Mortar: What’s Your Type?

Mortar nomenclature has developed over many years to its current form. Designations for mortar are found in ASTM C 270, Standard Specification for Mortar for Unit Masonry. In the United States, the three common types of mortar specified for new construction today are N, S, and M. These arbitrary designations were assigned by taking every other letter from the term “mason work.” Astute observers will notice that an “O” and a “K” also appear in that term. While these are recognized mortar types, they are typically used for non-load bearing walls and for tuckpointing or other repair work.

Mortars are differentiated primarily by their strength: M is the highest strength, S is next, and N is a moderate strength mortar. (O and K are lower strengths yet, which is important in repair work so as not to create a mortar that is stronger than the wall/units where it is being placed.)

If you think the strongest mortar is the best solution, think again. True, strong mortars do generally have increased durability and greater structural capacity. But, since much masonry is constructed as reinforced masonry today—there are steel bars added to the cavity then grouted solid to create a “concrete” wall—the reinforcement and grout become the more dominant structural elements. The mortar itself is less important for its load carrying capacity than for its other aspects, such as facilitating placement of units.

Rule of Thumb: Use a Type N mortar for all masonry work unless there is a compelling reason to choose another mortar. C 270 provides recommendations for mortars choices in a concise tabular format as shown here. Note that alternative mortar types are also suggested, whether for availability considerations or for minimizing the number of different mortar types on the job site. Consult the appendix of C 270 for tuckpointing mortar guidance.

 
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