Mortar

What is mortar?

Mortar is a bonding material that starts as a spreadable paste and cures into a hard, durable mass. In masonry, it sits between bricks, blocks, or stones to hold them together, seal gaps, and create a uniform, level bed. In tile work, the word mortar commonly refers to thin-set mortar, a cement-based adhesive used to bond tile to floors, walls, and countertops. While both share cement as a key ingredient, masonry mortar and thin-set are formulated for different tasks.

How mortar works

Traditional masonry mortar is made from a binder (portland cement and sometimes lime), clean sand, and water. When mixed, cement reacts with water and forms crystals that interlock with the sand particles and the pores of the masonry units. Lime improves workability and flexibility, helping the mortar accommodate small movements without cracking. Thin-set tile mortar uses very fine sand and may include polymer additives that boost adhesion and flexibility, especially over modern substrates like cement board or waterproof membranes.

Common DIY uses

• Building or repairing brick and block walls
• Setting natural stone or manufactured stone veneer
• Repointing: removing deteriorated joints and packing in fresh mortar
• Setting pavers and creating edging (with mortar or polymer-modified bed)
• Tiling floors, showers, backsplashes, and countertops (with thin-set mortar)
• Fireplaces and fireboxes using refractory mortar

Types of mortar

Mortars are tailored to the job. Choosing the right one matters for strength, durability, and the health of the material you are bonding.

Masonry mortar types (ASTM C270)

• Type M (high strength, roughly 2,500 psi): For heavy loads and hard stone, foundations, retaining walls, and below-grade applications. Too stiff for soft brick.
• Type S (medium-high strength, ~1,800 psi): Good for below-grade work, exterior walls, chimneys, and stone veneer. A common choice for structural and weather-exposed projects.
• Type N (medium strength, ~750 psi): The go-to for above-grade brick and block, chimneys, and general repointing. Balanced strength and flexibility.
• Type O (low strength, ~350 psi): Interior, non-load-bearing or historic restoration where softer mortar is needed to protect old, softer bricks.

These strengths are typical and vary by manufacturer. The idea is simple: match mortar strength to the masonry. Softer mortar protects softer bricks by being the sacrificial element.

Mix design shorthand

Traditional proportion mixes (by volume) are often listed as:
- Type M: 3 parts portland cement : 1 part hydrated lime : 12 parts sand
- Type S: 2 : 1 : 9
- Type N: 1 : 1 : 6
- Type O: 1 : 2 : 9
Most DIYers use bagged, preblended mixes labeled Type N, S, or M to keep it simple.

Thin-set tile mortars

• Unmodified (dry-set): Cement, fine sand, no polymers. Used over porous surfaces or with membranes that require it; often needs a liquid additive for modern installations unless the system specifies otherwise.
• Polymer-modified: Added polymers improve adhesion and flexibility; common for porcelain, large-format tile, and many floors and walls.
• Large-format tile mortar (medium-bed/large-and-heavy-tile): Formulated to support heavier, bigger tiles without excessive slump.
• Rapid-setting thin-set: Cures fast for quick turnarounds; shorter working time.

Specialty mortars

• Refractory mortar: Heat-resistant for fireboxes and flues.
• Lime mortar (non-cement): For historic masonry where breathability and low strength are crucial.
• Color-matched mortar: Pre-tinted or tintable for visible joints.

How to choose the right mortar

Ask these questions:
1. What am I bonding? Brick, block, soft historic brick, dense stone, or tile?
2. Is it above grade or below grade? Exposed to weather, freeze-thaw, or heavy loads?
3. What substrate and setting system am I using (for tile)? Cement board, uncoupling membrane, waterproofing?

Quick picks:
- Brick veneer or above-grade brickwork: Type N
- Stone veneer, retaining walls, steps, below-grade block: Type S
- Heavy structural or dense stone in severe exposure: Type M (only if the masonry is equally strong)
- Historic soft brick and gentle repointing: Type O or pure lime mortar
- Ceramic/porcelain tile on floors and walls: Polymer-modified thin-set; use large-and-heavy-tile mortar for big tiles
- Firebox and flue joints: Refractory mortar

Mixing and using mortar

• Measure water carefully: Start with about 70–80% of the recommended water, mix, then add water a little at a time. Aim for peanut-butter consistency for masonry, and creamy, ridgable consistency for thin-set.
• Mix time: 3–5 minutes for masonry mortar. Thin-set often needs a 5–10 minute rest (called slaking) after initial mixing; then remix without adding more water.
• Batch size: Mix only what you can use in 30–45 minutes for masonry and per the pot life on the bag for thin-set.
• Surface prep: Clean, sound, and slightly damp (SSD: saturated surface-dry) for masonry. For tile, follow the substrate manufacturer’s requirements and vacuum dust.
• Application: Butter masonry units and tool joints once the surface sheen disappears. For tile, trowel with the correct notch size, keep a consistent angle, and back-butter large tiles.
• Curing: Protect fresh mortar from direct sun, wind, and freezing. Keep masonry mortar damp for 24–48 hours if possible. Follow cure times before loading or grouting tile.

Care and maintenance

• Inspect joints annually for cracks or gaps. Repoint small areas before moisture gets behind the wall.
• Clean gently with a soft brush and water. Avoid aggressive pressure washing that can erode joints.
• Efflorescence (white powder) can be brushed off dry; persistent cases may need a mild masonry cleaner. Test first.

Safety

• Wear gloves and eye protection; wet mortar is caustic and can burn skin.
• Use a respirator when mixing; cement dust contains silica.
• Lift bags safely and use proper mixing tools.

Common mistakes to avoid

• Wrong type selection: Using Type M on soft, historic brick can cause brick spalling. Match mortar strength to the masonry.
• Too much water: Soupy mortar sags, shrinks, and weakens. Stick to the water range on the bag.
• Re-tempering after set: Adding water once mortar starts to set reduces strength. Discard and mix fresh.
• Skipping slake time (thin-set): Polymers need time to wet out; let it rest, then remix.
• Poor coverage under tile: For floors and showers, aim for near-full coverage; check by lifting a tile.
• Applying to dusty or frozen surfaces: Clean and condition the substrate; do not mortar below about 40°F without cold-weather procedures.
• Wrong adhesive in wet areas: Avoid tile mastic in showers and tub surrounds; use thin-set mortar rated for wet areas.

Related terms

• Concrete: Cement, sand, and coarse aggregate (gravel). Stronger in bulk; not a substitute for mortar joints.
• Grout: Fills spaces between tiles after they are set; not an adhesive.
• Mastic: Pre-mixed tile adhesive for dry, light-duty areas; not cement-based.
• Repointing: Removing and replacing deteriorated mortar joints.
• Masonry cement: Preblended cement and additives used to make mortar when mixed with sand.

Practical examples

• Building a garden wall: Use Type S for strength and weather resistance; tool concave joints for good water shedding.
• Repointing a 1950s brick facade: Use Type N and match the sand color and joint profile to blend repairs.
• Installing 12x24 porcelain tile: Use a polymer-modified large-and-heavy-tile mortar; key mortar into the substrate, comb in straight lines, and back-butter tiles.
• Setting stone veneer on a fireplace surround: Scratch coat the lath with Type S, let it firm up, then set stones with the same mix.
• Repairing a firebox: Use refractory mortar rated for high temperatures.