Chicago common brick built this metropolitan area. From the 1840s through the early decades of the twentieth century, most of the brick in the Chicago region came from local clay operations along the Chicago and Illinois River corridors. The material was fired from that local clay, produced in large volumes at relatively low cost, and used for nearly every structure built in the region during that period - commercial buildings, multi-family housing, the bungalow belt, the greystones, the North Shore estates.
The brick worked. Most of it is still standing. The problem is not the brick. The problem is what has been done to it since.
When soft common brick from the 1920s or 1930s gets repointed with modern Portland cement mortar, the brick starts failing. The failure is slow at first, then faster. By the time a homeowner notices the spalling faces, the damage has usually been progressing for five to fifteen years. Understanding why requires knowing what Chicago common brick is and how it was designed to function.
What Chicago Common Brick Actually Is
The term “common brick” distinguishes this material from face brick, which was manufactured to tighter tolerances for aesthetic use on visible facades. Common brick was a utility material - it built the structural core of walls, the non-visible courses, and in most residential construction, it served as the primary material on all exterior faces.
Chicago common brick was manufactured from clays found in the river valleys of northeastern Illinois. The deposits along the Chicago River, the Des Plaines River, and the Illinois River produced clays with specific chemical and mineral compositions that determined the fired brick’s color and physical properties. The salmon-to-buff color range characteristic of Chicago common brick comes from the iron content of these specific local clays and the relatively moderate firing temperatures used in nineteenth and early twentieth century kilns.
The BIA Technical Note 3A on brick material properties provides the framework for understanding how compressive strength, absorption rate, and durability classification vary across brick types. Soft-mud or hand-mold production through most of the nineteenth century produced bricks with slight dimensional variation and surface irregularities. As production mechanized through the late 1800s and early 1900s, stiff-mud extrusion became more common, but firing temperatures and clay composition remained consistent with the local material. The result was still a soft, relatively porous brick with the visual character that defines Chicago’s pre-war architecture.
Compressive strength of soft-mud Chicago common brick typically ranges from roughly 1,500 to 2,500 PSI, depending on clay source, firing temperature, and production era. Modern machine-pressed brick reaches 8,000 to 12,000 PSI or higher. The difference is not a defect. It reflects the material system these walls were designed around.
Chicago Common Brick on the North Shore: Evanston and Winnetka
The North Shore communities have the highest concentration of Chicago common brick in the suburban area, because they were the first suburban communities developed at scale and because their housing stock predates the transition to harder modern brick.
Evanston has the oldest residential brick stock on the North Shore, with median construction from 1939 and a significant share of homes dating to the 1890s and 1900s. The city documents prior Portland cement damage as a leading cause of brick failure across its residential stock - the repointing jobs from the 1970s and 1980s that used hard Portland cement mortar are now producing the visible spalling homeowners are calling about. The greystone two-flats and three-flats along Evanston’s major corridors are built with Indiana limestone facing on the front facade and soft Chicago common brick on the sides and rear - two materials in close proximity, each requiring different mortar formulations. A contractor who treats both surfaces with the same mix will damage one of them. For Evanston masonry work, this two-material problem is the defining challenge on greystone properties.
Winnetka’s housing stock, predominantly from the 1920s through the 1960s, uses soft Chicago common brick on the older homes that the city classifies as the primary brick type. Homes near Sheridan Road from the 1930s and 1940s show a consistent failure pattern: the gray Portland cement from a 1970s or 1980s repointing job looks intact. The brick around it is losing its face. The additional stress on Winnetka’s brick is direct Lake Michigan exposure - east-facing facades take wind-driven rain during northeast storms, lake-effect humidity, and sustained moisture loading that drives water deep into any joint that is not sealed correctly. On soft brick, repeated wetting and drying cycles are part of normal operation when the mortar system is correct. The same cycles with incorrect Portland cement mortar trap water inside the brick rather than allowing it to migrate through the mortar joint. For Winnetka masonry service, lake exposure is the amplifying factor for every mortar-compatibility problem on pre-war construction.
Why This Brick Demands Lime Mortar
The operating principle of a soft-brick wall is straightforward. The wall moves. Seasonal temperature changes cause expansion and contraction. Soil settlement occurs over decades. Moisture cycling causes dimensional changes in both brick and mortar. A wall system that does not accommodate this movement fractures.
The original lime-based mortars used with Chicago common brick were designed to be the weak link in the system. Lime mortar is flexible. It absorbs stress through micro-movement and gradual erosion at the joint face. When a wall shifts slightly, the mortar joint moves rather than the brick. Over time, the mortar erodes and requires replacement - this is the expected maintenance cycle. The brick is protected because it is harder than the mortar around it.
NPS Preservation Brief 2, authored by Robert C. Mack and John P. Speweik, establishes the technical basis for this principle: mortar must be softer than the masonry unit it joins. Applied to Chicago common brick, this means mortar in the Type O to Type N range (350 to 750 PSI minimum compressive strength per ASTM C270) is appropriate. BIA Technical Note 8 provides the full mortar selection framework for brickwork by exposure and structural application.
Portland cement mortars reverse this relationship. A Type S mortar with a minimum compressive strength of 1,800 PSI per ASTM C270 is harder than the brick it contacts in a soft Chicago common brick wall. When the wall moves, stress cannot travel through the mortar joint - the joint is too rigid. The stress transfers directly into the brick face. Water that enters the joint is blocked from migrating through the mortar by the impermeable Portland cement, so it concentrates inside the brick. Freeze-thaw cycling causes the trapped water to expand approximately 9 percent by volume and fracture the brick face.
For the broader context of how this failure sequence unfolds and what can be done at each stage, see what causes brick spalling and how to prevent it and lime vs. Portland cement mortar for tuckpointing.
The Bungalow Belt: Where Chicago Common Brick Is Most Concentrated
The Chicago bungalow belt runs through a broad arc of communities - Rogers Park, Skokie, Morton Grove, Niles, Lincolnwood, and through the close-in northwest suburbs. This corridor contains an estimated 80,000 bungalows built between 1910 and 1935, the peak production period for Chicago common brick in residential construction.
These are solid structures. One-and-a-half story brick homes built to last. The brick in them has survived over 90 years of Northern Illinois weather. The failures seen on these structures today are not material failures. They are system failures caused by subsequent incorrect repairs.
Skokie’s housing stock along the eastern border with Evanston uses soft common brick similar to the North Shore material - softer than the post-war hard brick used in most of Skokie’s later construction. Homes on Skokie’s eastern edge along Dempster and Oakton that received Portland cement repointing in the 1970s or 1980s show the characteristic spalling pattern: intact-looking mortar in the repointed joints, and crumbling or flaking brick faces at the joint edges. Road salt spray along these major corridors adds a second stressor: salt draws moisture into brick and accelerates freeze-thaw damage on foundation walls and front steps that sit close to the road. For Skokie masonry work, identifying the brick generation - soft near the Evanston border, harder post-war further west - is the first step before any mortar specification.
In Lincolnwood, the housing stock transitions between Chicago-style two-flats and early single-family homes from the 1940s through 1960s. The two-flat construction carries more vertical load, which adds a second variable to the mortar specification question. But the brick is largely the same soft material on pre-1945 structures, and the same Portland cement damage pattern appears. A block in Lincolnwood can have a 1942 two-flat next to a 1958 ranch - each requiring a completely different mortar approach. For Lincolnwood masonry service, this mixed construction profile means testing before specifying is not optional.
The Transition Year: Why 1920 Matters
Most historic masonry preservation standards reference 1920 as the approximate transition date between soft lime-mortar masonry and harder Portland cement masonry. Before 1920, residential construction throughout the Chicago metropolitan area used soft common brick with lime or lime-rich mortar. After 1920, industrial brick production scaled up, harder machine-pressed brick became increasingly available, and Portland cement mortar became standard.
This means any home built before 1920 should be assumed to have soft brick requiring lime-compatible mortar. Homes built between 1920 and 1940 require assessment - the transition was not instant, and local variation in brick sourcing means some construction from this period used soft common brick while other projects used transitional harder material. Homes built after 1940 are almost universally hard brick that tolerates Type S mortar.
The practical consequence: if you own a home built before 1920 in Evanston, the near North Shore, or the bungalow belt, any repointing work must use Type N or softer mortar unless a professional assessment of the actual brick hardness suggests otherwise. If your home was built between 1920 and 1940, get an assessment before any mortar work. Do not assume the later construction date means Portland cement is safe.
See type N vs. type S mortar for the full mortar selection framework, and brick types explained: common, face, pressed, and clinker for how to identify which material category your home falls into.
How Chicago Common Brick Fails: The Complete Sequence
Understanding the failure sequence helps you identify where your building is in it.
Stage one is joint erosion. Original lime mortar weathers back naturally, becoming recessed from the brick face. This is expected and eventually reaches the point where repointing is needed. The brick is unaffected. Joints at this stage feel sandy and crumble under finger pressure.
Stage two is water entry. When joints are recessed more than 1/4 inch, water enters during rain events. On soft brick with correct mortar, the water migrates through the mortar joint and evaporates. On soft brick with incorrect Portland cement mortar, water entering through any eroded section cannot exit through the hard impermeable mortar and concentrates inside the brick.
Stage three is brick face failure. Concentrated moisture inside the brick freezes, expands, and fractures the face. Fine crazing appears first. Then small flakes detach. Then concave depressions form where the face has fractured away. This stage is irreversible.
Stage four is interior water damage. Once face spalling has progressed enough, water enters the wall cavity, reaches interior surfaces, and causes damp walls, efflorescence on interior finishes, mold, and eventually structural damage to interior framing.
Most soft-brick buildings in the Chicago area that show up for assessment are in stage two or three. Stage one is the intervention point where tuckpointing with correct mortar stops the sequence. Stage three requires brick replacement in addition to mortar correction. Stage four requires masonry repair plus interior remediation.
Salvage Brick and Why Modern Brick Cannot Replace It
When Chicago common brick is damaged beyond repair and replacement is necessary, the replacement material must match the original. Modern brick cannot do this.
Modern brick is manufactured to different standards. It is harder, more dimensionally uniform, and produced in a narrower color range than soft common brick. The color and texture visible from the street would not match the original facade, and the compressive strength difference between a modern replacement brick and the surrounding original bricks creates a mechanical mismatch that generates new stress at the interface between old and new.
Salvage brick from demolition projects provides the only viable source for repair-quality matching material. Pre-1940 brick salvaged from Chicago-area demolitions carries the same clay composition, the same firing characteristics, and the same physical properties as the brick in the wall being repaired. After weathering for a season, a salvage brick repair on a soft-brick facade becomes nearly invisible.
For the sourcing and matching process in detail, see salvage brick: why old brick matters. For Evanston, which records salvage-only sourcing as necessary for its oldest soft common brick stock because modern brick is too hard and too uniform, this is a technical requirement, not a preference.
What the Right Repair Looks Like
Correct repointing of Chicago common brick requires four elements.
Mortar specification that matches the brick hardness. For most pre-1920 soft common brick, Type N (750 PSI minimum) or Type O (350 PSI minimum) is appropriate per ASTM C270. For very soft pre-1900 material in the oldest Evanston or bungalow stock, lime putty mortar may be the most compatible choice. The decision requires testing the brick and existing mortar hardness before specifying the replacement. BIA Technical Note 8 is the standard selection reference.
Joint preparation to a minimum depth of 3/4 inch per BIA Technical Note 7B. Shallow joint removal - less than 1/2 inch - is the most common cause of premature tuckpointing failure. The new mortar cannot bond correctly to a shallow ledge, and it will loosen and crack within a few winters.
Color and texture matching. Lime mortar comes in a wide range of formulations. The color of the replacement mortar should be matched to the weathered original visible in unaffected sections of the wall. A close match becomes invisible after a season. A poor match is permanent.
Brick replacement from salvage sources where needed. Any brick that has spalled past the point of structural integrity must be replaced. Adjacent bricks that remain solid but have the Portland cement mortar problem must have the mortar corrected even if the face is still intact, to prevent ongoing damage.
For the broader context of how Chicago bungalow-era homes require this approach throughout the bungalow belt, see Chicago bungalow masonry care for 1910 to 1935 brick.
Scheduling
Brick repair on Chicago common brick requires a contractor experienced with historic masonry systems, not a general tuckpointing crew applying standard Type S mortar to every job. The difference between the two approaches is the difference between a repair that lasts 30 years and a repair that causes accelerating brick damage within 5 to 10 years.
We work with lime-compatible mortar specifications on soft historic brick across the North Shore and northwest suburbs. We source salvage brick through established regional suppliers and test existing mortar and brick hardness before specifying any replacement material.
For homes in Evanston, Winnetka, Skokie, Lincolnwood, and the bungalow belt communities across the northwest suburbs, call (847) 713-1648 or contact us online. Since 1987, we have been the masonry contractor on soft-brick homes across Chicagoland’s North Shore and northwest suburbs.
Chicago common brick was not designed to live with Type S Portland mortar. The two materials in contact accelerate exactly the failure the repair was meant to prevent.