Brick Grades Explained: Why SW vs MW Brick Decides Whether Your Wall Spalls

Brick Grades: The Short Version

Brick is graded for freeze-thaw exposure. SW — severe weathering — is the grade that belongs on an exterior wall in Chicagoland. MW is moderate weathering, for milder or sheltered exposure. NW is negligible weathering, essentially interior use. Spalling happens when a brick with too much absorption for its exposure freezes while saturated, and repairs fail when replacement units do not match the original's absorption and hardness. Emerald Masonry LLC matches brick and mortar to the wall on every repair across Chicago and the Chicagoland suburbs — family-owned, licensed and insured, 40+ years of experience, free on-site estimates. Call (708) 288-1696.

Most homeowners have never heard of brick grades, which is reasonable — until a mason replaces twenty spalled bricks with units that look right and the repair starts failing in year four.

Why Grades Exist at All

Brick is fired clay. How hot and how long it is fired determines how vitrified — how glassy and fused — the material becomes, and that in turn determines how much water it absorbs.

Absorption is the whole story. Water inside a brick expands roughly nine percent when it freezes, and it has nowhere to go except outward through the face. A brick that drinks water freely and then freezes will eventually shed its face in layers. A dense, low-absorption brick going through the same winter barely notices.

So the grades are really a statement about how much freeze-thaw a unit can take while wet.

| Grade | Full name | Absorption | Where it belongs | |---|---|---|---| | SW | Severe Weathering | Lowest | Exterior walls in freezing climates; anything in contact with saturated ground; the Chicagoland default | | MW | Moderate Weathering | Moderate | Exterior in milder climates, or sheltered exposure above grade | | NW | Negligible Weathering | Highest | Interior, or fully protected from weather |

Two practical notes. First, grade is about exposure, not quality — an NW brick is not a defective brick, it is a brick in the wrong place if it ends up outside. Second, the harshest exposure on any building is at grade and below, where splash, snow banking and road salt keep masonry saturated for months.

The Fired Face Is the Whole Defense

Everything above depends on one fragile thing: the fired outer skin.

During firing, the surface of a brick vitrifies — it becomes denser, harder and far less absorbent than the body underneath. That skin is the weather protection. It is thin, and it does not grow back.

This is why sandblasting and high-pressure washing effectively downgrade a brick. Strip the skin and you expose the porous interior. A wall that performed adequately for sixty years can start spalling within a couple of winters after aggressive cleaning. The brick did not change grade on paper; it changed grade in reality.

It is also why painting brick is so damaging. Paint traps moisture inside a unit that needs to dry outward, and the water then spalls the face off from behind.

Chicago's Grade Problem: Common Brick

Chicagoland has an enormous quantity of brick that was never rated for what it ends up doing.

Chicago common brick — the soft, salmon-and-buff colored brick made from local clay from roughly the 1870s to the 1930s — was fired at lower temperatures and is dramatically more porous than face brick. It was utility material, used for the inner wythes of solid walls and for the side walls, rear elevations, alley faces, coach houses and garages nobody expected to look at.

Then it ends up exposed. A neighboring building comes down and a party wall stands in the weather. A rear addition is removed. A coach house always faced the alley.

Highly absorbent brick + full weather exposure + dozens of freeze-thaw cycles per winter = rapid surface loss. That is the entire explanation for why the side of a Chicago bungalow crumbles while its face brick front looks fine. Same building, same age, same weather, completely different absorption.

Matching: Where Repairs Actually Go Wrong

Here is the part that matters when you are hiring someone.

A replacement brick has to match on four dimensions, and most crews think about two.

1. Size — brick dimensions have changed over the decades; an off-size unit throws the coursing.
2. Color and texture — the visible match.
3. Absorption — how much water it takes on relative to its neighbors.
4. Hardness — how it behaves under stress relative to the surrounding units and mortar.

Get 1 and 2 right and 3 and 4 wrong, and the repair looks perfect on day one. Then it behaves differently from the wall around it through every wet-freeze cycle. A dense modern unit set among soft old brick does not flex or absorb the same way, and the stress goes into the neighboring original units.

On pre-1940 masonry, salvaged brick is often the better answer than new — not for authenticity but for physics. Salvaged units from the same era share the original's softness and absorption. And Chicago common brick is simply not manufactured any more, so salvage is the only real match.

And the Mortar Has to Follow

Brick grade and mortar type are the same decision viewed from two sides.

The governing rule: mortar must be softer than the brick it holds. Mortar is the sacrificial layer — it absorbs movement and moisture stress and wears out first, because replacing mortar is cheap and replacing brick is not.

So the absorption and hardness of your brick sets the ceiling on the mortar:

• Soft, high-absorption historic brick → lime-rich or lime-modified mix. Standard Type S here will spall the faces within a few winters.
• Dense post-war face brick → Type N for most above-grade work, Type S where conditions genuinely warrant.
• Below grade and at foundations → Type S, sometimes Type M, because moisture and soil pressure change the requirement.

Put hard mortar against soft brick and you invert the wall's design: the joints become the strong element and the brick takes the freeze-thaw stress. That inversion is behind an enormous share of the spalled masonry in Chicago's older neighborhoods.

Exposure Zones: The Same Wall Has Several

Grade is a property of the brick. Exposure is a property of the location — and one building has wildly different exposure zones, which is why damage clusters instead of spreading evenly.

At and below grade is the most severe zone on any property. Splash-back, snow banking against the wall for weeks, and road salt carried in by spray keep the lowest courses saturated for months. This is where SW-grade material genuinely matters and where you see damage first on almost every Chicagoland building.

Chimneys are the second-harshest. Exposed on four sides, above the roofline, with no heated interior drying them from behind. Chimney brick works harder than any wall brick on the same house.

Parapets on flat-roofed buildings are weathered on both faces and carry the water leaving the roof edge — the same problem as a chimney, spread across a whole roofline.

Horizontal surfaces — rowlock sills, wall caps, coping, steps and stoops — hold water rather than shedding it. Brick used flat-on takes far more punishment than the same brick standing in a wall.

North and shaded elevations dry slowly, so they enter each freeze cycle wetter than the sunny side of the same building.

Sheltered elevations under deep overhangs are the mildest zone, which is why an otherwise identical wall can look untouched there.

The practical consequence: specify for the zone, not for the building. A unit that performs fine on a protected elevation can fail quickly if it is reused in a sill, a chimney or the bottom three courses.

Warning Signs Your Brick Is Losing the Fight

• Spalling — faces flaking, shelling, popping or crumbling, worst near grade and on shaded elevations.
• Delamination — the face separating in layers.
• A sandy or gritty surface, leaving residue on your hand.
• Deep pitting, where units have lost real depth.
• Efflorescence — white chalky bloom proving water is moving through.
• A band of damage matching where somebody repointed a few years ago.
• Rapid deterioration after cleaning — the aggressive-cleaning signature.
• Damage concentrated under a gutter, below a sill, or at splash height — the water path is telling you where it goes.

Repair Process

1. Assess. Determine wall type, era, brick absorption and hardness, and where water is entering.
2. Fix the water source first — gutters, downspouts, grade, sills, flashing, joints. Otherwise the new brick starts the same clock.
3. Remove failed units carefully, without disturbing sound surrounding brick.
4. Source matched replacements — salvaged for older walls, current-production where a true match exists.
5. Match the mortar for hardness, color and sand.
6. Set and repoint, packing joints in lifts and tooling to the original profile.
7. Protect appropriately — breathable products only, if anything at all.

What Drives Cost

• Number of units and whether damage is localized or spread across an elevation.
• Match difficulty — unusual color, discontinued size, or salvage sourcing.
• Height and access — ground level versus staging or a lift.
• Underlying cause — a rusting lintel or failed sill adds scope.
• Wall type — solid masonry, veneer and soft historic brick each work differently.
• Extent of accompanying joint work.

We do not quote brick repair sight unseen. Free on-site estimate, written scope, and an honest read on what needs doing now versus what can wait.

Chicagoland Context

Freeze-thaw frequency is the reason grade matters so much here. It is not extreme cold that destroys masonry but repeated crossings of the freezing point with water present — dozens each winter, plus more in the shoulder seasons.

Lake-effect moisture keeps walls wetter for longer, particularly north elevations that dry slowly. Road salt drives salt crystallization damage into the lowest courses, where the most absorbent masonry usually sits. And the age of the stock means a large share of the region's brick is soft, porous and a century old.

Maintenance

• Keep water off the brick — gutters clear, downspouts extended, grade sloping away, splash controlled.
• Repoint on schedule with a matched mix, before spalling starts.
• Repair sills, crowns, coping and flashing promptly; they soak everything below them.
• Never sandblast or high-pressure wash, and never paint absorbent brick.
• Use breathable sealers only, and only after joints are sound.
• Inspect annually, especially at grade, on north elevations, and under gutters.

Related Services

Brick repair, brick replacement, tuckpointing and repointing, historic masonry restoration, masonry sealing and limestone and sill repair.

The Bottom Line

Brick grade is a statement about how much wet freezing a unit can survive. In Chicagoland, exterior walls want SW, and a great deal of the region's older masonry — especially common brick on exposed elevations — is effectively operating below the grade its exposure demands.

You cannot re-fire the brick you have. What you can control is how wet it gets, whether its fired face stays intact, and whether the units and mortar going into a repair actually match what is already in the wall. Those three things decide whether a brick repair lasts thirty years or three.

Emerald Masonry LLC is a family-owned, licensed and insured masonry contractor serving Chicago and the Chicagoland suburbs with 40+ years of experience in tuckpointing, chimney repair, brick repair and replacement, lintel and parapet repair, foundation and limestone/sill repair, caulking, sealing, and commercial, residential and historic masonry restoration.

Free on-site estimates. Call (708) 288-1696 or request an estimate.