What's That Stain on Your Brick? Efflorescence, Algae, Mold, and Mortar Smear Told Apart

A stain appears on your brick and the instinct is to clean it. Sometimes that's right. Sometimes the stain is the only warning you're going to get that water is moving through the wall, and scrubbing it off just removes the evidence.

The good news is that brick stains are unusually easy to identify by color, texture, and pattern — and once you know what you're looking at, you know whether it's a cosmetic nuisance or a symptom to trace. Paul Lally's Masonry has been reading Chicagoland brick since 1988. Here's the field guide.

Quick identification table

What you see	Almost certainly	Serious?
White, powdery or crystalline, brushes off, dissolves when wet	Efflorescence — mineral salts	The deposit no; what it indicates, yes
Green, fuzzy or slimy, on shaded damp areas	Algae	Mostly cosmetic; fix the moisture
Black or very dark grey, patchy, on damp surfaces	Mold or mildew	Mostly cosmetic; fix the moisture
Crusty, grey-green, flat circular patches	Lichen	Slow-growing, mostly cosmetic
Orange-brown streaks running downward	Rust from corroding steel	Often yes — see below
Dull grey or white haze, uniform, won't brush off	Mortar smear	Cosmetic
Dark saturation patches that never dry	Persistent water entry	Yes — trace the source
White, on an interior basement wall	Efflorescence from below-grade moisture	Trace it

The white one: efflorescence

What it is: as water moves through brick and mortar it dissolves naturally occurring mineral salts. When that water reaches the surface and evaporates, the salt is left behind as a white powdery or crystalline bloom.

The key insight: efflorescence is not damage. It's a receipt. It proves water travelled through your masonry and came out the other side. The deposit brushes off easily and often dissolves when you wet it — that's the simplest field test to separate it from growth.

Why it matters in Chicagoland: the water making that salt is the same water that freezes in winter, expands roughly nine percent, and pushes the hard fired face off your brick. That's spalling, and it's permanent. Efflorescence is an early warning of the conditions that cause it.

Common sources: failed or receding mortar joints, a leaking gutter or downspout, splashback where soil or pavement sits too high against the wall, a cracked chimney crown, failed coping on a flat-roofed building, grade sloping toward the house, or below-grade moisture on a foundation wall.

What to do: don't just clean it — find where the water is getting in. Cleaning without tracing the source means it returns, usually in the same place. Our deep dive on what causes efflorescence and how a mason removes it covers the removal side in detail.

The green and black ones: algae, mold, and lichen

What they are: living growth, not mineral deposit. Algae is green, often slimy or fuzzy. Mold and mildew read black or very dark grey in patches. Lichen forms crusty, flat, grey-green circular patches and grows extremely slowly.

How to tell them from efflorescence: wet a finger and touch it. Efflorescence tends to dissolve or smear away as powder. Growth stays put, and it feels slimy or fuzzy rather than chalky. Growth also concentrates in shade and damp; efflorescence follows water paths through the wall, which produces different patterns.

Where they show up: north elevations that get little winter sun, walls under overhanging trees, near grade where splashback keeps brick wet, beside downspouts, behind shrubs planted tight to the house, and in shaded gangways between buildings.

Are they serious? Generally not structurally — this is a surface issue and a moisture-management one. But persistent growth is telling you a surface stays wet, and constantly damp masonry deteriorates faster in freeze-thaw. Treat the growth as a prompt to improve drainage, trim vegetation back off the wall, and check that gutters aren't overshooting.

Cleaning: gentle methods and appropriate cleaners. Not a pressure washer at concrete settings — see the warning below.

The orange one: rust staining

This is the stain to take most seriously.

Orange-brown streaks running downward on brick mean steel is corroding somewhere above. The usual culprits:

• A steel lintel above a window, door, or garage opening — by far the most common
• Wall ties in a brick-veneer wall
• Anchors, shelf angles, or embedded fixtures
• A metal railing, sign bracket, or fastener

Why it's different from the others: the corrosion producing the stain is simultaneously destroying the masonry. Rust occupies several times the volume of the steel it came from, so as a lintel corrodes it expands and lifts the brick sitting on it. That's rust jacking, and its signature is a horizontal crack running along the joint just above an opening, frequently with step cracks radiating from the top corners — and rust streaks bleeding down below.

If you see rust staining paired with a horizontal crack above a window or garage door, that's not a cleaning question. See failing lintels and cracked brick above windows.

In brick veneer, rust staining without a crack above an opening can indicate corroded wall ties — the metal anchors holding the veneer flat against the structure behind it. That's worth investigating, because failed ties are what allow a veneer wall to bow outward. Our guide to brick veneer vs. solid masonry explains the difference.

The hazy one: mortar smear

What it is: a dull grey or white film left on the brick face when mortar residue wasn't properly cleaned during original construction or a later repair. Unlike efflorescence it's uniform rather than following water paths, and it doesn't brush off.

Is it serious? No — it's cosmetic. But it's worth naming because it's frequently mistaken for efflorescence, and the two want completely different responses. Efflorescence means investigate. Smear means clean, carefully, when you feel like it.

One caveat: heavy smear on a recent repair is a workmanship signal. Mortar riding up onto brick faces is also a hallmark of skim pointing — smearing new mortar over failed joints instead of cutting them out. If the haze arrived with a tuckpointing job that finished suspiciously fast, look closer. See why masonry repairs fail.

The dark patches that never dry

Not really a stain — a saturation pattern. Areas of brick that stay visibly darker than the surrounding wall for days after rain, or never fully dry at all.

This means water is entering and staying. Look directly above the patch: a failed joint, a cracked sill, a gapped sealant line at a window head, a downspout discharging onto the wall, a chimney or parapet issue overhead. Persistent saturation is the shortest path to spalling.

The warning: how NOT to clean brick

Every one of these stains tempts people toward the pressure washer, and this is where real, permanent damage gets done.

Fired brick has a hard outer face and a much softer interior. That face is the brick's weather protection, and it is thin. High-pressure washing at the settings people use on concrete and driveways strips it off along with the stain. What's left is porous, absorbent material that drinks far more water than it did before — so it effloresces more, spalls faster, and deteriorates on a much shorter clock.

Sandblasting is worse, and on historic Chicago common brick, greystone, or carved limestone it's genuinely destructive — it erases tooling detail permanently and dooms the material.

The rule: use the gentlest method that actually works. Soft washing, appropriate cleaners matched to the stain and the substrate, dwell time and rinsing rather than brute force. On century-old masonry, gentler still.

And a related trap: don't seal to hide a stain. A film-forming coating over efflorescing brick traps the moisture that's causing it, and the next freeze pushes the face off. Only breathable penetrating sealers belong on Chicagoland masonry, and only after the joints are sound. See is masonry waterproofing worth it.

How a mason diagnoses a stained wall

1. Identify the deposit by color, texture, pattern, and how it responds to water.
2. Look above it. Water runs downhill. The source of a stain is essentially always higher than the stain — often several feet higher, and sometimes on a different part of the building entirely.
3. Check the usual entry points: mortar joints, sills, lintels, window heads and sealant, the chimney crown and cap, coping on flat roofs, and flashing.
4. Check the water management: gutters, downspout discharge, grade, splashback, snow storage, and vegetation against the wall.
5. Assess the brick itself — is it already spalling, or still sound?
6. Distinguish surface from structural. Growth and smear are surface. Efflorescence and rust point inward.
7. Sequence the fix: stop the water first, repair the masonry, clean last, seal only if warranted.

Chicagoland context

Our climate produces staining unusually reliably, and for interlocking reasons.

Freeze-thaw cycling — repeated hard cycles, sometimes several in a week — drives water into masonry and then expands it. Wet springs and lake-influenced moisture keep walls damp long enough for growth to establish. Wind-driven rain pushes water at walls horizontally, finding joints that a vertical rain never would. And de-icing salt at grade adds its own crystallization damage, which is why the bottom courses beside driveways and walks look worse than anything above them.

The housing stock matters too. The pre-1930s city stock — bungalows, greystones, two-flats, brick storefronts — is soft, locally-fired common brick with lime-based mortar, more porous and more prone to efflorescence than modern units, and far more easily ruined by aggressive cleaning. The postwar suburban stock is largely brick veneer, where staining patterns can point to blocked weep holes, failed flashing, or corroding ties rather than the brick face itself.

Maintenance that prevents most staining

• Keep gutters clear and downspouts discharging well away from the foundation
• Maintain grade sloping away; don't let soil or mulch pile against the brick
• Trim vegetation back off the walls, especially on north elevations
• Don't pile snow against foundation walls
• Repoint failing joints before they recede deeply — the cheap moment
• Check the chimney crown and cap, and coping on flat-roofed buildings
• Look at the wall each spring, after the freeze-thaw season

Our masonry maintenance checklist for Chicago sets out a full cadence.

Related services

Depending on what your stain turns out to be: tuckpointing and repointing, brick repair, brick replacement, lintel repair and replacement, chimney repair and rebuilds, parapet wall repair, limestone and sill repair, caulking and joint sealant, foundation masonry repair, masonry waterproofing, or residential masonry restoration.

The bottom line

White and powdery means water is moving through your wall — trace it. Orange streaks mean steel is corroding — take that one seriously, especially above a window. Green and black mean a surface stays damp — fix the drainage and the shade. A dull uniform haze is mortar smear and is merely cosmetic.

And whatever the stain, resist the pressure washer. Removing the stain is easy; putting the brick's face back is impossible.

Paul Lally's Masonry Inc. is a family-owned, insured masonry contractor serving Chicago and the Chicagoland suburbs since 1988 — tuckpointing, brick repair and replacement, chimney repair and rebuilds, lintel replacement, masonry restoration, and waterproofing for residential and commercial properties. Built on Craftsmanship. Backed by Experience.

Send us a photo or have us walk the wall — we'll tell you what you're looking at and whether it points to something inside the wall. Our family name is on every job.

Free on-site estimates — call (708) 448-8866 or request an estimate online.