Find the working centre lines
Never start at a wall. No wall in a house is straight, and no two walls are at exactly 90 degrees. Start a tile floor from a pair of lines you snap yourself, and let the walls take whatever cuts they end up with.
Measure each wall and mark its midpoint. Snap a chalk line between opposite midpoints; do it in both directions. Where the two lines cross is your working centre. Then check it is actually square: from the crossing point, measure 3 feet along one line and 4 feet along the other. The diagonal between those marks must be 5 feet. If it is not, pivot one line until it is, and trust the squared line over the wall.
On a room longer than about 16 feet, use 6-8-10 instead of 3-4-5. The bigger the triangle, the smaller the error you can detect. A quarter inch out over a 5 foot diagonal becomes two inches of drift by the far wall, and by then the only fix is lifting tile.
For a rectangular room that gives you four quadrants to work in, each one starting from a known square corner. Run the numbers first with the tile calculator so you know how many tiles the field takes and where the cuts should land before you commit.
Dry lay before thinset
Lay a full row of tiles with spacers from the centre line out to each wall, dry, and look at what is left at the end. This takes fifteen minutes and it is the single highest-value thing you can do on a tile job.
What you are looking for: the size of the cut at each wall, whether the two opposite walls get similar cuts, and whether any cut is uncomfortably narrow. Write the numbers down on the subfloor in pencil — you will want them again when you come back to cut.
| Cut at the wall | Verdict | Action |
|---|---|---|
| Over 8 in | Good | Proceed |
| 4 to 8 in | Acceptable | Proceed, check both walls match |
| 2 to 4 in | Poor | Shift the layout by half a tile |
| Under 2 in | Sliver | Always shift; slivers break when you cut them |
Dry-lay with the spacers you will actually use. A 1/16 inch joint and a 3/16 inch joint over a 12 foot run differ by nearly an inch and a half of accumulated width, which is enough to turn a comfortable 5 inch perimeter cut into a sliver.
Avoiding sliver cuts
A sliver is any cut narrower than about a third of a tile. Three reasons to avoid them: they break in the saw, they have almost no bedding area so they come loose, and the eye goes straight to them.
The fix is always the same and always free: move the whole field by half a tile. If one wall is getting a 2 inch cut and the other a 10 inch cut, shifting the layout 6 inches gives you 8 inches at both. You have not changed the number of tiles; you have changed where the ugly part is.
The exception is a room with one wall people never see — behind a vanity run, under a kitchen toe kick, behind a wardrobe that is not moving. Push the small cut there deliberately and give the visible wall the full tile. Decide that before you dry lay, not after.
In a bathroom there is usually one more constraint: the tile in front of the vanity and the tile at the threshold are the two everybody looks at. If you can only make one of them come out well, make it the threshold.
Doorways and transitions
Doorways are where two rooms meet and where every mistake becomes visible at once. Three rules.
- Put a full tile in the doorway. It is the most looked-at tile in the floor. If the layout will not allow it, put the joint exactly under the closed door leaf.
- Align the grout line with the door casing. A grout line running 1/2 inch off the edge of a casing reads as a mistake even to people who could not say why.
- Plan the height change. Tile plus thinset plus membrane adds 5/8 to 3/4 inch. Check the door still swings and pick a transition profile before you tile, not after.
Undercut the door casings with a jamb saw before you start, using a scrap tile on a scrap of membrane as the height gauge. Tile slid under a casing looks built; tile scribed around one never does.
Reference lines
Chalk disappears the moment you trowel over it. Before you start setting, do two things: re-mark the main lines in pencil, and screw a straight length of timber to the subfloor along one line to set the first course against.
Set the first row hard against that straightedge. Once it has set enough to hold, pull the timber and fill in the last row underneath. This single step is what keeps a large floor running true — by tile forty, a quarter inch of drift has become a visible curve in the grout line, and there is no fixing it without lifting the field.
On floors over about 150 square feet, snap fresh control lines every four or five courses and check them with a tape from the original centre. Thinset moves under you, especially with large format tile, and catching a 1/16 inch drift at course five is trivial while catching it at course twenty is not.
Plan your exit too. Work in quadrants from the centre towards the walls so you finish at the doorway and never kneel on fresh tile. It sounds obvious until you find yourself stranded in a corner with 30 square feet of wet floor between you and the hall.
Before you order
The layout you choose in this planning stage decides the order. A grid and a herringbone in the same room with the same tile differ by seven points of waste, which is several boxes on a normal floor — the numbers are in tile waste factor by pattern.
If the job includes a shower, plan the walls and the pan at the same time as the floor so the grout lines relate: the method for measuring three walls, a curb and a niche is in the shower tile planning guide. Getting the floor grout lines to run into the shower without a jog is a detail that costs nothing at the planning stage and cannot be bought later.
Frequently asked questions
Where do you start tiling a floor?
At the intersection of two perpendicular lines through the centre of the room, adjusted so the cuts at opposite walls are equal and no cut is a sliver. Never start against a wall: no wall in a house is straight.
What is a sliver cut and why does it matter?
Any cut narrower than about a third of a tile. Slivers are fragile, hard to set flat and draw the eye straight to them. Shifting the whole layout by half a tile usually turns two slivers into two half tiles.
How long should thinset sit before I walk on the tile?
Twenty-four hours for foot traffic, 48 to 72 before grouting on a floor. Modified thinset under a large impervious tile can take longer; the bag tells you, and cold rooms are slower.
What if the room is badly out of square?
Square the layout to the most visible wall and let the error fall on the least visible one. In a bathroom that usually means squaring to the tub or the vanity run and pushing the taper behind the door. Never split the error across two visible walls: two slightly tapered cuts read worse than one obvious one.