What is Type S Mortar? | TSE Glossary

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Type S mortar is a high-strength masonry mortar with a compressive strength of 1800 psi at 28 days, classified under ASTM C270. It is specified for masonry below grade, structural exterior walls, and chimney bases that bear significant load. Its higher portland cement content gives it greater bond strength and resistance to lateral forces from wind, settlement, or hydrostatic pressure.

Type S is typically mixed at two parts portland cement, one part hydrated lime, and nine parts sand, yielding a mortar that develops strength rapidly and resists penetration of moisture. The trade-off is reduced flexibility: type S is too rigid for historic soft-brick walls, where it can crack the brick units rather than the joints. For modern hard brick or concrete masonry units used in load-bearing applications, the higher strength is appropriate.

In chimney work, type S appears at the base of the chimney where it transitions to the foundation, in chimney chase walls that bear roof and crown loads, and in any below-grade or grade-level masonry. Above the roof line, type N is generally preferred unless the original construction used type S throughout. Mixing types within a single chimney is common, with type S below the roof and type N above.

DFW’s expansive Blackland Prairie clay produces seasonal soil movement of two to four inches in some areas, putting lateral pressure on chimney bases. Type S mortar at the base resists that movement better than type N, and TSE technicians use type S during base tuckpointing on chimneys with documented foundation movement. Above the roof, type N remains the choice because the upper masonry needs flexibility for thermal cycling.

• [Type N mortar](/glossary/type-n-mortar/)
• [Refractory mortar](/glossary/refractory-mortar/)
• [Tuckpointing](/glossary/tuckpointing/)
• [Repointing](/glossary/repointing/)

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

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