Steel lintel repair is one of the more consequential masonry jobs on Chicagoland’s older housing stock, and it is consistently underdiagnosed. Walk down any block in Morton Grove, Niles, or Wilmette where homes date to the 1920s and 1930s, and you can read the signs from the sidewalk: a horizontal crack in the mortar joint directly above the window opening, rust staining bleeding down the brick face, or a slight outward bow in the courses above the opening.
All three signs point to rust jacking - the expansion of corroding steel within a masonry assembly. Rust physically lifts and cracks the masonry above the opening. Given enough time, a failed lintel means the window frame distorts, the opening loses structural integrity, and the damage that started as a hairline crack becomes a rebuilding job.
Lintel corrosion leaves signatures readable years before the damage becomes severe. Knowing what to look for and when to act is the difference between a modest repair and a major reconstruction.
What a Lintel Does
A lintel is the horizontal structural member that spans the opening above a window or door. In a solid masonry wall, every opening creates a gap in the continuous brick assembly. The brick courses above that opening cannot bear on air. They need something to carry their weight across the span until the masonry on either side picks up the load again.
In residential masonry construction from the 1910s through the 1950s, the standard solution was steel angle iron. A steel angle, typically 3 to 5 inches on each leg, was placed at the bottom of the opening and the brick was laid on top of it. The angle’s horizontal leg carried the load of the masonry above; the vertical leg faced outward at the bottom of the opening, sometimes visible as a thin steel edge, sometimes covered by brick trim or concealed entirely.
Stone lintels were used on more expensive construction, particularly on houses in Lake Forest, Winnetka, and the older neighborhoods of Glencoe. A single piece of limestone or granite cut to span the opening carries the load through compression rather than through steel’s tensile strength. Stone lintels fail differently than steel lintels, and they typically fail later.
Brick arches were used on some construction from the same period, particularly on buildings with more decorative detailing. A true arch transfers load laterally to the masonry on either side of the opening and does not require a lintel in the traditional sense, though a concealed steel tie or secondary lintel sometimes backs them.
For a house built in 1925 in Morton Grove - where homes have a median age of 1962 meaning the older bungalow stock significantly predates that median - the lintel above every window and every exterior door is almost certainly steel. In 2025, that steel is 100 years old. A century of moisture cycling, of winter water sitting against the metal, of the paint or parging that originally protected the steel deteriorating and leaving the metal exposed: all of that adds up. Corrosion is not a question of whether it will happen. For most of this housing stock, it is a question of how far along it is.
How Steel Corrodes in a Masonry Wall
Steel in contact with moisture and oxygen rusts. That process is basic chemistry. What makes lintel corrosion destructive beyond simple material loss is the volumetric expansion of the corrosion products.
When iron oxidizes to form rust, the resulting iron oxide compounds occupy significantly more volume than the original iron. The expansion is not trivial: depending on the form of rust and the degree of oxidation, the corrosion products can occupy several times the volume of the original steel they replaced. In an open environment, rust falls away from the surface and the loss is the mass of the original metal. In a confined space, the expansion has nowhere to go.
A steel lintel inside a masonry wall is confined on three sides. The masonry above bears down on the steel. The brick on either side constrains lateral movement. The only direction with any give is upward, into the masonry above the lintel, and outward against the face of the wall.
Rust jacking is what happens when that expansion pushes against the masonry. The expanding steel lifts the first course of brick above the lintel. That lifting cracks the mortar joint at the underside of the course directly above the opening. As corrosion continues and expansion increases, the crack widens, courses above are pushed progressively upward, and eventually individual bricks crack as the force exceeds the brick’s tensile capacity. BIA Technical Note 3A documents the physical properties of brick units - compressive strength, absorption, and freeze-thaw durability - that determine how quickly this damage propagates once rust jacking begins.
The visual signature on the wall face is a horizontal crack in the mortar joint at the top of the window opening. Because the expansion is uneven across the length of the lintel and across the cross-section of the steel, the crack is often irregular: wider at the center of the span, tighter toward the ends. Rust staining below the opening comes from the same steel; water carries the iron oxide through the mortar joints and deposits it on the brick face.
Stone and Brick Arch Lintels
Stone lintels do not corrode. They fail through a different mechanism: fracture under load, weathering, and in some cases salt scaling if the stone has absorbed deicing salt from nearby roads or walkways.
A stone lintel on a North Shore estate from the 1920s may show surface scaling from freeze-thaw damage, particularly on limestone, which is naturally porous. Deep weathering along bedding planes can split the stone over time. Cracks through a stone lintel that cross the full cross-section are structural cracks; surface scaling is not. The distinction requires close inspection.
On Lake Forest and Wilmette properties with original limestone lintels and surrounds, the approach to repair is preservation-oriented: consolidation of loose material, joint repair where the lintel has separated from the adjacent masonry, and protective treatment where the stone is actively scaling. Replacement with matching stone is possible but expensive; matching the profile, color, and finish of a cut 1920s limestone lintel requires sourcing appropriate material and custom cutting.
Brick arches that have lost their mortar or spread slightly at the crown may have cracking at the haunches or at the crown itself. These are repairable but require removing and relaying the affected brick with proper arch geometry, not just filling the cracks with mortar.
For homes in the bungalow era, including the housing stock throughout Niles, Morton Grove, and Evanston, the far more common situation is steel lintels, and the assessment focuses on corrosion. The guide to Chicago Bungalow Masonry Care covers the broader context of maintaining these homes and the specific material compatibility issues that affect the era.
The Visible Signatures
Reading lintel condition from the outside requires knowing what to look for and where to look. The following signs, assessed from ground level with some attention to angle and light, tell you whether to investigate further.
Horizontal crack at the top of the opening. This is the primary rust jacking signature. The crack runs along the mortar joint at the bed between the lintel and the first course of brick above it. Its location, specifically at the joint directly above the opening rather than at some point higher in the wall, is the diagnostic indicator. A horizontal crack higher in the wall has a different cause.
Rust staining below the opening. Iron oxide carried by water through mortar joints deposits on the brick face. The staining is typically brown to orange, concentrated at the base of the opening and sometimes running vertically down the brick face below it. Light staining may be mistaken for general soiling. Heavier staining localized around openings rather than distributed across the wall points to a source inside the wall.
Outward bulge in courses above the opening. As the masonry above a corroding lintel is pushed upward, the courses above the opening may develop a slight bow visible from the side. Stand at the edge of the wall and look along the surface: a slight outward curve at the window locations is visible even at a distance. This is a later-stage sign, indicating corrosion is already causing meaningful displacement.
Cracked brick units above the opening. Steel expansion strong enough to crack the mortar joints eventually cracks the brick units themselves. Diagonal cracks from the corners of the opening upward, or stepped cracking from the horizontal crack at the top, indicate that the load has exceeded the mortar’s capacity to absorb the movement. See How to Read Cracks in a Brick Wall for how to distinguish lintel-driven cracks from settlement and other causes.
Visible steel at the base of the opening. On some houses, the leading edge of the steel angle is exposed at the base of the window or door opening. If you can see it, you can assess it directly: rust flaking from the surface, pitting, paint that has bubbled and lifted, or visible section loss all indicate active corrosion.
Why Chicagoland’s 1910s-1930s Housing Stock Is at the Failure Age
Steel lintels embedded in masonry walls have a service life that depends on the moisture environment they live in, the quality of protection they had originally, and the permeability of the masonry around them. In a dry wall with good flashing and sound mortar joints, a steel lintel can last a century and beyond. In a wall that has had deteriorating mortar joints for decades, or where water has been entering above the opening through a deteriorating window head flashing, the corrosion environment has been more aggressive.
Chicagoland’s housing stock from 1910 to 1935 is reaching a threshold. Many of these homes are now 90 to 110 years old. Original mortar has been eroded or replaced, sometimes with incompatible materials. Original flashing at window heads has failed in many cases. Original paint or masonry protection on the steel has long since deteriorated.
The result is that steel lintels in this age group that were marginal five years ago are failing now, and lintels that were in reasonable condition five years ago are entering the active corrosion phase. A survey of windows across a typical block of 1920s bungalows in Highland Park, Wilmette, or Evanston will show some lintels with no visible symptoms, some with early cracking above the opening, some with visible rust staining, and occasionally one with significant displacement that has been developing for years without intervention.
Evanston’s documented masonry problem includes prior Portland cement repairs causing spalling on soft brick. That applies directly here: when mortar above a lintel gets repointed with Portland cement on pre-1920 soft Chicago common brick, the harder mortar traps moisture against the brick and accelerates both spalling and the lintel corrosion it was meant to stop. The repair compounds the original damage. NPS Preservation Brief 2 specifies that pre-1920 masonry should be repaired with lime or lime-rich mortar - softer than the brick it joins, not harder.
The inspection and assessment work we do on Chicago bungalows routinely includes lintel condition as a standard item. It is not possible to accurately assess a 100-year-old masonry home without looking at the openings.
Deerfield: Where Steel Lintel Rust Is the Documented Primary Problem
Deerfield deserves specific attention because window and door lintel rust causing brick displacement is the village’s documented second masonry problem, not an incidental finding. The housing stock that drives it is precise: colonials from the 1960s and 1970s where steel lintels have been in place for 50 to 60 years.
Deerfield’s median home age is 1970. Steel lintels above windows and exterior doors corrode after decades of moisture contact. Rusting steel expands, pushing the brick above it outward. The horizontal crack appears at the mortar joint directly above the opening. On Deerfield homes built between 1965 and 1975, this is the expected outcome for lintels that received no protective maintenance through 50-plus Illinois winters.
On a 1975 colonial near Deerfield Road, we replaced rusted steel lintels above three windows, reset the displaced brick courses above each opening, and tuckpointed the surrounding facade. From the street, the horizontal cracks had looked like settling. Up close, the brick above each opening had bowed outward by nearly 3/8 inch and the lintel steel had lost significant section. The brick reset alone was not the repair; the lintel replacement was.
Lintel replacement plus brick reset on a Deerfield colonial of this vintage runs $2,000 to $5,000 in the Chicagoland market, depending on how many openings are affected and how many brick courses above each have displaced. Deferring raises cost because bowed courses become harder to reset cleanly and additional brick may crack under sustained rust jacking pressure.
The builder-grade mortar failure that affects most Deerfield homes of this age often develops at the same time as lintel corrosion, so scheduling both in a combined inspection is the efficient approach.
How to Inspect Your Lintels
A lintel inspection is visual and takes less than an hour. Walk the exterior perimeter and photograph every window and exterior door opening. For each opening, note: the horizontal joint above the opening, any staining on the face below, any visible bulge when viewed from the side, and any exposed steel at the base of the opening. The best time is late spring after winter weathering has made new cracks visible, or early fall before the freeze-thaw season.
Categorize findings into three groups: no visible symptoms; early symptoms (hairline crack or light staining); and advanced symptoms (crack wider than 1/8 inch, significant staining, bulge, or exposed corroding steel). For advanced symptoms, removing one or two brick courses to inspect the steel cross-section is necessary. For early symptoms, mark the crack ends with pencil and a date, then check again in one season. For no visible symptoms on a pre-1950 home, schedule a return inspection in two to three years.
Repair Options
Repair approach depends on the extent of corrosion and the condition of the masonry above the opening.
Rust Treatment and Repointing
For early-stage corrosion where the steel has not lost significant section and the masonry above has not displaced, rust treatment may be appropriate. This involves removing brick courses above the lintel to expose the steel, mechanically removing loose rust, treating the cleaned steel with a rust-inhibiting primer, and rebuilding the brick above with mortar matched to the original - Type N at 750 PSI minimum per ASTM C270 for above-grade residential brick, or a lime-rich mix on pre-1920 soft brick where BIA Technical Note 8 and NPS Preservation Brief 2 require mortar softer than the brick it joins.
This is a maintenance repair. It arrests active corrosion and restores the mortar joints that have cracked. It does not add structural capacity to a lintel that has already lost section. For properties with soft historic brick, see Chicago Bungalow Masonry Care for the mortar compatibility framework.
Lintel Replacement
When a steel lintel has lost meaningful cross-section to corrosion, or when the masonry above has displaced, the lintel should be replaced rather than treated. The replacement sequence involves removing the brick courses above the opening to free the lintel, carefully shoring the masonry above the opening to prevent collapse during removal, extracting the old lintel, cleaning the bearing surfaces, setting the new lintel, and rebuilding the courses above it.
New lintel steel is typically specified to match the original bearing length and cross-section, with surface protection appropriate to the exposure conditions. This is structural work in the sense that it restores the original load-carrying capacity of the assembly.
The rebuilding of courses above the opening requires matching the existing brick as closely as possible. In the older housing stock across the North Shore and northwest suburbs, exact brick matches are often not achievable from current stock. A skilled mason working with salvaged brick from the same era can produce results that read as consistent from street level.
Rebuilding Courses Above the Opening
In cases where lintel corrosion has caused significant cracking or displacement of the brick above the opening, the repair scope extends to rebuilding the damaged courses. Displaced brick units may have fractured faces from rust jacking pressure; those bricks need replacement, not just repointing. The extent of rebuilding depends on how many courses above the opening have been affected.
This scope falls under brick repair work and can overlap with masonry repair if the damage is extensive. Full-scope lintel work on multiple openings is worth planning during a season when mortar can cure properly. Mortar curing in Illinois is temperature-dependent. Spring and early fall are the optimal windows. Avoid scheduling major masonry work in weather that will drop below freezing during the first 72 hours after mortar is applied.
Pair fall lintel inspection with the Fall Masonry Inspection Checklist for Illinois to identify what needs repair before winter. For context on the broader crack vocabulary on your home, see stair-step cracks in brick walls and why bricks crack.
Scheduling Lintel Repair
If you are seeing horizontal cracks above windows, rust staining, or bulging brick on your Chicagoland home in Evanston, Highland Park, Wilmette, Deerfield, or the surrounding communities, the earlier you have it assessed the better the repair options.
Delta - Masonry and Tuckpointing has been working with the older masonry housing stock in this region since 1987. We assess lintel condition as part of our standard masonry inspection, handle brick repair and masonry repair scopes from repointing through course rebuilding, and match historic brick and mortar specifications for pre-1940 construction.
Call (847) 713-1648 or contact us online to schedule a free on-site assessment. We work across Niles, Morton Grove, and throughout Chicagoland’s North Shore and northwest suburbs.
Corroding steel does not just weaken the lintel. It physically lifts the masonry above it as the rust expands. That is why you see a horizontal crack exactly at the top of the window opening.