Mortar types: M, S, N, O, K

One-line orientation

Mortar bonds masonry units and accommodates movement — the exam tests selecting the right type by location and load, and the fact that stronger is not automatically better.

Key points

• Strength order (strongest → weakest): M > S > N > O > K. Mnemonic MASON WORK — read the capital letters in order.
• Type M — highest strength; below-grade, high-load, structural. Foundations, retaining walls.
• Type S — second highest; exterior load-bearing walls, reinforced walls, outdoor masonry. Good flexural bond strength.
• Type N — balanced workability + moderate strength; the general above-grade choice: non-loadbearing veneers, chimneys, interior load-bearing walls.
• Type O — low strength; interior, non-loadbearing repair work.
• Type K — weakest, largely obsolete; historic/heritage masonry only.
• Two binder systems:
  • Portland-cement (Portland-lime) mortar — Portland cement + hydrated lime + aggregate; Portland cement Types I, II, III selected for strength/durability.
  • Masonry cement — Portland cement + blended hydraulic cement + plasticizers + admixtures; better workability but lower strength and higher water permeability.
• Mortar covers roughly 20% of a masonry surface and stays workable about 2.5 hours after mixing.

Mortar types M · S · N · O · K — strongest to weakest

Rank M > S > N > O > K and match the type to location and load, not to maximum strength.

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Over-strong mortar drives cracking through the brick unit instead of through the sacrificial joint.

Confusions / comparison

Type	Relative strength	Best use	Watch for
M	Highest	Below-grade, high-load, foundations, retaining walls	Stiff; least movement accommodation
S	2nd	Exterior load-bearing, reinforced, outdoor	Strong + reasonable bond
N	Moderate	General above-grade veneer, chimneys, interior load-bearing	The default exterior veneer choice
O	Low	Interior non-loadbearing, repair	Not for exterior/structural
K	Weakest	Historic/restoration only	Essentially obsolete

Related

→ Mortar joints (this module): joint profiles and weathering · Brick types & grades (this module): units the mortar bonds · CMU & grout (this module): mortar beds units, grout fills cells.