About one in four tile failures I get called to inspect comes down to a homeowner using grout where mortar belonged, or vice versa. The two products look similar in the bag — both are gray, both are sand-and-cement based, both mix with water — but they do completely different jobs. Mortar vs grout comes down to one core distinction: mortar is the adhesive that bonds tile to the substrate, while grout is the filler that locks the joints between tiles. Confuse them and the floor either falls off or cracks open within months. This guide breaks down the types, the right applications, and the mistakes that separate a 20-year tile job from a 2-year one.
What Mortar Actually Does
Mortar is a structural adhesive. It transmits load from the tile down into the substrate, accommodates minor movement, and resists shear forces from foot traffic. Modern thinset mortar is a blend of Portland cement, fine sand, and polymer modifiers that boost flex and bond strength. The polymer is the key — straight cement-and-sand mortars went out with the 1980s for tile work because they could not handle the slight deflection of modern subfloors.
You apply mortar with a notched trowel, comb it to a uniform thickness, then press the tile in with enough pressure to collapse the ridges and achieve at least 80 percent contact for dry areas, 95 percent for wet areas like showers.
What Grout Actually Does
Grout fills the joints between already-installed tiles. It is not an adhesive. Its jobs are to seal the tile field against moisture intrusion, accommodate thermal movement, and finish the installation visually. Grout is weaker than mortar by design — it has to crack before the tile does when something flexes.
You force grout into joints with a rubber float held at a 45-degree angle, wait 15 to 30 minutes for initial set, then sponge off the haze with clean water and a tile sponge.
Mortar Types: S, N, and M
Walk into any masonry supply and you will see bags labeled by letter, but those letters refer to ASTM C270 mortar for brick and block work — not tile. Still, homeowners need to know them because they show up in repointing, chimney work, and stone veneer:
- Type M: 2,500 psi, very high strength, used below grade for foundations and load-bearing exterior masonry.
- Type S: 1,800 psi, high strength, the standard for exterior walls, soft brick repointing, and shower pan mud beds.
- Type N: 750 psi, medium strength, the most common for interior brick and above-grade exterior masonry.
- Type O and Type K: low-strength mortars for restoration of historic soft-brick structures.
For tile work specifically, you do not buy Type S or Type N — you buy thinset mortar (ANSI A118.1 or A118.4 modified) or medium-bed mortar for tiles larger than 15 inches.
Grout Types: Sanded vs Unsanded vs Epoxy
Grout choice depends entirely on joint width and exposure:
Unsanded Grout
For joints 1/16 to 1/8 inch wide. Smooth texture won’t scratch polished marble, glass, or metal tiles. Works on shower walls and backsplashes where joints are tight.
Sanded Grout
For joints 1/8 to 1/2 inch wide. Sand bulks up the mix and resists shrinkage cracking on wider joints. Standard for floor tile and most porcelain installs.
Epoxy Grout
For high-stain or chemical-exposure areas. Two-part epoxy resin grout like Laticrete SpectraLOCK Pro Premium ($65 per minijar) is stain-proof, mold-resistant, and never needs sealing — but it sets in 45 minutes and is unforgiving to install.
Where Each One Goes
The roles never reverse. Mortar always goes between the tile and the substrate. Grout always goes between adjacent tiles. The one place this gets confusing is shower pans: a mud bed (deck mud) is mortar that gets sloped to a drain, but that is a structural sub-layer, not the bonding mortar above it. A pre-slope mud bed plus a waterproof membrane plus a setting bed of thinset is the conventional shower floor stack.
Color, Sealing, and Maintenance
Mortar color is irrelevant — it is hidden. Grout color is everything. Match grout to the tile for a monolithic look, contrast it for a graphic look. Cement-based grouts need to be sealed 72 hours after install with a penetrating sealer like Aqua Mix Sealer’s Choice Gold ($40 per quart). Reseal annually in showers, every 2 to 3 years on floors. Epoxy grouts skip this step entirely.
Common Mistakes That Kill Tile Jobs
Three mistakes account for most failures I see in mortar vs grout work. First, using mastic adhesive in a wet area — mastic dissolves in standing water and the tile sheets off the wall. Second, mixing thinset too wet to make spreading easier; the polymer chains never set up correctly and bond strength drops by half. Third, grouting before the mortar has cured at least 24 hours; mortar still off-gassing moisture pushes the grout out of the joint and causes pinholes.
Frequently Asked Questions
Can I use grout as mortar in a pinch?
No. Grout has no bonding additives and will not hold tile to a wall or floor. The tile will pop loose within weeks.
Can I use mortar as grout?
Also no. Mortar has too much polymer and not enough fine sand for joints. It cracks, pinholes, and stains permanently.
How long does each take to cure?
Thinset mortar reaches initial set in 24 hours and full cure in 28 days. Grout is walkable in 24 hours, sealable at 72 hours, and fully cured at 28 days.
What is medium-bed mortar?
A thicker-bodied mortar designed for large-format tiles 15 inches and bigger. It will not slump under the weight of a heavy tile, where standard thinset would let the tile sag and lippage develop.
Do I need to seal mortar?
No. Mortar under tile is already encased and protected. Only grout needs sealing.
