Yes, there’s a serious problem: using Type S mortar on a 1920s soft red brick house is likely to cause damage.
Soft, historic bricks from that era are low-fired and porous, designed to move slightly with thermal and moisture changes. The original mortar was lime-rich and soft—typically Type O or even a lime-sand mix—to act as a sacrificial, flexible layer that accommodates movement and allows moisture to escape.
Type S mortar is high in Portland cement, making it very hard, dense, and impermeable—far stronger than the brick. When used as repointing mortar, it creates a rigid joint that:
- Prevents moisture from escaping through the joint, forcing it into the brick.
- Causes spalling, cracking, or face delamination of the soft brick during freeze-thaw cycles.
- Transfers stress to the brick instead of absorbing it, accelerating deterioration.
Correct approach: Use a mortar softer than the brick—typically Type N or, better yet, a custom lime-based mortar (e.g., 1:2:9 lime:cement:sand or straight NHL 3.5 lime) to match the original’s permeability and flexibility. Grinding to 1/4" depth is acceptable if done carefully to avoid damaging brick edges.
Bottom line: Stronger is not better here. Using Type S risks irreversible damage to historic masonry. Recommend removing the Type S and repointing with a compatible, softer mortar. If unsure, consult a preservation mason or refer to ASTM C270 and Secretary of the Interior’s Standards for Rehabilitation.
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