Mortar is one of the oldest building materials in continuous use — Roman aqueducts have stood for 2,000 years thanks to it. But many homeowners don’t know which type goes where, and using the wrong mortar on a residential project causes failures that show up years later as cracked joints, loose tile, or settled brick. Asking what is mortar used for deserves a real answer because the wrong mix ruins a weekend project and a wallet. Here’s the practical breakdown after two decades of masonry, tile, and stone work.
What Mortar Is Made Of
Mortar is a paste of cement, sand, and water that hardens into a stone-like binder between masonry units. Modern mortar adds hydrated lime to the mix, which improves workability and slightly reduces strength — a deliberate trade-off that lets the mortar move with thermal expansion instead of cracking the bricks themselves.
The four standard ASTM C270 mortar types — M, S, N, and O — differ in cement-to-lime ratios and resulting compressive strength. Type M is strongest at 2,500 psi. Type S hits 1,800 psi. Type N is the residential workhorse at 750 psi. Type O is weakest at 350 psi and used for repointing historic soft brick. Each has specific applications where it performs and others where it fails.
Brick and Block Walls
The most common answer to what is mortar used for is laying brick or concrete block walls. For above-grade exterior walls — exterior brick veneer, garden walls, chimneys above the roofline — Type N mortar is the right pick. It’s strong enough to hold the wall together but flexible enough to absorb thermal cycling without cracking.
Below-grade walls, retaining walls, and any wall under heavy load needs Type S mortar. The added strength and bond come from a higher cement-to-lime ratio. Type S is also right for areas with high seismic activity or hurricane wind exposure where stronger ties between units matter for structural performance.
Tile Setting: Different Material Entirely
Setting ceramic or porcelain tile uses thinset mortar, which is technically a different product from masonry mortar. Thinset is Portland cement plus a polymer additive plus very fine sand, and it’s specifically formulated to bond tile to substrates like cement board, concrete slab, or properly prepared mortar bed.
For most residential floor and wall tile, modified thinset (Custom Building Products VersaBond, Mapei Ultraflex 2, Laticrete 254 Platinum) is the standard. Large-format porcelain over 15 inches square needs a large-and-heavy-tile mortar like Mapei Ultraflex LFT or Custom MegaFlex. Don’t substitute masonry mortar for thinset — the fine sand and polymer additives are essential for tile bonding.
Stone Veneer and Cultured Stone
Manufactured stone veneer (Boral Cultured Stone, Eldorado Stone) installs over a metal lath and scratch coat using Type S mortar. The scratch coat is a 1/2-inch base layer applied to the lath, then the stone units bed into a fresh mortar setting bed and the joints are pointed with a grout bag.
Real natural stone requires the same Type S structural mortar but often with a polymer-modified setting bed for added bond strength. Heavy stones like full-bed sandstone or limestone benefit from a bonding agent applied to the back of each piece before setting. Skip that step on heavy stone and pieces release from the wall as the bond ages.
Repointing Old Brick
Repointing is grinding out failed mortar joints on existing brick walls and refilling them with fresh mortar. The critical principle is matching the new mortar strength to the original. Old soft brick (pre-1900) was laid with very soft lime mortar and fails catastrophically if you repoint with modern Portland-rich Type S. The hard new mortar transfers stress to the soft brick, which spalls and breaks.
For pre-1900 historic brick, use Type O mortar (1 part lime, 2 parts cement, 9 parts sand) or pure Type K (almost all lime, very little cement). For 20th-century brick, Type N is usually the right match. Match the joint profile too — concave, V-joint, or struck — to maintain the historic appearance.
Concrete Block (CMU) Construction
Concrete masonry units stack with Type S or Type N mortar in 3/8-inch joints. The joint thickness is critical — 3/8 inch is the design assumption that lets the wall meet its rated bearing capacity. Thinner joints crack from inadequate cushioning. Thicker joints are weaker than the block units they bind.
For load-bearing CMU walls, Type S is standard. For non-load-bearing partition walls and garden walls, Type N is fine. Reinforced CMU walls also need grout (a different product than mortar) poured into the cells around vertical rebar — don’t confuse the two when ordering materials.
Repairing Cracked Mortar Joints
Cracked mortar joints in residential brick are common after 30 to 50 years. The crack pattern tells the diagnosis. Stair-step cracks following the joints often indicate foundation movement and need a structural assessment before repair. Vertical cracks through brick units typically indicate point-load failure and also need professional review.
Surface-cracked mortar joints with sound brick are repointable. Grind out the joint to 3/4 inch deep with a tuckpoint grinder (Dewalt DWE46101 or similar), brush out dust, dampen the joint, pack new mortar in 1/4-inch lifts, and tool the joint to match the original profile. Done correctly, the repair is invisible from 6 feet away.
Mixing Mortar Correctly
Pre-blended mortar mixes (Sakrete Type N, Quikrete Type S) are the easiest path for small projects. Add water gradually — the mortar should hold its shape on a trowel without slumping but fall off when shaken. Mix only what you can use in 90 minutes. Mortar that starts setting can’t be re-tempered with water without losing strength.
For larger projects, mix from raw ingredients. Type N is 1 part Portland cement, 1 part hydrated lime, and 6 parts mason sand. Type S is 1 part Portland, 1/2 part lime, 4-1/2 parts sand. Always pre-blend dry ingredients thoroughly before adding water. Add water last, mixing with a hoe or paddle in a wheelbarrow until the consistency is right.
Common Mortar Mistakes
Adding too much water is the top failure. Wet mortar is easy to work but loses dramatic strength as it cures. Stiff mortar that’s slightly hard to spread cures stronger and bonds better than soupy mortar. Aim for the consistency of stiff peanut butter.
The other major mistake is laying brick or block in the wrong weather. Fresh mortar can’t freeze before initial set — even one hard frost destroys the bond. Don’t lay below 40°F without protection. Direct sun and wind also cause rapid moisture loss; mist freshly tooled joints with water for 24 hours in hot dry weather.
Knowing what is mortar used for matters because no single mortar product covers every job. Type N for above-grade brick veneer, Type S for structural and below-grade work, thinset for tile, and historic-matched soft mortars for old brick. Match the product to the job, mix it stiff, and protect it from weather extremes during the cure. Done right, mortar lasts longer than the brick it holds.
