lines in both directions, keeping the intersection accessible as long as possible so you can re-reference it. Check the leading edge against a laser or string every three or four rows rather than every ten — a 1 mm drift per row is invisible until it is 12 mm at the wall. Levelling clips will pull neighbouring faces flush but they will not pull a wandering line straight, and they mask the drift while you make it.

Joint width was decided back at the line

Grout joint width is a tolerance decision made during the dry lay, not a styling choice made on grouting day. Facial dimension variation and edge warpage permitted by ANSI A137.1 American National Standard Specifications for Ceramic Tile have to be absorbed by the joint; the wider the tile and the greater its bow, the wider the joint needs to be to stop lippage and stepped joints reading along a run. Rectified edges allow tighter joints, but rectified edges on a floor that is not flat simply expose the substrate more honestly.

Match grout to the joint and the exposure. Cement grouts fall under ANSI A118.6 and polymer-modified grouts under ANSI A118.7, while chemical-resistant, water-cleanable epoxies fall under ANSI A118.3 for commercial kitchens, industrial floors and anywhere a wash-down regime rules out cement. Joints need to be raked clean to a consistent depth and free of adhesive before grouting, and the mortar underneath must have cured, not merely gone hard on top — grouting over a green bed traps water and shows as patchy shading for months.

Colour and quantity are a batch problem. Mix to a consistent water ratio across the whole floor, keep to a single batch of powder where the run allows, and clean with a barely damp sponge rather than flooding, because water drawn out of a fresh joint is the usual cause of blotching. Estimate the volume from the actual joint width and depth you settled on, tile size and the floor area.

Where the field meets everything else

Perimeters are joints, not gaps to fill with grout. Leave the field clear of every restraining surface — walls, columns, kerbs, thresholds, pipe penetrations — and close those gaps with a flexible sealant, hidden behind skirting or a silicone bead. Grout hard against a wall transfers the whole floor's thermal and moisture movement into the tile edge, and the tell-tale is a line of cracked grout or tented tile running parallel to that wall a season later.

Thresholds deserve their own setting out. Line the transition on the door stop so the joint disappears when the door is closed, check the finished height against the leaf and any undercut needed for a mat or seal, and confirm the slip performance of the specified tile against ANSI A326.3 Test Method for Measuring Dynamic Coefficient of Friction of Hard Surface Flooring Materials where the floor will be walked on wet. Underfloor heating adds one more constraint: bring the system up slowly and only after the adhesive and grout manufacturers' stated cure periods, never to force a floor dry ahead of handover.

Failures, traced back up the line

Hollow-sounding tiles almost never start with bad mortar. They start with a skinned bed, a notch too small for a ribbed back, or combing in circles so air had nowhere to go. Sound the floor with a rod before the grout goes in, while lifting and re-bedding is still a half-hour job rather than a demolition.

Lippage complaints trace back further, to flatness readings that were never taken or to a joint too narrow for the tile's warpage. Both are layout-stage decisions. The ANSI A108 series ties allowable lippage to joint width, which is precisely why a wider joint on a bowed large-format tile is a technical choice rather than a concession.

Tapering cuts along a wall, a 10 mm sliver at the shower kerb, a pattern that drifts out of line with the cabinets — every one of those is the control line, arriving late. Nothing downstream repairs them: not a better adhesive, not levelling clips, not a careful grout. Spend the time at the start, and the rest of the floor is only labour.