Mortar Types
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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.
Location |
Building Segment |
Recommended Mortar |
Alternative Mortar |
Exterior, above grade |
Load-bearing walls
Non-load bearing walls
Parapet walls |
N
O
N |
Sor M
N or S
S |
Exterior,
at or below grade |
Foundation walls, retaining walls, manholes,
sewers, pavements, walks and pations |
S |
M or N |
Interior |
Load-bearing walls
Non-load bearing walls |
N
O |
S or M
N |
From ASTM C 270
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