What is Type N Mortar? | TSE Glossary

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Type N mortar is a medium-strength masonry mortar with a compressive strength of 750 psi at 28 days, formulated to ASTM C270. It is the most common mortar specified for above-grade chimney exteriors and is preferred by preservation specialists because its lower strength accommodates the soft historic brick found in older masonry without cracking the units. ASTM C270 lists it as the appropriate choice for general above-grade exterior work.

Type N is mixed at roughly one part portland cement, one part hydrated lime, and six parts sand. The lime content gives the mortar flexibility, allowing it to flex slightly with seasonal thermal movement and minor settling. Lower compressive strength is a feature, not a bug: in a brick-and-mortar wall, the mortar should fail before the brick, so cracks form in repointable joints rather than splitting expensive units.

For chimneys, type N is used on the exterior shell above the roof line and in tuckpointing repairs. It is not appropriate for fireboxes or flue tile joints, which require refractory mortar, nor for below-grade work or load-bearing walls subject to high lateral loads, where type S is specified. Sweeps and masons match the original mortar type during repointing to preserve compatibility with surrounding masonry.

DFW’s freeze-thaw cycles and clay-soil movement put significant stress on chimney mortar joints. Type N is the workhorse for tuckpointing repairs on the upper sections of older Dallas chimneys, where flexibility matters more than strength. TSE technicians match mortar to existing brick hardness during all tuckpointing work because using too-strong type S mortar against soft 1920s brick is a common cause of accelerated brick spalling in historic neighborhoods.

• [Type S mortar](/glossary/type-s-mortar/)
• [Refractory mortar](/glossary/refractory-mortar/)
• [Tuckpointing](/glossary/tuckpointing/)
• [Repointing](/glossary/repointing/)
• [Spalling](/glossary/spalling/)

• ASTM C270 mortar specification
• IRC 2021, Section R607.2
• NFPA 211 (2024), Section 10.4

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