The Difference Between Sanded and Unsanded Grout and When to Use Each

I spent three days grinding concrete on a job last month just so the floor wouldn’t click like a castanet. Most guys skip the leveling compound. They think the underlayment will hide the dip. It won’t. I was out there with a seven inch diamond cup wheel and a vacuum system that still couldn’t catch all the dust. Once that slab was flat to within an eighth of an inch over ten feet, the client asked if we could use a specific designer grout they saw on a blog. I had to explain that grout is not a paint color. It is a structural component. If you choose the wrong aggregate size for the joint width, that beautiful tile will be failing within eighteen months. I have seen it a hundred times. People focus on the ceramic or the stone, but the grout is what manages the movement and the moisture. It is the shock absorber of the flooring world. If you treat it like an afterthought, you are asking for cracks and water infiltration. This guide breaks down the physics of sanded versus unsanded grout from the perspective of someone who actually has to warranty the work.

The physical anatomy of a grout joint

Grout joints serve as the transition zones between rigid tiles and the subfloor system. Sanded grout contains fine silica sand that acts as a reinforcing filler, while unsanded grout relies on a high concentration of polymers and Portland cement to fill narrow gaps without the risk of scratching delicate surfaces. When we talk about grout, we are talking about a cementitious material that must bridge a gap while resisting shrinkage. Most people think grout is there to glue the tiles together. That is a lie. The thin-set mortar under the tile does the heavy lifting of adhesion. The grout is there to fill the voids, prevent debris from getting under the tiles, and provide a sacrificial surface that can handle the expansion and contraction of the building envelope. Without a properly filled joint, the edges of your tiles are vulnerable to chipping. This is especially true in high traffic areas where the weight of a person can cause micro-deflection in the subfloor. If that joint is empty or filled with the wrong material, the tile loses its lateral support.

“A floor is only as good as the subfloor beneath it; deflection is the enemy of every joint.” – Master Flooring Axiom

Why sand is the structural backbone of wide joints

Sanded grout is the industry standard for any joint wider than one eighth of an inch because the sand particles create a bridge. These silica aggregates prevent the cement paste from shrinking as the water evaporates during the hydration process. Without sand, a wide joint would crack and pull away from the tile. Think of sanded grout like concrete. Concrete has gravel and sand to give it mass and stability. In a grout joint that is a quarter inch wide, you have a lot of volume to fill. As Portland cement cures, it undergoes a chemical reaction that consumes water. This causes the volume of the mixture to decrease. If you don’t have sand in there to hold the space, the grout will develop hairline fractures. I have seen installers try to use unsanded grout in wide joints because they wanted a smoother look. Within a week, the grout looked like a dried up lake bed in a drought. The sand also provides a degree of slip resistance and wear resistance. It is tough. It can handle the friction of boots and the weight of furniture better than a pure cement paste ever could. However, that sand is abrasive. If you are installing polished marble, limestone, or glass, those sand grains will act like sandpaper and ruin the finish of your tile during the installation process.

The delicate chemistry of unsanded mixtures

Unsanded grout is designed for joints ranging from one sixteenth to one eighth of an inch in width. It lacks the sand aggregate, which allows it to flow into tight spaces that sanded grout cannot reach. It is the primary choice for rectified tiles and soft natural stones. When I am working with a rectified porcelain where the edges are ground to a sharp ninety degree angle, the goal is often a tiny joint. You cannot jam sand into a sixteenth inch gap. It just won’t go. Unsanded grout has a higher polymer content to compensate for the lack of sand. These polymers help with stickiness and flex. Because there is more cement and less filler, this grout is much stickier and easier to apply to vertical surfaces like shower walls. It hangs onto the tile. But you have to be careful with the water ratio. If you add too much water to unsanded grout, you weaken the polymer chain and end up with a powdery, weak joint that will wash away the first time someone hits it with a scrub brush. It is a more temperamental material than its sanded cousin. It requires a precise hand and a lot of patience during the clean up phase because it tends to leave a thicker haze on the tile surface.

How joint width determines your grout type

The selection process for grout begins with a tape measure rather than a color palette. You must measure the gap between tiles to ensure the aggregate can physically occupy the space. Sanded grout is mandatory for joints over one eighth inch, while unsanded is for joints under one eighth inch. Many DIYers try to split the difference at exactly an eighth of an inch. In my shop, if it is an eighth of an inch, we usually go with sanded. I want that extra strength. The only exception is when the tile is too soft. If you are running a glass mosaic in a shower, you stay with unsanded even if the joints are a bit wider, but you have to be prepared for some minor shrinkage. The physics of the installation dictate that the wider the joint, the more structural integrity you need. If you have a rustic saltillo tile with half inch joints, you actually move beyond standard sanded grout and into masonry mortar or wide joint grout mixtures that have even larger aggregates. Choosing the wrong one is a recipe for a callback. I have seen floors where the installer used unsanded grout in three eighths inch joints on a slate floor. The grout just fell out in chunks after three months of foot traffic. It didn’t have the internal skeleton to hold itself together.

The friction between tile surfaces and abrasive fillers

Every tile material has a Mohs hardness rating that determines its resistance to scratching. Sanded grout contains silica which has a hardness of seven, meaning it will scratch any material with a lower rating like marble, travertine, or polished limestone. This is where a lot of guys mess up. They think they can be careful with the sponge and avoid scratches. You can’t. When you are spreading grout with a rubber float, you are applying significant pressure. That sand is being ground into the face of the tile. If you are working with a soft stone, you will see the swirl marks as soon as the light hits the floor at an angle. For those materials, you must use unsanded grout or one of the newer high performance small aggregate grouts. Even some ceramic tiles have a delicate glaze that can be marred by sand. I always tell my crew to do a test patch in a closet. If the grout scratches the test tile, we switch to a different product immediately. You cannot buff out scratches in a tile glaze. Once it is gone, the floor is ruined. This is why the chemistry of the grout must match the geology of the stone.

Moisture migration and the porous nature of cement

Both sanded and unsanded grouts are naturally porous because they are cement based products. They act like a hard sponge that can wick moisture into the subfloor if not properly sealed or modified with latex additives. In a shower environment, this is a major concern. Water doesn’t just sit on top of the grout; it moves through it. This is why the waterproofing layer behind the tile is more important than the grout itself. However, the density of the grout matters. Sanded grout, when packed tightly, creates a very dense matrix that resists water better than a poorly mixed unsanded grout. I prefer using a high performance grout that uses a specialty cement or an epoxy resin in wet areas. Epoxy grout is a different animal entirely. It is waterproof, stain proof, and doesn’t require sealing. But for standard cement grout, you have to wait for the full cure, usually three to seven days, before applying a high quality penetrative sealer. If you skip the sealer, your white grout will be orange or black within a year because of mineral deposits and soap scum. While most people want the thickest underlayment for comfort, too much cushion actually causes the locking mechanisms on LVP to snap under pressure, and similar logic applies to tile. If the subfloor flexes too much because of a soft underlayment, the grout joints will crumble regardless of whether they are sanded or unsanded.

Why your shower floor depends on aggregate density

Shower floors represent the most hostile environment for any grout choice because of constant thermal cycling and moisture exposure. The density of the grout joint determines how well the system handles the weight of a standing person and the erosive power of hot water. For most shower floors using small mosaic tiles, unsanded grout is the standard because the joints are tiny. However, the small size of the tiles means there are hundreds of linear feet of grout. This grout is the only thing keeping the tiles from shifting. I always use a liquid latex additive instead of water when mixing grout for a shower floor. This increases the bond strength and makes the grout less permeable. You want that joint to be as dense as possible. If the grout is soft or porous, water will sit in the joints, leading to mold growth that you can never truly clean away. It starts from the bottom up. Once the mold is in the cement matrix, you are looking at a regrout job or a full tear out. It is better to spend the extra money on a high performance grout now than to pay me three thousand dollars to scrape out your old grout in two years.

• Check the subfloor for deflection before laying a single tile.
• Verify the tile material hardness to avoid sand scratches.
• Measure the narrowest and widest parts of your grout joints.
• Mix the grout to a peanut butter consistency without excess water.
• Use a dedicated grout float to pack joints completely from bottom to top.
• Clean the tile with a damp, not dripping, sponge to prevent washing out pigment.
• Wait for the manufacturer recommended cure time before sealing.

The danger of choosing aesthetics over engineering

Focusing on grout color before selecting the correct aggregate type is a fundamental error that leads to structural failure of the floor system. Engineering requirements for joint width and tile protection must always override the visual preference for a specific texture. I see people on social media showing off these ultra thin joints with sanded grout because they like the look of the heavy grit. Or they try to use a smooth unsanded grout on a wide joint because they want it to look like a solid slab. It doesn’t work. The laws of physics do not care about your Pinterest board. If you put unsanded grout in a quarter inch joint, it will crack. If you put sanded grout on a marble floor, it will scratch. You have to respect the materials. The best looking floor is the one that stays together. A cracked joint is an ugly joint, no matter what color it is. When I walk onto a job site, I am looking for the technical specs. I am looking at the TCNA handbook. I am making sure the expansion joints are placed every twenty feet so the whole floor doesn’t tent in the summer heat. Grout is part of that system. It is the final seal on a complex piece of engineering. Treat it with the respect it deserves, and your floor will outlast your mortgage. Treat it like a decoration, and you will be calling me to fix it before the year is out. That is just the way the business works. You can do it right once, or you can do it twice and pay double. The choice is yours, but the physics of the cement won’t change for anyone.