Uneven Grout Lines: Causes, Prevention and Fixes After Install

Uneven grout lines almost always trace back to setting, not grouting: tile that varies more than the joint can absorb, spacers used to force a width the tile cannot hold, layout that drifts without control lines, or walls and floors that are not flat or plumb. Most of it is preventable; after the thinset cures, fixes are limited.

Two kinds of "uneven"

It helps to name the problem precisely, because the causes differ.

• Wavy or crooked lines. The joint keeps a fairly constant width, but the line itself bends, steps or drifts off square. This is a layout and control problem.
• Varying widths. The line runs straight, but the joint is wider in some places than others, sometimes tapering along a single tile. This is mainly a tile tolerance or joint sizing problem.

Often both appear together. Under ANSI A108.02, the correct outcome on tile with normal variation is straight centerlines with slightly varying joint widths, which is the second kind, held within a range the eye accepts. When joints are sized correctly, that variation is small enough to disappear once grouted.

The main causes

Tile size variation

Every fired tile varies a little. When the joint is too narrow for the variation, each tile forces a decision: keep the width and let the line drift, or keep the line and let the width change. Across 15 or 20 tiles, a few hundredths of an inch per tile adds up to a quarter inch or more of drift. Mixed calibers are a frequent culprit. Calibrated tile is sorted into size groups, and a delivery containing two caliber codes can include tiles that differ by more than the joint can absorb. Check caliber and lot codes on every box before setting.

Out-of-square tile adds a taper, and edges that bow in plan make a joint narrower in the middle than at the ends. Both are allowed within limits by ANSI A137.1, and both look worse in narrow joints. For rectified tile specifics, see rectified tile grout lines.

Spacer misuse

Spacers are useful, but misuse causes more crooked lines than any other single habit.

• Forcing spacers into every joint on variable tile. The spacer insists on a fixed width, so the tile variation goes into the line instead. On calibrated or handmade tile, pros often use spacers as a guide and adjust tiles by eye to the layout lines, sometimes pulling spacers out of joints that are fighting the line.
• Spacers placed at intersections on tile with eased edges. Cross spacers can ride up on rounded corners and give a false width. Standing spacers on edge in the joint (a "T" or vertical position) gives a more consistent reading and makes them easier to remove.
• Mixing spacer sizes or brands. Nominal 1/8 inch spacers from different sources are not always identical.
• Using leveling clips as spacers. Many clip systems are not designed to set width, and their straps vary. Use them for alignment of faces, with separate spacing control.
• Leaving spacers in too long or grouting over them. Buried spacers leave shallow, weak spots in the joint that often show as a lighter patch or crack.

Layout drift

Setting tile off one wall and following the last course is the classic cause of wandering lines. Walls are rarely straight or square, and any small error in one course is copied into the next. Without fixed reference lines, errors only grow. Rooms that are out of square make this worse, because a layout started parallel to one wall will run off square to the other.

Wavy walls and uneven floors

When the surface under the tile is not flat, each tile sits at a slightly different angle. Edges that should be parallel tilt toward or away from each other, so joints open at one end and close at the other, and lines wander in a gentle wave. On walls, bowed studs and patched drywall are common causes. On floors, humps and dips do the same, and they also cause lippage. The fix is flattening before tiling, since thinset is not meant to be used as a leveling layer beyond what the mortar's TDS allows.

Tiles moving after placement

Wall tile slides down if there is no support for the first course or if the mortar is not designed for non-sag use. On floors, walking or kneeling on freshly set tile can shift neighbors. Tiles bumped while thinset is still soft drift without anyone noticing until grout highlights it.

How to prevent uneven grout lines

1. Measure the tile and size the joint. Pull tiles from several boxes, measure the spread and set the joint width at three times that, rounded up. If the design wants a narrower joint, that is the moment to change tile, not later.
2. Blend boxes. Work from three or four boxes at a time so variation is spread out rather than concentrated in one area.
3. Dry-lay a section. Lay a few rows without thinset to check how the tile and joint behave, and to plan cut sizes at walls and thresholds.
4. Snap a grid of control lines. Establish two square reference lines (check with a 3-4-5 triangle or a laser) and then snap lines every few tiles, for example every 3 to 4 feet. Each tile is set to the line, not to its neighbor.
5. Use a story pole or tape check. Mark tile-plus-joint increments on a straight board and check them against the work every few courses. Drift shows up as marks that no longer match.
6. Support the first course on walls. Screw a straight ledger board to the wall at the height of the second course, set upward from it, and fill the bottom course last.
7. Flatten the substrate first. Check with a long straightedge and correct with patch or self-leveler on floors and with shimming or skim coats on walls before tiling.
8. Check as you go. Sight down lines and hold a straightedge along joints after every few tiles while the thinset is still workable.

Can you fix uneven grout lines after install?

It depends almost entirely on timing.

While the thinset is still workable

Tiles can be nudged back to the line within the mortar's adjustability time, which for many standard thinsets is on the order of 15 to 30 minutes after placement, and shorter for rapid-setting mortars or in hot conditions. Check the mortar's TDS. If a tile has to be lifted, scrape and re-butter it rather than pressing it back onto skinned mortar, or it will not bond.

After the thinset has cured

Options are limited and none is perfect.

Grout color is the most effective cosmetic lever. A contrasting grout outlines every tile and makes width variation obvious; a grout close to the tile's color lets the eye read the floor as a field. The grout color guide explains how contrast affects perceived line quality. If the existing grout is already in place, see how to regrout tile or grout colorant.

Widening a joint to straighten it is not practical: it requires cutting the tile edges in place, which chips glazes and leaves raw edges. Grinding or sawing joints wider is sometimes done on stone by specialists, but it is not a fix for a typical ceramic or porcelain floor.

When "uneven" is normal

Handmade tile, zellige, saltillo and tumbled stone are expected to have irregular joints. Their charm is the variation, and trying to hold perfectly straight, even joints with them is futile. For those materials, the goal is consistent average width and visually balanced lines, not precision. See grout for handmade and zellige tile and wide grout joints.