Rusted Steel Lintels: The Crack Pattern to Know
A horizontal crack at the head of a window or door, sitting in the bed joint directly above the opening, with stair-step cracks running diagonally off both upper corners, almost always means the steel lintel is corroding. Rust occupies several times the volume of the steel it came from, so the lintel swells and lifts the brickwork above it.
Most cracks in a brick wall are ambiguous. They could be settlement, thermal movement, a bad repair or something that stopped moving fifty years ago. One pattern is not ambiguous at all, and once you have seen it you will spot it from the sidewalk on half the blocks in the city.
What does the pattern look like?
Three features appear together above a window or a door.
A straight horizontal crack. It runs along a bed joint, usually the first or second joint above the opening, and it spans roughly the width of the window. It is level and it is straight, which is what separates it from settlement cracking.
Stair-step cracks off the upper corners. From each end of the horizontal crack, cracking runs diagonally up and outward, jumping along head joints and bed joints in a staircase. They usually rise at about 45 degrees and die out a few courses up.
Displacement you can feel. Run a hand along the wall at the crack. The brickwork above often sits proud of the brickwork below by an eighth of an inch or more. Sometimes the whole course above the opening is visibly lifted, or a brick end has been pushed out.
Add rust staining weeping down from the joint, or a visible steel angle at the head of the opening that has scaled and swollen, and you have all the evidence you need.
Why does rusting steel crack brickwork?
When steel corrodes, the iron oxide that forms takes up several times more space than the metal it replaced. Common figures put it at up to around ten times the original volume. The steel does not simply thin out. It expands.
A steel angle lintel sits in a slot with brick bearing on it at each end and brick resting on the outstanding leg across the opening. There is nowhere for that expansion to go except into the masonry. The horizontal joint directly above the steel is the weakest path, so it opens first and gives you the straight crack. The masonry over the opening acts like a rigid block being jacked upward, and the stair-step cracks off the corners are the wall tearing along the joints where that block separates from the wall around it.
The trade name for it is rust jacking, and it is self-feeding. The crack lets more water to the steel, the steel corrodes faster, the expansion grows.
Why is this so common on St. Louis buildings?
Two reasons.
Openings in 19th century St. Louis buildings were often spanned with brick arches or stone lintels, which do not rust. From roughly the 1900s onward, steel angles became the standard way to carry brickwork over a window, and a great deal of the early and mid 20th century stock here uses them. That steel is now anywhere from sixty to a hundred and twenty years old.
The second reason is that the steel was usually bare. Galvanizing became routine much later, and the industry guidance today recommends galvanized steel for harsh exposures for exactly this reason. Bare steel with a coat of shop primer, sitting behind a brick face with no flashing over it and no way for water to escape, is a corrosion experiment running on a twenty-four hour schedule.
Apartment blocks around the Delmar Loop in University City and the brick four-families across the city show this pattern constantly, because they have a lot of openings and a lot of steel.
How do you tell this apart from other cracks?
| Pattern | Likely cause | Direction of travel |
|---|---|---|
| Straight horizontal crack at the head of an opening, stair-steps off both corners | Rusting steel lintel | Brickwork above pushed up |
| Stair-step cracks only, running down toward a corner or a pier | Foundation settlement | Brickwork below dropping |
| Vertical crack at a building corner, widest at the top | Thermal movement or differential settlement | Wall pulling apart |
| Fine cracks tracking the mortar joints over a whole wall | Mortar shrinkage or a hard previous repoint | No structural movement |
| Horizontal crack low on a basement wall, wall bowing inward | Lateral soil pressure | Wall pushing in |
The tell is direction. A lintel problem lifts the masonry above the opening. Settlement drops the masonry below. If you can feel the course above the crack standing proud, you are looking at the lintel.
How urgent is it?
Not an emergency in most cases, and not something to leave for five years either.
The slow answer is that the wall above the opening is gradually losing its bearing. As the steel scales and swells, its effective section thins and the bricks it carries are being pushed out of plane. Left long enough you get loose brick over the window, a sagging head course, a window frame that racks out of square and starts sticking, and eventually brick falling.
Signs it needs attention sooner: brick you can move with a hand, a gap you can see daylight through, the lintel visibly deflected, or a window that has become hard to operate over one or two seasons.
What does the repair involve?
The steps are the same whether the opening is a basement window or a storefront.
- Shore the opening. Needles are set through the wall above the lintel and supported on props at both sides, so the masonry above is carried while the steel comes out. This step is the whole job. Skipping it drops the wall.
- Take down the masonry over the lintel. Enough courses are removed to release the steel and to expose sound bearing at each end. Sound brick is saved for reuse.
- Cut out the corroded steel. The old angle usually comes out in pieces.
- Clean and rebuild the bearing. The pockets at each end are cleaned back to solid masonry and rebuilt where needed. Adequate end bearing matters more than the size of the angle.
- Set new galvanized steel. Sized for the span and the load above it, with a proper bearing length at each end.
- Flash over the lintel and provide weeps. This is what keeps the next lintel from doing the same thing. Water that gets into the wall has to reach daylight above the steel.
- Rebuild the brickwork with salvaged or matched brick, then point the area with a mortar compatible with the wall.
- Strike the shoring only once the mortar has set.
Typical lintel replacement in our market runs $950 to $3,200 per opening, reviewed September 2026. Span, the amount of load above the opening and how much brick has to come down set the number.
In the City of St. Louis this is structural masonry repair, so a building permit applies, and your contractor should pull it.
What about just grinding and painting the steel?
It works only when you catch it very early and there is no cracking yet. Where the steel is lightly surface-rusted and the masonry has not moved, cleaning the exposed face back to bright metal, priming it properly and sealing the joint above buys real time.
Once the wall has cracked, the expansion has already happened inside the joint where no brush will reach. Painting the visible face at that point covers the evidence and changes nothing.
Getting it looked at
If you have a straight horizontal crack over a window with stair-steps running off the corners, get it assessed before the brick starts moving. We photograph the opening, check the bearing at each end and tell you whether it needs replacing now or can be watched for a season. The lintel replacement page covers how we support an opening and what the finished detail includes.