Mortar for Old Masonry

There’s no one-size-fits- all mix. Choosing the right mortar depends on the age, material, and condition of the masonry.

- Justin Fink Fink & Son Historic Restoration

Chances are good that if you own or work on old houses, you'll come across masonry in some form—in the foundation, chimney, or other details made from stone, brick, or tile. Mortar is the “glue” that holds these materials together. Over time, mortar is pushed and pulled by freeze/thaw cycles and eroded by weather, and the joints need to be scraped and refilled in a process known as pointing. “Mortar” is a general term for a range of mixtures made from different ingredients in varying ratios. These ingredients and their proportions determine how the mortar performs. Avoiding problems like cracking and spalling means understanding what goes into mortar and how the formulas have changed over time.

Here’s an overly simplified timeline:

Up until the mid-1800s, masons used a mixture of lime putty and sand. Lime putty was made on the jobsite by combining lime or quicklime with water, a practice known as slaking. By 1900, masons slowly began incorporating portland cement—a fast-curing, dense, hard-setting hydraulic cement—into the mortar mixture as an additive to speed set time and increase strength. By 1950, portland cement was dominant; knowledge of using lime mortars was largely lost.

A SACRIFICIAL SOLUTION

If all mortar did was hold a wall together, then we would have abandoned those sand- and-cement mixtures and switched to a space-age adhesive. Aside from aesthetics, the reason we continue to use mortar is that it acts as a sacrificial component. Expansion and contraction, freezing and thawing, and structural shifts all put strain on masonry assemblies. If the mortar is denser and stronger than the masonry units it's holding together, then it’s the masonry units that will fail. Using mortar softer and more vapor-permeable than the masonry units means the repairable mortar will absorb the damage.