Foundation Spalling:
What Flaking Concrete on Stem Walls Actually Means

What Spalling Actually Is

Spalling is the flaking, scaling, or chipping of concrete from the surface of a foundation wall. The material that breaks away is not the full thickness of the concrete, it is the outer layer, anywhere from a few millimeters to an inch or more deep, separating from the substrate beneath. What remains is a pitted, rough, or cratered surface where smooth formed concrete used to be.

In a crawlspace or basement, spalling appears on the interior face of the concrete stem walls, the vertical walls that sit on top of the concrete footing and support the structure above. Spalling on these surfaces is visible as areas where the concrete surface has broken away in irregular pieces, often with sharp edges, leaving a rougher texture beneath. In more advanced cases, you can see the aggregate (the gravel or rock inside the concrete mix) exposed at the surface where the cement paste has broken away around it.

Spalling is not the same as cracking. Cracks are linear separations through the concrete. Spalling is surface deterioration, the concrete is breaking apart from the outside in, rather than splitting along a plane. Both can appear on the same wall, but they represent different conditions and require different evaluations.

What Causes It

Spalling does not happen randomly. It is always the result of a process, usually moisture-related, working on the concrete over time. Understanding the cause is important because the cause determines whether the spalling is likely to continue and what the correct repair approach is.

Freeze-Thaw Cycling

This is a common cause in the Portland metro and SW Washington. Concrete is porous, it contains a network of tiny capillaries that absorb water from the surrounding soil and from moisture in the air. When that absorbed water freezes, it expands by approximately nine percent. That expansion puts internal pressure on the concrete matrix surrounding each frozen water-filled pore. Over time, repeated freeze-thaw cycles fracture the concrete from within, and the surface layer begins to flake away.

Portland and SW Washington do not have severe winters by national standards, but most winters bring nights below freezing followed by days above it. Each of those cycles is a stress event on concrete that is already saturated. Concrete that was mixed or cured with any deficiency, too much water in the mix, curing interrupted by rain, finishing work done too early, is more susceptible to this mechanism.

Carbonation

Over decades, carbon dioxide from the air and from soil gases reacts with the calcium hydroxide in the cement paste. This carbonation process gradually reduces the alkalinity of the concrete, which in turn reduces the passive protection that alkaline concrete normally provides to any embedded steel reinforcement. While carbonation itself does not cause immediate spalling, it changes the concrete’s chemistry in ways that make it more vulnerable to other deterioration mechanisms. In older concrete foundations, those from the 1920s through the 1950s, carbonation of the outer layer is common and is part of why surface spalling can progress more readily in these walls than in newer concrete.

Reinforcing Steel Corrosion

When spalling progresses deep enough to expose the steel reinforcing bar (rebar) inside the concrete, the exposed steel begins to corrode. Corroded steel expands, iron oxide (rust) occupies a larger volume than the original steel. That expansion puts pressure on the surrounding concrete from inside, cracking and spalling the concrete away from the rebar in a pattern that is distinctive: long, straight cracks running parallel to the embedded rebar, with spalling on either side. This is the most serious form of spalling because it becomes self-accelerating, the more concrete spalls away, the more steel is exposed, the more the steel corrodes, the more concrete spalls.

If rebar is visible in any area of spalling on a foundation stem wall, a structural engineer or qualified foundation contractor evaluation is warranted before closing. This is not a condition to simply patch and move on from without understanding whether the structural integrity of the wall section has been compromised.

Original Concrete Mix and Curing Deficiencies

Concrete placed with too high a water-to-cement ratio, which increases workability but reduces strength and durability, is more porous and more susceptible to freeze-thaw damage. Concrete that was not properly cured (kept moist long enough after placement to allow full hydration of the cement) develops more surface microcracking. Both conditions reduce the concrete’s resistance to the moisture ingress that drives spalling. In homes built before modern quality control standards were common, these mix and curing deficiencies are more likely, which is part of why older foundations show spalling more frequently than newer ones.

How Serious Is It, Reading the Spectrum

Foundation spalling covers a wide range of severity. The most important thing a buyer can do after receiving this finding is understand where on that spectrum the specific condition falls. The inspection report documents what was observed; a concrete contractor or structural engineer evaluation quantifies the severity and recommends the appropriate response.

Spalling Severity Spectrum, What Each Level Means

Surface Scaling

Thin flaking of the outermost concrete layer, typically 1/8 to 1/4 inch depth. Aggregate not exposed. Wall thickness and structural integrity not meaningfully affected. Correct with surface preparation and repair mortar, followed by penetrating sealer. Common finding on older Portland-area foundations.

Moderate Pitting

Concrete surface deeply pitted, aggregate exposed in affected areas, some loose material. Depth of 1/4 to 3/4 inch. Structural concern is low if no rebar is visible and the wall section is otherwise intact. Requires more thorough surface preparation and patch depth. Moisture source contributing to the condition should be identified and managed.

Deep Spalling with Exposed Rebar

Spalling has progressed to or through the rebar depth. Steel is visible, corroded, or showing rust staining at the wall face. This is the most serious condition, structural engineer evaluation warranted before proceeding. Repair involves treating exposed steel and re-establishing concrete cover, not simply patching the surface.

What makes this evaluation require a professional rather than a visual impression from a showing is that the depth of spalling is not always visible without probing the surface. A wall that looks moderately pitted from a distance may have significantly compromised depth in specific locations that are only apparent under close inspection with a proper tool. Conversely, a wall that looks concerning from a photograph may turn out to have only superficial surface scaling when a contractor evaluates it hands-on.

What Russ documents on inspection is what is visible and observable from the accessible crawlspace or basement: the location, approximate extent, and apparent depth of the spalling, along with any visible rebar, active crumbling, or related conditions such as water staining or efflorescence at the spalled areas. The professional evaluation translates that documented condition into a scope and cost.

What Correction Looks Like

Repairing spalled concrete is well-understood work that qualified concrete contractors perform regularly. The process differs based on severity, but the general sequence is the same regardless of depth.

Remove All Loose Material

Before any repair material can bond, every loose, delaminated, or friable piece of existing concrete must be removed. This is typically done with a chipping hammer or angle grinder with a grinding wheel, working back to sound concrete. The temptation to simply apply repair mortar over a loose surface is exactly what produces failed repairs, the new material bonds to the loose layer, which then fails with the repair on top of it.

Address Any Exposed Steel

If rebar is visible after removing loose material, the steel must be cleaned of rust with a wire brush or mechanical grinder to bright metal, then treated with a rust-inhibiting coating before patching. Patching over corroded steel without treating it leaves an active corrosion process inside the repair, which will eventually crack and spall the new material from within, the same cycle that created the original problem.

Apply Bonding Agent

A concrete bonding agent, typically an acrylic or latex-modified primer, is applied to the prepared substrate before placing repair mortar. The bonding agent chemically bridges the interface between old and new concrete, significantly improving adhesion and reducing the risk of the repair delaminating over time. Skipping this step is a common shortcut in low-quality repairs and is a major reason why patch repairs fail prematurely.

Apply Repair Mortar

Polymer-modified concrete repair mortars are the appropriate material for foundation spalling repairs. These pre-mixed products are formulated to bond to existing concrete, resist shrinkage cracking, and perform in moist environments. They are worked into the prepared area in appropriate lift thicknesses and finished to match the surrounding wall face. Standard Portland cement mixed on site is not the right repair material for this application, it lacks the adhesion and flexibility of purpose-formulated repair mortars.

Allow Full Cure, Then Seal

After the repair mortar has cured, typically a minimum of 28 days for full strength, though initial hardening happens much sooner, applying a penetrating concrete sealer to the stem wall surface reduces future moisture absorption. Silane or siloxane-based penetrating sealers absorb into the concrete matrix and reduce moisture ingress without forming a surface film that can peel. This is an important final step, particularly in the Pacific Northwest climate where the stem walls will be exposed to sustained moisture every wet season.

What Repair Does Not Fix

Surface repair addresses the symptom but does not eliminate the cause. If the spalling is being driven by sustained moisture contact from poor exterior drainage, inadequate grading, failed downspout extensions, or a perimeter drain tile system that has stopped functioning, then repairing the spalled surface without addressing the moisture source means the same process will continue working on the concrete behind the repair.

A complete approach to foundation spalling on an older Portland-area home often involves both surface repair and drainage improvement: correcting the grading adjacent to the foundation, extending downspouts, and in some cases evaluating the condition of the perimeter drain tile. The repair mortar stops the current damage. The drainage work slows the mechanism driving future damage.

What a Home Inspector Can and Cannot Determine

Russ evaluates the accessible foundation surfaces from the crawlspace interior and the basement. What he looks for: the location and approximate extent of spalling, the depth of deterioration (surface scaling versus deep pitting), any visible rebar, the presence of moisture staining or efflorescence at or near the spalled areas, and any related cracking patterns that suggest movement rather than surface deterioration alone.

What a home inspector cannot determine from a visual assessment: the precise depth of deterioration across the full wall section, the structural integrity of the wall at the affected locations, and whether the spalling is active and progressing or has stabilized. These determinations require hands-on probing, coring, or structural engineering assessment, tools and expertise that go beyond the scope of a general home inspection.

When spalling is documented in a report, the next step is a qualified concrete contractor evaluation. This is not the same as getting a repair quote, it is an evaluation of the actual condition, which then produces a scope and a quote. A contractor who skips the evaluation and goes straight to quoting a repair has not assessed what they are actually repairing. Ask specifically: what is the depth of deterioration, is there exposed or corroding rebar, and is the condition active or stable?

What This Means Before Closing

Foundation spalling is a common finding on older Portland-area homes. The finding calls for a professional evaluation and a real cost number so you understand what you are dealing with. A qualified concrete contractor can assess the specific condition, determine whether the spalling is active or stable, and provide a written scope of work and estimate.

The severity matters for understanding cost. Light surface scaling on a 1940s foundation with no visible rebar and no active moisture intrusion is a maintenance-level condition. Deep spalling with visible rebar or active moisture involvement is a more significant finding that warrants a structural contractor evaluation before any repair work is scoped. Understanding the difference helps you ask the right questions and make an informed decision.

One thing worth flagging specifically: a foundation stem wall that has been recently parged, meaning a thin coat of fresh concrete or stucco has been applied to the surface, can conceal spalling that was there before the parging. Fresh parging on an older foundation is not automatically suspicious, but it is worth noting. Parging applied over active spalling without proper preparation is a temporary cosmetic cover, not a repair. If you see fresh parging in a home where the foundation age suggests spalling is likely, ask your inspector what, if anything, was visible at the edges or any areas where the parging is incomplete.

For more context on related foundation conditions, see the existing post on foundation cracks in Portland homes, which covers the crack patterns that are distinct from spalling and how to evaluate their significance. If the inspection report documents both spalling and cracking, the two findings together give a fuller picture of the foundation’s condition than either finding alone.