When it comes to maintaining historic buildings composed of stone, repointing mortar joints is one of the most critical tasks to prevent further wear and structural damage. Mortar joints are the spaces between bricks or stones that are filled with mortar. Mortar is a mixture of cement, sand, and water, where additional materials like lime, gypsum, and polymers can be added to enhance the durability and weather resistance of the bond. Mortar joints in historic buildings are essential for preserving the structural integrity and historical significance; they act as a crucial defense against moisture and can deteriorate prior to the masonry unit, protecting the stone from damage. Over time, mortar joints can deteriorate due to weather, pollution, moisture, and structural movement. Left unaddressed, failing mortar joints can lead to many structural issues, mainly water infiltration and instability.
Historic Mortar Joints
In historic buildings, the primary material choice was lime mortar, which is more permeable and flexible than modern cement by allowing moisture to escape easily and accommodating the natural movements of bricks. Where the more modern material for mortar is Portland cement, which offers faster set times and strength, but does have some significant downsides to it. This material is less permeable than lime mortar, which can lead to potential moisture trapping and damage, causing risk for long-term wear and repair costs.
When it comes to typical joint profiles for historic buildings, the most popular choices were V-struck, weather-struck, or raked joints. Whereas today, concave and convex joints are more commonly used. The impact of a joint profile can determine how it stands up over time; if it’s too slicked or compressed, it can become less breathable, leading to cracking or moisture damage.
Examples of different mortar joint profiles.
What Is Mortar Repointing?
Repointing is the process of removing deteriorated mortar from the joints of a masonry wall and replacing it with new mortar. In historic buildings, this task must be approached with care to ensure that the new mortar matches the original in composition, color, texture, and strength. Using the wrong type of mortar during repointing can damage the surrounding masonry and compromise the integrity of the building. Improper repointing can lead to cracked or spalling masonry, trapped moisture and frost damage, loss of historical authenticity, and increased repair costs. It is most common during stone conservation and restoration projects to encounter a damaged mortar joint. Typically, historic stone conservation and restoration projects will require a historic assessment, traditional craftsmanship, and accurate materials to properly repair the damage. Below, we will cover the basics of repointing historic mortar joints with traditional craftsmanship to produce a result that blends with the historic material and protects it from future damage.
During the mortar repointing process for our interior restoration project at Boston College’s Gasson Hall.
After the interior restoration of Gasson Hall.
How to Repoint Historic Mortar Joints
Repointing historic mortar joints is not just a matter of maintenance, it’s a process that requires a deep respect for the original materials and craftsmanship. The overview of steps includes the initial assessment, removal of damaged mortar, preparation and development of the new historically accurate mortar, and the application, staining, and curing of the new mortar.
Assessments
Before any work begins, a thorough assessment is crucial. This includes determining the type of mortar used (lime, cement, etc.), evaluating the condition of both the mortar and masonry units, and taking mortar samples for analysis to replicate the composition. Mortar analysis is often conducted to ensure that the new mortar will match the original material both visually and chemically.
Examples of damaged masonry and mortar joints before our restoration for the Pilgrim Hall Museum at Steinway Library.
Removal
Mortar removal should be done carefully and to the correct depth, typically 2 to 2.5 times the width of the joint. Hand tools are preferred over power tools when working on historic structures to avoid damaging the masonry. Common tools include mortar rakes, chisels, hammers, and pointing trowels. Using hand tools helps maintain the authenticity of the structure and reduces the risk of vibration-related damage.
Images of the damaged mortar removal process at the Steinway Library.
Preparation for Repointing
After removal, the joints should be brushed clean of dust and debris and then lightly misted with water to ensure proper adhesion of the new mortar. This prevents the dry masonry from drawing moisture too quickly from the new mortar, which can cause cracking and poor bonding. Matching the new mortar to the old is essential. This includes using similar sand in color and texture, matching the binder (lime, cement, etc.), and maintaining appropriate ratios for workability and strength.
Applying and Curing the New Mortar
Repointing mortar to historic joints requires precision, patience, and an understanding of traditional masonry techniques. Once the old mortar has been carefully removed and the joints cleaned and dampened, the new mortar should be packed into the joints in thin layers. Each layer must set slightly before applying the next to prevent air pockets and ensure a strong bond. The mortar should be pressed firmly into the joint using appropriate tools such as a pointing trowel or jointer to achieve full contact with the surrounding masonry. The final layer should be tooled to match the original profile, whether it’s flush, concave, or weathered, it should blend seamlessly with the historic technique. Special care must be taken to avoid smearing mortar on the face of the bricks or stones, as this can cause staining and alter the appearance of the historical material.
Once the new mortar is applied and dried, it’s possible for the color to slightly differ from the existing mortar. In this case, staining the mortar is needed to resolve any color difference. The surface must be cleaned and free of any dust so the custom-tinted masonry stain can be applied in thin, controlled layers using a brush or sponge. The process may require multiple applications and adjustments to achieve a seamless blend. Once the joints are finished with the appropriate profile to match the original, the final step is curing to ensure the material is properly sealed.
Up-close view of the craftsman staining the new mortar to match the existing mortar.
Curing historic mortar properly is essential to ensure its long-term durability and compatibility with original masonry. Newly repointed joints should be kept moist and protected from direct sun, wind, and freezing temperatures for several days to weeks, depending on the climate and mortar type. This can be done by misting joints regularly with water and covering them with damp burlap or plastic sheeting to retain moisture. Inadequate curing can result in shrinkage cracks, poor bonding, or a weakened mortar structure, all of which can compromise the integrity and appearance of the historic masonry.
After restoration at the Steinway Library, including the repointing historic mortar.
Done correctly, repointing historic mortar joints can breathe new life into a building, safeguard its structure, and preserve its historical authenticity. Large or historically significant projects should always involve professionals with experience in historic preservation. Understanding the nuances of working with traditional mortars and masonry systems, ensuring each repointing project is carried out with integrity and precision, is vital to a successful result. John Canning & Co. is highly experienced in this area and is more than happy to offer assistance. If you have a project you would like to discuss or have any questions at all, don’t hesitate to contact us.
