What Is Grout? What It Does, How It Works and Why Tile Needs It

Grout is a dense, pigmented filler packed into the joints between tiles after the tile is set. It locks tile edges together, protects them from chipping, keeps dirt and water from collecting in open gaps, and gives the surface a finished look. It does not glue tile down, waterproof the floor, or absorb structural movement.

A working definition of grout

In tile work, grout is the material that fills the grout joints, the gaps deliberately left between tiles. Most grout is cement-based: portland cement, a graded filler (fine sand in sanded grout, finer mineral powder in unsanded grout), pigments and, in modern products, polymers and other admixtures. Resin grouts replace the cement binder with epoxy, urethane or acrylic polymers. The ingredients and what each one does are covered in what grout is made of.

The word "grout" also gets used in masonry and concrete for flowable fills that go into block cores or under machine base plates. Those products are designed for strength and flow in thick sections, not for narrow, visible, cleanable joints, and they do not belong between tiles. If a bag does not reference a tile grout standard (ANSI A118.6, A118.7, A118.3 or A118.8, or ISO 13007-3 classes such as CG1, CG2 or RG), it is not a tile grout. The standards themselves are explained in grout standards: ANSI and ISO.

Where grout sits in a tile assembly

A tiled floor or wall is a stack of layers, and each one has a single job. Grout is the top-most layer in the joints, but it is the least structural of all of them. Understanding the stack explains most grout failures, because the grout is usually where problems in lower layers first become visible.

Grout and setting materials are often confused because both are cementitious powders mixed with water. They are formulated very differently. Thinset is engineered for bond and open time, grout for joint packing, low shrinkage, color and cleanability. The side-by-side breakdown is in grout vs thinset vs mortar.

What grout does mechanically

Grout looks decorative, but in a well-built installation it does real mechanical work. Here is what it actually contributes.

It supports and protects tile edges

Tile edges are the weakest part of a tile. An open joint leaves each edge free to chip when a chair leg, cart wheel or dropped pan hits it. A fully packed joint backs up the edge so that a point load on the corner is shared with the grout and the neighboring tile instead of snapping off a sliver of glaze. This is why ungrouted tile in traffic areas chips along its edges within weeks, and why grout that has eroded below the tile surface leaves edges exposed again.

It distributes lateral load between tiles

On floors, rolling and point loads push tile sideways as well as down. Grout transfers some of that lateral load from one tile to the next, so the field behaves more like a continuous surface. It does not replace adhesion: if the thinset has poor coverage or a tile is hollow, the grout around it will crack as the tile rocks. A cracked joint in an otherwise sound floor is often the first visible symptom of a bond problem below, which is why the crack diagnostic in why grout cracks starts by tapping the tile.

It keeps joints from becoming dirt traps

An unfilled joint is a narrow channel that collects grit, food, hair and water. Grit in joints is abrasive and works against tile edges every time the floor is walked on. Grout raises the joint to just below the tile face so that the surface can be swept, mopped and squeegeed.

It makes the surface look finished and corrects for variation

No batch of tile is perfectly uniform. Even rectified porcelain varies by small fractions of an inch, and handmade tile varies a great deal. The joint absorbs that variation so rows stay straight, and grout fills the joint so the variation is not visible as uneven gaps. Grout color also controls how the layout reads: a contrasting color emphasizes the grid, a matching color makes the field look more monolithic. See the grout color guide.

What grout does not do

Most grout disappointment comes from expecting grout to do a job that belongs to a different layer.

• It is not an adhesive. Grout bonds to the sides of the tile and to the setting material at the bottom of the joint, but it contributes almost nothing to holding tile on a wall or floor. Loose tile needs to be reset, not regrouted.
• It is not waterproofing. Cement grout is porous, and even epoxy grout does not make a tile installation watertight because water reaches the substrate through cracks, fixtures and penetrations. See does grout waterproof tile.
• It is not a movement joint. Cement grout is rigid. Where tile meets a wall, a tub, a cabinet or another plane, or crosses a structural joint, the joint gets a flexible sealant per TCNA EJ171, not grout. Details are in caulk vs grout for movement joints.
• It is not a leveling layer. Grout cannot hide lippage (uneven tile edges). Overfilling a joint to "ramp" between two tiles leaves thin feathered grout that breaks off.

Why tile needs joints at all

If grout is just filler, why not butt the tiles together and skip it? Three reasons.

1. Tile size varies. ANSI A137.1 allows dimensional tolerances on tile, and even calibrated or rectified tile is not identical piece to piece. Butted tile accumulates those differences across a room, so rows wander and edges collide. ANSI A108.02 requires a joint at least three times the actual facial dimensional variation of the tile, and never less than 1/16 inch (about 1.6 mm). The detail is in minimum grout joint width.
2. Edges need room. Tile edges that touch can chip against each other during setting and in service, and they cannot be adjusted for flatness without one tile pushing on another.
3. The assembly moves. Temperature, moisture and structural deflection all move tile and substrate by small amounts. A grout joint does not absorb large movement, but it keeps tiles from bearing directly on one another and gives the field a slightly less rigid grid.

Tile without joints is covered in its own article, tiling without grout lines.

How much of a tiled surface is actually grout

Grout is a small share of the area of most floors, but that share climbs fast as tiles get smaller. That matters for stain risk, cleaning time and how much grout performance you should pay for. The figures below are straightforward geometry for square tile with a single joint width.

A mosaic shower floor can be more than ten times as much grout by area as a large-format floor. That is one reason resin and high-performance grouts are common on mosaic shower floors and countertops while basic cement grout is often fine on a large-format living room floor. For more combinations see the grout joint area analysis.

The main families of grout

Grout choice is a decision of its own, covered in which grout do I need. At the level of fundamentals, the families differ by binder.

Within cement grouts, the sanded versus unsanded split is about joint width. The common rule of thumb is unsanded for joints under 1/8 inch and sanded for 1/8 inch and wider. The full comparison is in sanded vs unsanded grout.

What makes grout perform

Two installations with the same bag of grout can perform very differently. In order of how often they cause trouble, the variables are:

• Water content. Extra mixing water raises porosity and lowers strength in cement grout. It is the single most common cause of soft, chalky or blotchy grout. See why excess water weakens grout.
• Joint preparation. Thinset squeezed up into joints leaves the grout too shallow to hold. Joints need to be clean and reasonably deep before grouting.
• Packing. Grout must be forced fully into the joint with the float held at a steep angle, not just spread across the top. See grout compaction and joint filling.
• Cleanup. A sopping sponge washes cement and pigment out of the joint face, leaving low, sandy or light-colored grout.
• Curing conditions. Cement grout gains strength by hydration over days, not by drying. Heat, wind and very low humidity can starve it of water; cold slows it.
• The layers below. Stiff, flat substrate and full mortar coverage under the tile matter more to grout longevity than the grout brand.