Why are my floor tiles cracking?

Floor tile is hard and stiff, and it does not bend. When it cracks, something underneath has usually moved more than the tile could take: a wood subfloor flexing under foot traffic, a concrete slab cracking or shrinking, tile expanding against a wall with no room to move, or a tile that was set without enough mortar under it to support it. Floors and walls shift over time, causing cracks beneath the finished tile.^[7] The pattern of the cracks is the best clue to which one you have.

Read the pattern

• One tile, a starburst or a chipped corner: usually an impact, such as a dropped pan or a tool. Replace the tile and move on.
• A crack line running across several tiles: movement underneath, most often a crack or joint in the slab or a seam in the subfloor.
• Cracks in the middle of a room or between joists, with some bounce: a subfloor that flexes too much.
• Cracks and tented or lifted tiles along a wall or doorway: tile with no room to expand at the edges.
• Scattered cracked tiles that sound hollow: poor bond or too little mortar.

A floor that flexes too much

Wood-framed floors bend a little under load, and tile needs that bend kept small. Custom Building Products notes that building codes recommend that floor deflection should not exceed L/360, which is slightly less than 1/2 inch over a 15-foot span,^[1] and that floors built on joists 19.2 inches apart and trusses 24 inches apart, with deflection greater than L/360, have become common.^[1] Stone is less forgiving: the Marble Institute of America recommends a stiffer floor, deflection no more than L/720, for soft stone such as marble and travertine, stone larger than 12 by 12 inches, heavily veined stone and stone thinner than 3/4 inch.^[1] That is why marble on a bouncy upstairs bathroom floor is a risk.

What sits between the joists and the tile matters as well. LATICRETE explains that a tile underlayment in a bathroom adds support to prevent the tile and grout from cracking when walked on.^[8] Custom's plywood bulletin says plywood sheets butted tight together with no gaps do not allow for expansion and contraction,^[2] and the buckling plywood edges can show through a backer board and cause bond failure and cracked tiles.^[2] It lists oriented strand board (OSB), particle board, luan and interior-grade plywood among the materials not suitable for bonding tile directly,^[2] and warns that any shrinkage of a wet subfloor after tile goes down will result in bond failure.^[2] A slow leak from a toilet or tub that soaks the subfloor can loosen or crack tile before it shows as a stain below.

A concrete slab that moves

Basement floors and slab-on-grade houses put tile on concrete, and concrete cracks. Custom's movement-joint paper explains that control joints in concrete can keep opening and closing,^[3] so the movement joint in the tile should sit directly over the one in the concrete.^[3] Tile laid straight across a slab joint or an existing crack often cracks along the same line.

Expansion matters too: concrete that expands more than it shrank can bend and dome slightly, resulting in cracks in the assembly.^[3] And timing matters on new concrete. Custom notes that most makers of tile-setting products require a slab to cure 28 days before tile is bonded directly to it,^[4] and that tile bonded too soon can come loose or crack as the concrete shrinks.^[4] Cracks that appear in the first months after a new slab was tiled often trace to this.

No room to move at the edges

Tile, grout and the floor under them all expand and shrink with temperature and humidity. Custom's grout data sheet states: "Movement joints are required for perimeters and other changes of plane in all installations."^[5] The same sheet says cement grout is not for use in movement joints or changes of plane,^[5] so a floor grouted tight against the walls, the tub or the cabinets has nowhere to move. The movement-joint paper adds that floors exposed to direct sunlight need movement joints more often, every 8 to 12 feet,^[3] which is worth knowing for a sunroom or a floor below a big south-facing window.

Filling those joints with the wrong caulk causes the same trouble. On what happens when the wrong caulk is used in a movement joint, Custom says: "The damage can be as minor as cracks in the cement grout, to major cracks through the tile or loose tile."^[3]

Not enough mortar under the tile

A tile only partly bedded in mortar is supported at a few points, and a chair leg or a dropped can of paint can snap it over the gap. Custom lists a recommended minimum of 80% mortar coverage under tile in dry areas and 95% in wet areas and exteriors.^[6] Tiles set on only a few dabs of mortar often sound hollow when tapped and can crack one at a time over the years.

What it means for the repair

If it is one tile cracked by an impact, replace it; see how to replace a cracked tile. If the pattern points to movement, a new tile in the same spot will usually crack again, and the fix is to deal with the cause: a movement joint, a crack isolation or uncoupling membrane, or a stiffer subfloor. How to keep tile from cracking covers what to ask for.

Who fixes it depends on how deep the cause goes. Tile and subfloor work falls under Virginia's flooring and tile contractor specialties, but the flooring specialty explicitly excludes floor joists and other structural parts of the floor,^[9] which are a job for a builder. Compare hiring a handyman or a tile installer.

Stop and call a pro if

• Cracks run across many tiles, keep growing, or come with a floor that sags, slopes or bounces noticeably.
• You see damp wood, water stains or sagging joists from the basement or crawlspace below.
• Doors start to stick, or cracks appear in walls or the foundation at the same time.
• Tiles are cracked or loose in a shower or around a toilet or tub, which can mean water under the floor.

Browse small tile jobs or subfloor repair, find flooring businesses around Blacksburg, or read the other handyman answers. When you know what you are dealing with, describe the tile repair and local businesses quote it.