The Masonry Repair That Can Make a Historic Chimney Worse

Photo By: Hayden Mills

For homeowners living in a house built more than a century ago, deteriorating mortar and aging chimney infrastructure can look like a straightforward maintenance problem. A few cracked joints, crumbling masonry or a leak around the chimney may seem like the kind of repair any contractor or mason can handle.

But historic masonry does not always respond well to modern repair methods.

That is an issue Chimney Scientists, a Philadelphia-based chimney company founded by microbiologist Joe Ochal, has encountered while working on older homes. The company has developed specialized training around historic masonry after finding that many contractors are familiar with modern Portland cement-based materials but have less experience identifying and working with historic lime mortar.

For homeowners, the distinction can have major consequences. A repair can appear successful while setting the stage for more serious deterioration years later.

When the repair becomes part of the problem

Many homes constructed before approximately 1929 were built with lime-based mortar. Unlike modern Portland cement, historic lime mortar is relatively soft and permeable. It was designed to work with porous brick and allow moisture to move through the masonry.

That system matters particularly on chimneys, which are exposed to rain, snow, temperature changes and moisture generated by the home itself.

Replacing deteriorated lime mortar with a much harder cement-based material can change how the wall handles water. Instead of moving through the masonry and evaporating, moisture can become trapped.

The damage may not be immediately apparent.

According to Chimney Scientists, also one of the largest chimney companies in the U.S., one of the recurring problems it encounters is the “patch it” approach: a leaking historic chimney receives a modern cement-based repair, the leak temporarily improves and the homeowner assumes the problem has been resolved. After subsequent rainy seasons, the leak can return, sometimes with more extensive masonry deterioration behind it.

That can leave homeowners paying for essentially the same repair more than once.

Why the brick can suffer

The problem is not limited to moisture.

Portland cement is significantly harder and denser than many historic bricks. Older masonry was generally constructed so that the mortar would be the more sacrificial component, meaning it could deteriorate and be replaced without destroying the surrounding brick.

When a harder material is introduced, that relationship can change.

As masonry expands and contracts with temperature, rigid mortar and softer brick respond differently. Over time, the stress can contribute to cracking and deterioration of the brick itself. The outer face of a brick may eventually break away, a condition known as spalling.

Once that protective surface has been lost, the brick becomes more vulnerable to water. What began as deteriorating mortar can therefore become a much larger masonry restoration.

Chimney Scientists has identified this problem as one reason historic chimney repairs require material identification before repair work begins. The company trains its technicians to recognize structures that require historic lime mortar and has pursued hands-on training with specialized suppliers including Cathedral Stone Products and Limeworks.

That expertise is particularly relevant because historic mortar cannot always be identified simply by its appearance.

An old house can contain several generations of repairs

Historic chimneys rarely exist in exactly the condition in which they were originally built.

A homeowner may be looking at original brick combined with several different generations of mortar, patches, stucco or other repairs. Some may have been applied decades ago, meaning a visual inspection alone may not reveal the full history of the structure.

Portland-based mortar is often gray or smoother in appearance, while historic lime mortar can appear lighter and more granular. But those characteristics are not reliable enough on their own to determine what a structure contains.

Proper identification requires training and experience with historic masonry.

That is why the question for a homeowner should not simply be, “How much will it cost to repoint this chimney?” It should also be, “What is already here, and what material will you use to repair it?”

The cheapest quote may not be the cheapest repair

Historic restoration can cost more upfront. Chimney Scientists estimates that historic lime materials can cost several times more than Portland-based alternatives, although material is only one part of a masonry project’s total cost. Labor, scaffolding, equipment and access can make up a significant portion of the final bill.

There is another cost homeowners should consider: having to do the job twice.

Removing incompatible material, repairing damaged brick and restoring a chimney that has been allowed to deteriorate can be substantially more involved than addressing the original mortar problem correctly.

The issue becomes especially important during a larger renovation. Chimney Scientists recommends evaluating chimneys before or during the early stages of renovation rather than discovering a major masonry problem after walls have already been finished. Chimney repairs can involve significant dust, equipment and access requirements, making them considerably more disruptive once a home has been completed.

Historic preservation also extends beyond mortar. Chimney Scientists cautions against impermeable coatings and masonry paint on historic chimneys because they can interfere with the movement and evaporation of moisture. Renovations can introduce moisture problems of their own, including condensation from improperly positioned HVAC ductwork against masonry.

For homeowners, the lesson is not that every old chimney requires an expensive restoration. It is that age changes the equation.

A century-old chimney was built as a particular masonry system. Before replacing deteriorated material with whatever product is most familiar or readily available, homeowners should determine what the original structure was designed to use.

A repair should protect the building that is still standing, not simply make the damage disappear.