Why the Wrong Mortar Is Destroying Your Historic Brick Home

If your home was built before 1920, there's a detail hiding between your bricks that most contractors get wrong — and getting it wrong can permanently destroy brick that has survived a century of Illinois winters. That detail is the mortar. Old homes were built with soft, breathable lime mortar. Modern tuckpointing crews almost always show up with hard Portland cement mortar. The two materials are not interchangeable, and using the wrong one on historic brick is one of the most common — and most expensive — mistakes we see across McHenry County and Lake County.

At RCC Masonry & Concrete, our historic masonry restoration work regularly involves undoing cement repointing jobs that did more damage in ten years than the previous hundred. Here's the science behind why the wrong mortar destroys old brick, how to spot the damage, and how to repair it correctly.

A Quick History: Why Old Homes Used Lime Mortar

For thousands of years — from Roman aqueducts to Chicago's post-fire brick boom — masonry was laid with lime mortar: burned limestone (quicklime) slaked with water and mixed with sand. Lime mortar cures slowly by absorbing carbon dioxide from the air, stays relatively soft, and remains slightly flexible for its entire life. Portland cement, patented in 1824, didn't dominate American construction until roughly the 1920s and 1930s. That means nearly every brick building constructed in Northern Illinois before that era — farmhouses, storefronts, churches, schoolhouses — was built with lime mortar.

The bricks themselves were different too. Pre-1920s bricks were fired in coal or wood kilns at lower, less consistent temperatures. The result is a brick with a hard outer "fire skin" and a softer, more porous core. That softer brick and soft lime mortar were engineered — intentionally or not — to work as a system.

How the Lime Mortar System Actually Works

In a healthy historic wall, the mortar is deliberately the weakest, softest, most porous element. That gives the wall three critical abilities:

• Breathability: Moisture that enters the wall — from rain, rising damp, or interior humidity — evaporates out through the soft mortar joints rather than through the brick faces.
• Flexibility: Lime mortar accommodates seasonal expansion, contraction, and minor foundation movement without cracking the brick. It can even self-heal hairline cracks as free lime redeposits.
• Sacrificial wear: The joints erode slowly over decades and are simply repointed every 50–100 years. The mortar is the maintenance item; the brick is preserved.

When a wall like this is repointed with hard, waterproof Portland cement, every one of those functions is reversed — and the brick becomes the sacrificial element instead.

What Happens When Portland Cement Meets Historic Brick

Portland cement mortar is typically two to four times harder than the old brick surrounding it, and it's nearly impermeable to water vapor. In a Northern Illinois climate that delivers 80+ freeze-thaw cycles a winter, the failure sequence is predictable:

• Moisture gets trapped. Water still enters the wall — it always does — but it can no longer escape through the sealed cement joints. The only exit path is through the porous brick faces.
• Salts and freezing water attack the brick. As moisture migrates through the brick and evaporates at the surface, dissolved salts crystallize inside the brick's pores. In winter, trapped water freezes and expands by 9%. Both processes tear the brick apart from the inside.
• The brick faces pop off. This is spalling — the hard fire skin flakes away, exposing the soft inner core, which then erodes rapidly. Once a historic brick loses its face, it cannot be repaired, only replaced.
• Rigid joints crack the wall. Because cement mortar can't flex, normal seasonal movement concentrates stress in the bricks themselves, causing cracks that run through brick units instead of harmlessly through mortar joints.

The cruel irony: a cement repointing job often looks crisp and solid for the first few years. The damage develops underneath, and by the time spalling is visible, the wall may need hundreds of brick replacements — with salvaged brick that's increasingly hard to source and match.

Warning Signs Your Historic Brick Has the Wrong Mortar

If you're seeing brick failure with intact joints, that's the signature of a cement-on-lime mistake. It's the opposite of normal masonry aging, where joints wear first and brick survives.

The Right Fix: Lime Mortar Repointing

Restoring a historic wall properly is a fundamentally different process from standard tuckpointing:

• Mortar analysis first. A sample of the original mortar is examined (or lab-tested) to determine its lime content, sand color, and aggregate size so the new mortar matches in strength, color, and texture.
• Hand removal, not grinders. Existing cement mortar is carefully removed with hand chisels or thin-blade tools. Aggressive angle grinding — the standard on modern brick — widens joints and scars irreplaceable historic brick.
• Softer-than-the-brick mortar. The replacement mix — typically a Type O or K lime-rich mortar, or a natural hydraulic lime (NHL) blend — is deliberately weaker than the brick, restoring the wall's breathability and flexibility.
• Matched joint profiles. Historic joints were often struck, grapevine, or beaded. Reproducing the original profile keeps the repair visually seamless.
• Slow curing and protection. Lime mortar needs days of damp curing and protection from frost and direct sun — which is why proper restoration work is scheduled around the weather, not around the crew's calendar.

Expect lime repointing to run roughly $12–$25 per square foot in Northern Illinois — a premium over the $8–$15 typical for conventional tuckpointing. That premium buys work that protects the brick for another 75–100 years instead of destroying it in ten.

Questions to Ask Before Anyone Touches Your Historic Brick

• "What mortar mix will you use, and how will you match the original?" (A red flag is any answer that starts and ends with "Type N bags from the supply house.")
• "How will you remove the old mortar?" (Hand tools on historic brick; grinders only with extreme care, if at all.)
• "Have you done lime mortar restoration before? Can I see examples?"
• "How will you cure and protect the new joints?"

Most tuckpointing crews are honest and skilled at what they do — modern masonry. But historic restoration is a specialty, and a contractor who doesn't know the difference will confidently do the wrong thing.

Protect a Building That's Already Survived a Century

A pre-1920s brick home in McHenry or Lake County has already outlasted every roof, furnace, and window it was built with. The only thing that can take it down prematurely is the wrong repair. If your historic brick is showing spalling, cement patches, or crumbling joints, RCC Masonry & Concrete's historic masonry restoration service provides mortar matching, hand repointing, and salvaged brick replacement across Northern Illinois. Call (224) 441-5284 or request a free assessment — we'll tell you honestly whether your wall needs restoration work or standard tuckpointing.