Mortar Types Explained: Choosing the Right Mortar for Your Project

Updated March 2026 · By the MasonryCalcs Team

Walk into any building supply store and you will find bags labeled Type N, Type S, and Type M mortar. Pick the wrong one and you compromise either the strength or the flexibility your project needs. Mortar is not just glue between bricks — it absorbs movement, distributes load, and resists water penetration. Each type has a different ratio of Portland cement to lime, which controls its compressive strength, bond strength, and flexibility. This guide explains what each type does, where to use it, and why the strongest mortar is not always the best choice.

The ASTM Mortar Classification System

The American Society for Testing and Materials classifies mortar into five types: M, S, N, O, and K (remembered by every other letter in the phrase MaSoNwOrK). Each type has a different cement-to-lime ratio and minimum compressive strength. Type M is the strongest at 2,500 PSI. Type K is the weakest at 75 PSI. But compressive strength is only one factor — and not always the most important one.

The key insight is that higher-strength mortar is also more rigid and more brittle. A mortar that is too strong for the masonry units it bonds can actually cause cracking because it cannot flex with the wall under thermal expansion, settling, or wind loads. The mortar should always be weaker than the masonry unit it bonds — this ensures that stress-related cracking occurs in the repointable mortar joints rather than through irreplaceable bricks or stones.

Type N Mortar: The General-Purpose Standard

Type N is the most commonly used mortar for above-grade construction. It has a compressive strength of 750 PSI, excellent workability, and good bond strength. Its moderate cement content (1 part cement to 1 part lime to 6 parts sand) gives it enough flexibility to absorb minor wall movement without cracking.

Use Type N for exterior brick veneer, interior partition walls, chimneys above the roofline, and most above-grade masonry work. It is the default choice whenever building codes do not specify a stronger type. Type N is also the best choice for repointing historic brick because its flexibility and lower strength are compatible with the softer bricks used in older construction. Using Type S or M mortar to repoint old brick can cause the bricks themselves to crack.

Type S Mortar: For Below-Grade and High-Stress Work

Type S has a compressive strength of 1,800 PSI and a higher cement-to-lime ratio (2 parts cement to 1 part lime to 9 parts sand). The additional cement gives it superior bond strength and better resistance to lateral forces from soil pressure. It is stiffer than Type N and slightly harder to work with, but the performance difference matters where loads are higher.

Specify Type S for foundation walls, retaining walls, below-grade masonry, brick paving, and any application where the mortar faces soil contact or lateral loading. It is also the standard choice for areas with high wind loads or seismic activity because its bond strength resists the racking forces that earthquakes and hurricanes impose on masonry walls.

Type M Mortar: Maximum Strength Applications

Type M delivers 2,500 PSI compressive strength with the highest cement content of the standard types (3 parts cement to 1 part lime to 12 parts sand). It is the strongest and most durable mortar, but also the most rigid and least workable. Type M is overkill for most residential projects and can actually harm masonry assemblies that need flexibility.

The appropriate applications for Type M are limited: heavy-load-bearing walls, structural foundations below grade, stone retaining walls with massive units, and masonry in contact with the earth where durability against soil chemistry matters. Some building codes require Type M for below-grade foundation walls in areas with aggressive soil conditions. If your structural engineer specifies Type M, use it. If no type is specified, you almost certainly do not need it.

Type O and Type K: Specialty and Restoration

Type O mortar (350 PSI) has a high lime content that makes it very flexible and soft. It is primarily used for interior non-load-bearing walls and for repointing historic buildings where the original mortar was lime-based. The flexibility of Type O allows old masonry to breathe and move without cracking the units — critical for structures built before Portland cement was widely used.

Type K mortar (75 PSI) is the softest available and is used almost exclusively in historic preservation. Many pre-1920 buildings used pure lime mortar with no Portland cement. Repointing these buildings with modern Type N or S mortar traps moisture in the soft old bricks and causes spalling. Type K, with its high lime and low cement content, matches the original mortar performance and prevents this damage.

• Type M (2,500 PSI): heavy loads, below-grade foundations, aggressive soil
• Type S (1,800 PSI): retaining walls, below-grade, high wind and seismic zones
• Type N (750 PSI): general above-grade, veneer, chimneys, most residential work
• Type O (350 PSI): interior non-load-bearing walls, some restoration work
• Type K (75 PSI): historic preservation, lime mortar matching

Mixing and Working with Mortar

Pre-mixed mortar bags are the most convenient option for small to medium projects. They contain the correct proportions of cement, lime, and sand — you just add water. Mix to a consistency that holds its shape when sliced with a trowel but is wet enough to spread easily and bond to the masonry unit. The texture should be similar to thick peanut butter.

Work time for mixed mortar is 90-120 minutes in moderate conditions. Heat, wind, and low humidity shorten this window. Once mortar begins to stiffen in the mortar board, do not add water to retemper it — the hydration process has already started, and adding water produces a weaker, inconsistent bond. Discard stiffened mortar and mix a fresh batch. For large projects, mix only what you can use in 30-45 minutes to avoid waste.