Protect Historic Masonry Buildings from Permanent Damage Caused by Portland Mortar
Historic masonry buildings are very different from modern buildings. Historic bricks were fired at lower temperatures and are much softer and more permeable than modern bricks. Historic buildings constructed with these softer bricks were designed to absorb moisture and then release it. A key component of this design was the lime mortar historically used in masonry applications, a mortar that was also soft and readily allowed moisture to pass through.
In the late 1800’s, a new mortar debuted in the U.S. at the height of the Industrial Revolution. Favored for all the qualities a mass-production revolution could ask for (fast-curing, inexpensive, and less work for masons), Portland quickly gained popularity with masons and by the early 1900’s most buildings had some Portland mortar in their masonry surfaces – usually as an additive to traditional lime mortar. By the mid-1900’s Portland was no longer used as an additive and became the predominant ingredient in mortar mixes. Historic buildings were not immune to the new technology and masonry repairs on historic buildings in the 1900’s were predominantly made with Portland mortar.
If your historic building has been re-pointed in the last sixty years, it very likely was re-pointed with a Portland cement mortar mix.
A common mistake, Portland mortar applied to historic buildings doesn’t just erode the historic fabric of the building, it causes physical damage that is often permanent. Traditional mortars worked with the softer historic masonry materials to expand and contract together as temperatures and moisture levels changed, creating a wall and masonry surface that “breathed” to expel excess moisture. Applying a Portland mortar mix to a historic masonry surface disrupts that relationship and traps moisture in the wall and historic bricks.
Moisture trapped within walls will not easily pass through Portland cement mortar and will be forced through the soft brick instead, a path of much less resistance. When the water evaporates, salt deposits are left behind that crystallize that destroys the protective shell of the bricks. Once this outer surface is damaged, the softer insides of historic bricks rapidly disintegrate.
Moisture issues caused by Portland mortar on a historic building can begin to destroy historic masonry within just a few years.
The historic bricks on masonry buildings are not the only things threatened by Portland cement mortar – structural elements, interior features, and occupant health are also compromised by the moisture issues associated with Portland mortar. Remember, historic
masonry materials and mortars were designed from a construction approach that created buildings that “breathed”, allowing moisture both in and out. Modern masonry materials and mortars are designed from a watertight construction approach that aims to keep water from passing through.
Combining a material from the system designed to let a house “breathe” with a material from the system designed to prevent water from passing through is a recipe for disaster.
Historic mortar differs significantly at a molecular level from modern mortar. This difference makes modern mortar incompatible with historic masonry materials, permanently damages historic masonry materials and structural elements of masonry buildings, and traps moisture in walls lowering energy efficiency and endangering air quality inside the building.
Here’s a tool you can use to evaluate your historic building’s masonry for potential problems and problem indicators. For a printer-friendly version of this checklist, click on the picture.