Lime Mortar and Repointing Historic Brick in Charleston: An Owner's Guide
Walk down almost any street on the Charleston peninsula and you are looking at old masonry: garden walls, chimneys, foundation piers, whole brick houses, and stuccoed facades with brick underneath. Much of that work was laid in a soft mortar made from lime and sand, and a good deal of it has lasted because the mortar was meant to give way before the brick did.
That idea surprises a lot of owners, and it explains one of the most common and costly mistakes on older Charleston buildings: patching soft historic brick with hard Portland cement mortar or cement stucco. Here is why historic mortar is soft, how cement causes damage, what proper repointing involves, and how to vet a mason before anyone picks up a chisel.
Why Charleston's Old Brick Was Laid in Soft Lime Mortar
Handmade brick, hand-mixed mortar
According to Historic Charleston Foundation's guide to dating historic building materials, early Charleston brick was "made locally on several plantations nearby," shaped by hand from local clay in wooden molds. Those bricks tend to be slightly irregular and smaller than modern brick, often show black spots from firing, and sometimes still carry the fingerprints of the people who formed them, both free and enslaved. The same source describes the mortar between them as a mix of lime and aggregate, usually sand, and notes that it was soft and therefore compatible with soft handmade brick.
Lime from the coast
Lime comes from burning a source of calcium carbonate, and the Lowcountry had one in abundance: oyster shells. The South Carolina Encyclopedia's entry on tabby describes the region's tabby construction as distinguished by oyster shell aggregate and lime derived by burning shells, and notes research indicating that tabby manufacture was understood around Charleston before 1726. Historic Charleston Foundation adds that some historic mortars here used crushed shells as the aggregate, so flecks of shell in an old joint are part of the local story, not a sign of sloppy work.
Mortar as a bedding layer, not glue
The National Park Service's Preservation Brief 2, Repointing Mortar Joints in Historic Masonry Buildings, by Robert C. Mack and John P. Speweik (revised 1998), explains that historic mortar acted as a bedding material, much like an expansion joint, rather than as a glue. High lime mortars are more permeable than denser cement mortars, and because lime mortar is slightly water soluble, it can re-seal hairline cracks. The brief states that stresses in a wall "should be relieved by the mortar rather than by the masonry units."
How Portland Cement Damages Soft Historic Brick
Portland cement was first manufactured in the United States in the early 1870s, and Preservation Brief 2 notes that by the 1930s most masons used a mix of equal parts Portland cement and lime putty. Historic Charleston Foundation notes that standardized bricks with Portland cement mortar usually point to construction after the 1890s.
The trouble starts when that harder, denser material meets older, softer brick. Preservation Brief 2 describes what happens when new mortar is less permeable than the brick around it: moisture stays in the wall until water pressure pops the surface off the bricks. Instead of drying through the joints, water and salts move through the brick faces, and the faces spall. Hence the brief's rule: new mortar "must have greater vapor permeability and be softer (measured in compressive strength) than the masonry units."
Historic Charleston Foundation notes that preservation professionals call Portland cement mortar "historic brick's greatest enemy." England's national heritage body agrees: its guide Repointing Brick and Stone Walls states that unsuitable repointing "is amongst the most frequent causes of damage to the character and fabric of historic buildings."
Cement stucco and sealers add to the problem
The Park Service's Preservation Brief 22 on historic stucco explains that the differing expansion and contraction rates of lime stucco and Portland cement stucco will normally cause the stucco to crack. A Post and Courier column by Robert Behre on the craftspeople who repair Charleston's walls puts it in local terms: "While Portland is stronger, it doesn't mesh well with softer, earlier stucco or masonry." The column adds that elastomeric paint, which trapped water and did not let a wall breathe, caused further damage.
Signs a Wall Needs Repointing
Preservation Brief 2 lists the conditions that usually signal a repointing job:
Disintegrating mortar
Cracks in mortar joints
Loose bricks or stones
Damp walls
Damaged plasterwork
Also look at the source of the water. The brief cautions that repointing alone will not solve problems caused by something else, such as a leaking roof, and Historic England notes that the parts of a wall most in need of attention are usually those most exposed to weather or affected by leaking rainwater pipes or gutters. Fix the gutter first, then the joints.
You may not need to repoint the whole wall. Preservation Brief 2 says it is preferable to repoint only the areas that require work, though if 25 to 50 percent or more of a wall needs it, doing the entire wall may be more cost effective. Cracking that suggests the wall itself has moved is a question for a structural engineer, and on older Charleston houses that conversation often includes the iron plates and rods described in our post on earthquake bolts and iron stars.
How a Proper Repointing Job Is Done
Matching the mortar
Preservation Brief 2 recommends studying unweathered samples of the old mortar. Sand is the largest ingredient by volume, so the sand must match the historic sand to get the color and texture right. A laboratory mortar analysis can be useful, but the brief notes that specifications should not be based solely on a lab report.
Raking out the joints
Old mortar is removed to a minimum depth of 2 to 2 1/2 times the width of the joint, about 1/2 to 1 inch for typical brick. The brief describes hand chisels and mash hammers as the method posing the least threat to historic masonry, and warns that power saws or grinders "almost always will result in damage to the masonry units by breaking the edges."
Filling, tooling and the joint profile
Deep joints are filled in layers of roughly 1/4 inch, each allowed to become thumb-print hard before the next. The final layer is tooled to match the original profile. Historic joints may be flush, beaded, raked or grapevine, and the brief notes that front walls often received more careful detailing than side and rear walls, so one house can have more than one profile. Masons should avoid leaving a thin featheredge of mortar on the brick face, which breaks easily and lets water in.
Curing and the test panel
Lime mortar needs time and moisture to cure. Preservation Brief 2 recommends misting the work once joints are thumb-print hard, as often as every hour at first and then every 3 to 4 hours, covering the wall with burlap for the first 3 days, and keeping wall temperatures between 40 and 95 degrees F.
Before the full job, the brief recommends a test panel using the same materials and techniques, usually 3 feet by 3 feet for brickwork, which becomes the standard for the rest of the work. Done right, a repointing job is meant to last at least 30 years, and preferably 50 to 100 years.
Stucco on Charleston Buildings and Lime-Based Repair
Many Charleston houses are brick buildings finished in stucco. Preservation Brief 22 explains that until the late 1800s stucco was primarily lime-based, and that after about 1900 most stucco was primarily Portland cement mixed with some lime.
The brief's repair process mirrors good repointing. Unsound areas that have lost their key echo with a hollow sound when tapped gently. Deteriorated stucco is removed down to the masonry, and the new material is matched in composition, color and texture, typically in three coats. The Post and Courier column profiles stucco contractor Joe Faust, a recipient of Historic Charleston Foundation's Stoney Craftsmanship Award, who describes the goal: "Most of the time, our job is to make it look like the repair never happened."
How to Vet a Mason for Historic Brick
Preservation Brief 2 suggests requiring bidders to have at least five years' experience repointing historic masonry buildings, and asking for a list of past repointing projects you can see in person. A few more questions help:
How will you remove the old mortar? Listen for hand tools.
How will you match the mortar, including sand, lime, color and joint profile?
Will you build a test panel first?
What is your curing plan in hot weather?
Have you worked on projects reviewed by the city's preservation staff?
Charleston also has local resources. The American College of the Building Arts, founded in 1998, teaches Preservation Masonry and Plaster Working among its six specialties, and Craft in America notes it was inspired by master blacksmith Philip Simmons, the subject of our Philip Simmons gates walking guide. Historic Charleston Foundation's Charter Day Awards include the Stoney Craftsmanship Award, which "recognizes craftspeople who have kept alive Charleston's tradition of excellence in the building trades."
How Charleston's BAR Treats Masonry Work
In the city's historic districts, the Board of Architectural Review reviews exterior work visible from a public right-of-way; our post on what Charleston's BAR reviews covers the process. For masonry, the city's Board of Architectural Review General Guidelines, dated March 2024, line up closely with the Park Service guidance:
Damaged bricks and mortar should be replaced to match the existing.
Repointing mortar should match the existing in color, texture and strength. The guidelines point to Preservation Brief 2 and warn that a mix with a higher concentration of Portland cement can cause significant damage.
Joints should match the existing type, such as concave tooled, v-shaped or flush.
Brick generally should not be painted unless previously painted, and waterproof coatings on historic masonry are not permitted.
Stucco repairs should match the original in strength, color, texture and composition, and new stucco should never be applied over existing stucco.
The guidelines list repairs and in-kind replacements among items staff may review. Requirements depend on the building and scope, so confirm with the city's preservation staff before work begins.
FAQ
Can a mason use standard modern mortar on my historic brick?
Preservation Brief 2 says replacement mortar must be softer and more vapor permeable than the brick. The city's guidelines warn that a higher concentration of Portland cement can cause significant damage, so ask your mason to explain the mix.
How long should repointing last?
Preservation Brief 2 says a good job is meant to last at least 30 years, and preferably 50 to 100 years.
Do I need BAR approval to repoint or repair stucco?
Exterior work visible from a public right-of-way in Charleston's historic districts falls under BAR review, and staff may review repairs and in-kind replacements. Check with the city's preservation office for your property.
Should I paint or seal old brick?
The city's guidelines say brick generally should not be painted unless previously painted, because paint can trap moisture in the wall, and waterproof coatings on historic masonry are not permitted.
Conclusion
Soft lime mortar is not a weakness in Charleston's old brick. It is the part of the wall built to absorb movement, let moisture out and be renewed every few generations so the brick can stay. Repairs that respect that, with matched lime mortar, hand-raked joints, the right profile and patient curing, can last for decades. If you are buying an older home, masonry belongs on your list of questions, along with the items in our guide to the South Carolina property condition disclosure.
If Charleston's brick and stucco streetscapes draw you in, browse our neighborhood guides, take a look at our current listings, or reach out to our team with questions about a historic home you are considering.