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What's That Stain? The Exterior Stain Identifier for Northern Homes

By the Up North Pressure Washing crew · Duluth, MN · Updated June 2026 · Free to cite with a link

Sixteen things stain Northland home exteriors, and almost every one gets misdiagnosed — which matters, because the fix for one stain is the worst possible treatment for another (scrub oxidation and you'll strip the finish; pressure-wash artillery fungus and you'll drive it deeper). Filter by where the stain is, find your match, and you'll know what it is, whether it's hurting anything, and what actually removes it.

Green film, spreading from shaded areas

Siding · north walls

What it is: Algae (Chlorella and relatives) feeding on the dust film on your siding. The defining Northland stain. Full guide here.

Damage: slow — but holds moisture, stains over years, and reads as neglect.

Fix: Soft washing — kills the colony at the root. Never high pressure on siding.

Black or dark gray spots, peppered, won't budge

Siding · low on walls near mulch

What it is: Artillery fungus — a wood-mulch fungus that fires sticky spore packets up to 20 feet onto siding and cars. The dots are glued on with one of nature's strongest adhesives.

Damage: cosmetic but borderline permanent once cured. The most stubborn stain on this list.

Fix: Fresh spots sometimes release with careful manual work; cured spots usually don't come fully off any cleaning method — honest answer. Prevention is the play: swap wood mulch near walls for stone or rubber.

Chalky white powder that rubs off on your hand

Siding · older vinyl & aluminum, sunny sides

What it is: Oxidation — UV breaking down the siding's surface resins. Wipe a finger across it: white residue = oxidation.

Damage: it IS the surface degrading. And here's the trap: washing it carelessly leaves permanent streaks and shiny spots.

Fix: Specialized low-pressure oxidation washing with the right chemistry and even technique. This is the one stain where a well-meaning DIY wash makes the wall look worse. Details here.

Rust-orange trails below fixtures

Siding · under railings, AC lines, nail heads

What it is: Iron oxide washing off corroding metal above — fasteners, flashing, railing bolts, or well water from sprinklers.

Damage: the stain is cosmetic; the corroding fixture above it is the real story.

Fix: Oxalic/specialty rust removers — bleach-based cleaners set rust deeper. Fix or seal the metal source or it returns. More here.

Greenish-black dots inside panel laps and around windows

Siding · joints & shaded seams

What it is: Mildew/mold colonizing the dampest micro-zones — often a sign water is sitting where it shouldn't.

Damage: potentially — heavy concentrations can mean moisture behind the panel. Worth investigating, not just washing.

Fix: Soft wash kills surface growth; check gutters/downspouts/sprinklers above the worst patches for the moisture source.

Black vertical streaks down roof slopes

Roof · north & shaded slopes

What it is: Gloeocapsa magma — cyanobacteria eating the limestone filler in your shingles. Full guide.

Damage: slow shingle degradation, faster on the streaked slopes.

Fix: Roof soft washing only. Pressure washing strips granules and can void the manufacturer warranty — see what GAF and Owens Corning allow.

Green cushions on roof edges and valleys

Roof · edges, valleys, shade

What it is: Moss — and unlike algae it has root-like rhizoids physically lifting your shingles.

Damage: high. Freeze-thaw + moss = pried-up shingles and early slope failure.

Fix: Soft wash treatment kills it; dead moss sheds with weather over following weeks. Never scrape it dry and never pressure wash it.

Crusty pale-green or gray discs bonded to shingles

Roof · older roofs

What it is: Lichen — fungus + algae in partnership, chemically bonded to the granule layer.

Damage: removal done wrong tears granules off with it; left alone it slowly pits the surface.

Fix: Chemical treatment, then patience — lichen releases gradually over weeks. Anyone offering same-day lichen removal is removing your roof surface with it.

White haze or crusty film, worst near the garage

Concrete · driveways

What it is: Road salt residue + efflorescence after the spring thaw. Why it matters more than it looks.

Damage: active — chlorides in the pores multiply freeze-thaw damage all year.

Fix: Hot-water surface cleaning to lift salt out of the pores, then penetrating sealer.

Dark drip-line and circular spots

Concrete · driveway center, parking spots

What it is: Oil and automotive fluids soaked into the slab's pores.

Damage: minimal structurally — but the #1 driveway eyesore and a flag in listing photos.

Fix: Degreaser dwell + hot water. Cold-water rentals smear it; heat releases it. Old, deep stains lighten dramatically but may ghost.

Green/black slippery film

Concrete & wood · shaded, damp areas

What it is: Algae and mildew on a surface that stays damp — same biology as the siding version, plus a genuine slip hazard on walkways and steps.

Damage: safety issue first, surface wear second.

Fix: Concrete: surface clean + post-treat. Wood: low-pressure wash with wood-safe cleaner — high pressure furs the grain.

Vertical black stripes on gutter faces ("tiger striping")

Gutters · exterior faces

What it is: Electrostatically bonded grime — overflow pulls roof tar and pollutants over the edge, and each drip lane bakes on.

Damage: cosmetic, but it's the visible signature of chronic gutter overflow — check inside.

Fix: Pressure won't touch it (it just removes paint). Dedicated oxidation-safe cleaners + hand brushing. And clean the clog causing the overflow. Related: the ice dam connection.

Uniform silver-gray on wood

Deck & fence · horizontal surfaces first

What it is: UV-degraded surface wood fibers — the natural graying of unprotected lumber.

Damage: the gray layer itself is dead fiber; underneath, unprotected wood is absorbing water.

Fix: Clean + brighten brings the color back dramatically; then stain inside the July–August window to keep it.

Cloudy spots that survive window cleaner

Windows · lower panes, sprinkler side

What it is: Hard-water mineral deposits — and in stage three, permanent etching. The three stages explained.

Damage: progressive — early film is removable, etched glass is not.

Fix: Stage-dependent: pure-water cleaning, mineral-remover restoration, or pane replacement. Get an honest per-pane assessment.

Fog or moisture BETWEEN the panes

Windows · double-pane units

What it is: A failed insulated-glass seal — humid air is getting inside the unit and condensing.

Damage: the window's insulating value is gone, and no cleaning can reach it.

Fix: IGU replacement (often just the glass unit, not the whole frame). We'll tell you when we see it — and we never pressure wash windows, which is how many seals fail early.

White fluffy crystals on brick, block, or stone

Masonry · foundations & chimneys

What it is: Efflorescence — water moving through masonry, dissolving salts, and leaving them on the surface as it evaporates.

Damage: the crystals are harmless; the water path creating them deserves attention (grading, gutters, sealing).

Fix: Dry brushing + specialty wash for the deposit. Persistent recurrence = active moisture migration; find the source.

Can't find your stain? Text a photo to 218-576-8610 and we'll identify it for free — even if the answer is "that's a painter's problem, not ours."

Diagnosed it? We'll handle it.

Free written quote with the correct method for your exact stain — Guardsman-founded, insured, and honest about what won't come off.

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Reference this guide: writers, realtors, and inspectors are welcome to cite or link this page. Related: The PSI Chart · 2026 Price Guide · Spring Checklist