How Mortar Types Are Classified
In Canada, masonry mortar is classified under CSA A179, which aligns closely with the ASTM C270 standard used in the United States. Both standards define mortar types by proportion specification (the ratio of cement, lime, and sand by volume) and by property specification (the minimum compressive strength achieved at 28 days).
The commonly used types in residential and light commercial construction are Type N, Type S, and Type M. A fourth type, Type O, exists for low-load interior applications and is rarely specified in Canadian construction due to its limited freeze-thaw resistance.
Type N Mortar
Type N is a general-purpose mortar formulated with one part portland cement, one part hydrated lime, and six parts masonry sand by volume. It achieves a minimum 28-day compressive strength of 5.2 MPa under property specification.
The relatively high lime content in Type N produces a workable, flexible mortar that accommodates minor thermal and moisture movement in the masonry assembly. This flexibility is an advantage in applications where the wall system needs to breathe — above-grade exterior brick veneer being the primary example. In above-grade veneers, a mortar that is slightly weaker than the brick allows the joint to absorb movement and cracking before the brick face itself is damaged.
Type N is appropriate for:
- Above-grade exterior brick veneer walls
- Interior non-load-bearing partitions
- Soft stone masonry (limestone, sandstone) where the stone's compressive strength is lower than a hard mortar would allow
- Tuckpointing and repointing heritage masonry where the original mortar was lime-based
Type N is not appropriate for below-grade applications, foundations, or areas in contact with soil.
Type S Mortar
Type S uses a ratio of one part portland cement, one-half part hydrated lime, and four and one-half parts masonry sand. It achieves a minimum 28-day compressive strength of 12.4 MPa — roughly 2.4 times stronger than Type N.
The reduced lime content and higher cement proportion give Type S a harder, denser joint with greater resistance to water penetration and lateral forces. It is the standard specification for most structural masonry applications in Canadian residential construction.
Type S is appropriate for:
- Below-grade masonry in contact with soil, including foundation walls
- Exterior retaining walls subject to soil pressure
- Masonry in high-wind or seismically active zones (required in many BC municipalities)
- Pavement applications and brick walkways subject to vehicle traffic
- Chimneys above and below the roofline
Type M Mortar
Type M is the highest-strength masonry mortar in common use, produced at a ratio of three parts portland cement, one-quarter part hydrated lime, and twelve parts masonry sand, achieving a minimum compressive strength of 17.2 MPa. The very low lime content produces an extremely rigid, dense joint with high bond strength but low flexibility.
That rigidity is a limitation in most above-grade applications. A Type M joint is harder than many masonry units it binds — if differential movement occurs, cracking follows the face of the brick or stone rather than the joint, which is the opposite of what is needed for maintainable masonry. Repointing Type M joints requires mechanical removal, which risks damaging the surrounding units.
Type M is appropriate for:
- Below-grade foundations and footings in contact with soil and groundwater
- Manholes, catch basins, and utility masonry below grade
- Masonry in severe frost exposure conditions where maximum density is needed to limit water absorption in the joint
Type M is generally not recommended for above-grade exterior walls, heritage masonry, or any application where thermal or settlement movement is anticipated.
Lime Content and Freeze-Thaw Performance
A consistent observation in Canadian masonry practice is that mortar fails faster than it should when lime content is reduced to increase early strength. Pre-mixed masonry cement products, which substitute portland-lime blends with proprietary additives, have variable freeze-thaw performance depending on the manufacturer's formulation.
For projects in climate zones experiencing more than 50 freeze-thaw cycles per year — which includes Edmonton, Winnipeg, most of Ontario north of Highway 7, and much of Quebec — specifying mortar by the CSA A179 proportion specification (rather than property specification alone) ensures that the lime content is predictable. This is particularly relevant when repointing existing masonry, where the new mortar should not be significantly harder or less permeable than the masonry units it contacts.
The Canadian Masonry Contractors Association technical notes address freeze-thaw mortar selection in detail for applications in ASHRAE climate zones 5 through 8, which cover most of Canada's populated regions.
Pre-Mixed vs. Site-Mixed Mortar
Pre-mixed dry mortar bags (sold under product names at masonry supply stores) offer convenience but require checking that the type designation — N, S, or M — matches the application requirement. Some products labelled as "all-purpose" or "general masonry" do not correspond to a standard CSA A179 type and may not perform equivalently.
For larger jobs (typically 500 bricks or more), site-mixing from portland cement, hydrated lime, and washed masonry sand provides more control over water content and workability. The key variable in site mixing is the sand — it must be washed masonry sand free of organic material, not bank-run sand or unscreened aggregate, which alter the mix ratio unpredictably.
Key Takeaways
- Use Type N for above-grade brick veneer, soft stone masonry, and heritage repointing.
- Use Type S for below-grade, structural masonry, chimneys, retaining walls, and paving.
- Use Type M only for below-grade foundations in contact with soil or water — avoid in above-grade applications.
- In high freeze-thaw climate zones, specify by proportion rather than property specification to ensure predictable lime content.
- Confirm that pre-mixed products carry a standard CSA or ASTM type designation before using on structural or below-grade applications.
Last updated: May 14, 2026