What Type of Mortar Is Best for Old Brick Homes?

The mortar types, simplified

ASTM C270 defines mortar by strength, from hardest to softest: M, S, N, O, and K. Older brick was typically laid in lime mortar, which is softer than any of these.

• ·Type M/S: high strength, used for below-grade and structural new work
• ·Type N: general-purpose, suitable for many post-1930 homes
• ·Type O: softer, good for many older, exposed brick walls
• ·Lime / Type K: historic and very soft brick

Why softer is often better

Mortar should be the sacrificial element. When it's softer than brick, movement and moisture pass through the joints. When it's harder, the brick edges crack and faces spall.

Mortar analysis

For historic buildings, a lab or field analysis of the original mortar identifies binder and sand so the new mix matches in performance and appearance, as recommended by NPS Preservation Brief 2.

Frequently asked questions