Concrete homes in the Florida Keys develop problems differently than homes anywhere else in the country. The combination of salt air, humidity, and older construction stock means that what looks like a cosmetic issue on the exterior wall of a CBS home is often something that started inside the concrete months or years before it became visible.
Spalling is the most common of those problems. It affects a significant portion of the older concrete construction in Monroe County, and it is one of the most misread conditions homeowners encounter. People see rust stains and assume something spilled. They see cracks and assume normal settling. They see a chunk of concrete missing and assume it got knocked off.
None of those assumptions are usually correct. The signs of spalling have a specific pattern, and once you know what to look for, they are readable. This guide walks you through each of them so you can assess your own home before you call anyone — and understand what you are looking at when you do.
What You Are Actually Looking At
Before getting into the specific signs, it helps to understand what is happening inside the wall when spalling is occurring. The first post on this site covered the mechanics in detail, but the short version matters here: concrete homes built before roughly 1990 in the Keys have steel rebar running through them. Salt air carries chloride ions that migrate through the porous concrete over time. When enough chloride accumulates at the rebar surface, the steel begins to corrode.
Rust expands. When the rebar inside a concrete wall or column rusts, it takes up more volume than the original steel did. That expansion puts pressure on the surrounding concrete from the inside out. The concrete cracks, delaminates, and eventually breaks away. What you see on the exterior of the wall is the surface expression of that internal process.
This matters for reading the signs because the visible damage is always downstream of the actual problem. By the time you can see something on the outside of your wall, the corrosion has been running inside for a while. The earlier you catch the surface signs, the smaller and less expensive the repair.
Sign 1: Rust Stains Running Down Exterior Walls
Rust stains are usually the first thing homeowners notice, and they are almost always misread as something external. A stain that originates at a crack or a small surface fracture and runs downward in a brown or orange streak is not coming from outside the wall. That color is oxidized iron from the rebar inside bleeding outward through the concrete.
The stain itself does not indicate structural failure. It does indicate active corrosion. The rebar is rusting, and the byproduct is migrating to the surface. The location of the stain tells you approximately where the corrosion is occurring inside the wall.
On stucco-covered homes, which describes most CBS construction in the Keys, rust stains sometimes appear on the stucco surface without any visible cracking underneath. The stain is finding its way through the stucco before the substrate has cracked enough to show. That is actually an earlier-stage sign than cracks, and it is worth noting.
Look for rust stains:
- Running vertically down exterior walls, often starting at a hairline crack
- Around the base of columns or support posts
- On the underside of soffits or balcony edges
- Near window and door frames, where concrete tie beams are often located
A single faint stain in an isolated location is worth monitoring. Multiple stains across different walls or structural elements suggest the corrosion process is active across a wider area.
Sign 2: Cracks That Follow a Pattern
Not every crack in a concrete or stucco wall is spalling. Settlement cracks, thermal expansion cracks, and cracks from minor impact are common and usually not related to rebar corrosion. The difference lies in the pattern.
Spalling-related cracks follow the rebar. Since rebar runs horizontally and vertically through walls and columns in a grid pattern, the cracks that form as it expands tend to run parallel to those lines. You will often see long horizontal cracks running along a wall at consistent heights, or vertical cracks running the length of a column. The cracks may be spaced in a way that reflects the spacing of the rebar beneath.
Settlement cracks tend to be diagonal, random in direction, and concentrated at corners or transitions between materials. They do not follow a linear grid pattern.
Cracks that are associated with spalling also tend to widen over time. If you marked the ends of a crack six months ago and it has grown, or if a crack that was hairline-thin is now wide enough to see a gap, the underlying process is still active. Active cracking means active corrosion.
What to look for:
- Horizontal cracks running along the length of exterior walls, particularly near the top or bottom of a wall section
- Vertical cracks running down columns or support posts
- Cracks that appear in parallel pairs running the same direction
- Any crack with rust staining at or near it
- Cracks that have changed since you last noticed them
A network of cracks in a grid-like pattern on a concrete surface, particularly combined with rust staining, is a reliable indicator of rebar-driven spalling rather than ordinary concrete cracking.
Sign 3: Bubbling, Blistering, or Raised Areas on the Surface
This sign gets missed more often than any other because it does not look dramatic. Before concrete chunks break away from a surface, the bond between the concrete and the outer layer typically begins to fail. The surface lifts slightly, creating a raised area or a blister that you can see or feel when you run your hand along the wall.
On stucco-finished homes, this often presents as a slight puffing or bubbling of the stucco surface, sometimes with a fine network of cracks radiating outward from the center of the raised area. The stucco has separated from the substrate beneath it. The substrate itself may have already delaminated from the rebar zone.
This is actually a useful stage to catch the problem. The surface has not yet broken away, which means the repair involves less removal and restoration work than if chunks are already missing. Once you know what a delaminating surface looks and feels like, you can identify it with a slow walk-around of your exterior walls.
Run your hand along columns, the edges of walls, soffits, and balcony faces. Any area that feels raised, soft, or spongy compared to the surrounding surface is worth pressing on lightly with your thumb. If it flexes or sounds hollow when you tap it, the concrete has delaminated underneath.
Sign 4: Concrete Chunks Breaking Away or Already Missing
This is the stage most homeowners recognize as spalling. A section of concrete or stucco has broken off entirely, leaving a rough depression in the wall surface. In more advanced cases, you can see the rebar itself, dark with rust, running through the cavity where the concrete used to be.
This stage is not necessarily a structural emergency in every case, but it is past the point where monitoring is appropriate. Once concrete has delaminated and broken away, the rebar is no longer protected by the alkaline environment of the surrounding concrete. It corrodes faster. Adjacent sections are now more vulnerable because moisture and chlorides have a direct path deeper into the wall.
The location of the missing concrete matters. A chunk gone from the middle of a flat wall section is a different situation than deterioration at the base of a load-bearing column or at a tie beam above a window opening. Structural elements that carry load deserve more urgency than flat wall sections that do not.
What to document when you find missing concrete:
- Where on the structure it is located
- Approximate size of the affected area
- Whether rebar is visible and what condition it appears to be in
- Whether the surrounding area shows cracks or delamination signs
Photographs are useful here. A licensed contractor doing an assessment will want to see the scope and distribution of the damage, and photos help establish that before the site visit.
Sign 5: The Hollow Sound Test
This is a simple diagnostic that does not require any tools or expertise, and it covers ground that visual inspection alone can miss. Concrete that is sound and intact produces a solid, dense sound when tapped. Concrete that has delaminated internally produces a hollow, higher-pitched sound.
Walk the perimeter of your home with your knuckles or a small rubber mallet and tap the exterior walls methodically — columns, wall sections, soffits, balcony edges, the areas around window and door frames. You are listening for the shift from solid to hollow.
A hollow section does not always mean spalling in the rebar corrosion sense. It could indicate a void in the original construction, a section where stucco was applied over a gap, or other conditions. But hollow-sounding areas in the locations where spalling typically occurs, particularly when combined with any of the other signs on this list, are worth flagging for a closer look.
The underside of soffits and balconies is especially worth doing this on. These surfaces are exposed to salt air from below as well as above, and delamination there often develops before it becomes visible on vertical wall faces.
Where to Look First on a Keys Home
Not all surfaces on a concrete home are equally at risk. Salt air exposure, geometry that traps moisture, thin concrete cover, and structural loading all influence where spalling shows up first. These are the locations to check carefully before moving on to the rest of the exterior.
Soffits and the underside of overhangs. The concrete is thinner here, the geometry traps moisture and salt air against the surface, and the underside faces outward in all directions. Spalling under soffits often develops before it appears on vertical wall faces.
Columns and support posts. These are load-bearing members. Corrosion in a column is a structural concern in a way that corrosion in a flat wall panel may not be. Check columns at the base, around the mid-section, and at the cap where they meet a beam or slab above.
Tie beams above and below windows and doors. Tie beams are concrete structural elements that run above and below window and door openings. They contain rebar, they are sometimes thinner than wall sections, and the transition between the beam and the surrounding wall is a place where moisture can concentrate.
Balcony edges and railings. The edges of concrete balconies and the bases of railing posts set into concrete are exposed on multiple sides and often show signs early.
Walls facing prevailing wind. In the Lower Keys, prevailing winds typically come from the east and southeast. Windward-facing walls receive more direct salt air exposure than leeward faces. If your home has a wall that takes the weather, start there.
Areas near ground level. Salt spray from rain and wind can concentrate near grade, and the ground-to-wall transition is a place where moisture stays longer.
What to Do With What You Find
A walkthrough of your own exterior is a starting point, not a professional assessment. What it gives you is a sense of whether something is there, roughly where it is, and how much of the exterior shows symptoms. That is genuinely useful information before you call a contractor.
If you found nothing: a concrete home in the Keys built before 1990 benefits from a periodic professional inspection regardless. The corrosion process runs ahead of the visible symptoms. An inspection gives you a baseline.
If you found rust stains or hairline cracks in one location: document it with photos, note the date, and check it again in a few months. A single isolated sign warrants monitoring and a professional look, but it is not necessarily urgent.
If you found rust stains, cracks following a linear pattern, and hollow-sounding areas in multiple locations: this is past the monitor-and-wait stage. The indicators together point to active, distributed corrosion. Getting a licensed contractor out for an assessment gives you an accurate picture of the scope and what repair actually involves.
If you found missing concrete with exposed rebar: call a contractor. The damage is past the early stage, the exposed rebar is corroding faster now, and adjacent sections are more vulnerable. An assessment will tell you how far it extends beyond what you can see.
Document everything before the assessment: photographs of each sign, approximate locations on the home, and your observations from the hollow sound test. The more specific you can be, the more useful the contractor’s site visit will be.
Frequently Asked Questions About Spalling Signs in the Florida Keys
Is rust staining always a sign of spalling?
Rust staining on a concrete or stucco exterior almost always indicates rebar corrosion inside the wall. There are occasional exceptions, such as rust from an external metal fixture running down the wall surface, but when the staining originates at a crack or appears to bleed through the wall material itself, it is coming from the rebar. If you are not sure, a contractor can tell you in a site visit.
Can I have spalling without any visible signs yet?
Yes. The corrosion process runs inside the concrete before it produces surface symptoms. Older homes in the Keys, particularly those built in the 1950s through 1970s, can have active rebar corrosion that has not yet produced visible cracking or staining. A professional inspection that includes a hammer tap survey of the exterior can identify delaminated areas that are not yet visible to the eye.
My home has stucco over concrete block. Does that change what I’m looking for?
The signs are the same, but the stucco layer can mask early symptoms. Rust staining and cracking will work their way through the stucco eventually, but the stucco may show bubbling or delamination as an earlier indicator before the underlying concrete substrate shows visible damage. Run your hand along stucco surfaces and feel for raised or soft areas in addition to looking for stains and cracks.
How serious is spalling on a non-structural wall versus a column?
The underlying corrosion process is the same in both cases. The urgency differs. A flat exterior wall panel that is not carrying structural load can have moderate spalling without creating an immediate safety concern. A column, tie beam, or other load-bearing element with active corrosion warrants faster attention because the structural capacity of the member is directly affected as the rebar corrodes and concrete delaminates around it.
I found spalling signs on my neighbor’s property — can it spread to mine?
Spalling does not spread between properties. Each structure’s corrosion timeline depends on its own construction, age, and exposure. What your neighbor’s home tells you is that the conditions in your area are aggressive enough to drive spalling, which is worth knowing for your own home’s maintenance planning.
What does a professional spalling assessment involve?
A licensed contractor will do a visual inspection of the exterior, a hammer tap survey to identify hollow or delaminated areas, and an evaluation of any visible damage. For more extensive suspected damage, an engineer may be involved. The assessment gives you a scope of what is present, which areas are structurally significant, and what a repair would involve. In Monroe County, any repair work will require a permit, and a licensed contractor handles that process.