The Short Answer
Black roof streaks are algae, not dirt — they need soft washing, not pressure. Green on siding is algae accelerated by Austin’s humidity and live oak shade. Brown staining under trees is tannin from organic debris. Hard water spots on windows are mineral deposits from Austin’s limestone-filtered water supply. Rust on driveways needs a chemical pre-treatment before pressure washing.
Every one of these has a correct removal method and several incorrect ones. The incorrect ones either don’t work or damage the surface. Surface type matters as much as stain type.
How to Identify What You’re Looking At
Before anything else, identification. The wrong diagnosis leads to the wrong treatment — and in some cases to permanent surface damage. Here’s a quick reference:
| What it looks like | What it probably is | Where you’ll find it | Correct approach |
|---|---|---|---|
| Black streaks running down from ridge | Gloeocapsa magma algae | Asphalt shingle roofs | Soft wash only — no pressure |
| Green slime or film, often slimy to touch | Algae (various species) | North-facing siding, shaded concrete | Soft wash with sodium hypochlorite |
| Black fuzzy or powdery patches | Mold or mildew | Shaded siding, wood decks, concrete | Soft wash with biocide solution |
| Brown staining under trees or gutters | Tannin from oak, cedar, pecan | Concrete, stucco, limestone near trees | Alkaline pre-treatment + pressure wash |
| Reddish-brown on driveway or siding | Rust from irrigation or metal fixtures | Driveways, siding near irrigation heads | Oxalic acid rust remover before washing |
| White chalky spots on windows or stone | Hard water mineral deposits | Windows, glass, stone near irrigation | Mild acid solution or DI/RO water finish |
| Yellow film over everything in spring | Cedar and live oak pollen | All exterior surfaces, worst in Feb–March | House wash with surfactant; rinse thoroughly |
| Dark oil spots on garage floor or driveway | Vehicle oil or grease | Concrete driveways, garage floors | Commercial degreaser + hot water pressure wash |
The Stains We See Most Often in Austin & the Hill Country
The black streaks on roofs in Austin and the Hill Country are almost universally Gloeocapsa magma — a blue-green algae that colonizes asphalt shingles by feeding on the limestone filler used in their manufacture. It spreads downward from the ridge (where birds land and deposit spores) in characteristic dark streaks that darken and widen over time.
Beyond aesthetics, Gloeocapsa degrades the shingle granules that protect your roof from UV radiation. Once the granules go, shingle lifespan drops quickly. GAF, CertainTeed, and Owens Corning all specify soft washing as the only approved removal method — high-pressure washing voids the warranty.
In Lakeway, Westlake Hills, and Bee Cave, where live oak canopy keeps roof surfaces shaded for much of the day, algae spreads faster than on sun-exposed roofs. If you’re seeing streaks, address them now rather than at your next annual service — the colony grows significantly in 6 to 12 months.
Green algae and black mold on exterior siding in Austin have the same root cause: moisture that doesn’t dry quickly enough. North-facing walls in particular stay damp long after rain because they get almost no direct sun. Add live oak canopy and cedar pollen — which accumulates in the texture of siding and acts as an organic nutrient base — and you have ideal conditions for rapid algae and mold growth.
The distinction between algae and mold matters for treatment but not for diagnosis from a distance. Algae is typically green and slimy; mold tends to be black, gray, or fuzzy. Both respond to sodium hypochlorite-based soft wash solutions that kill at the root rather than just washing the surface.
Pressure washing alone — without a biocide solution — removes surface growth but leaves the root structure in the material. Regrowth typically appears within 4 to 6 months. Soft washing with proper dwell time kills the organism, which is why results last 12 to 24 months rather than a season.
Tannin staining is one of the most common and least understood exterior problems in Austin and the Hill Country. Tannins are natural polyphenol compounds found in high concentrations in live oak leaves, acorns, cedar elm seeds, and pecan hulls. When these decompose on a wet surface — concrete, stucco, limestone — they release water-soluble tannins that absorb quickly into porous materials and stain brown.
Tannin staining is particularly common under gutters where leaves accumulate and break down in water, on driveways under live oak canopy, and on limestone retaining walls and patios. The longer it sits, the deeper it penetrates.
Standard pressure washing does not remove established tannin staining. It requires an alkaline pre-treatment (typically sodium hydroxide or potassium hydroxide based) applied to the surface before washing. The alkaline solution breaks down the tannin molecule, making it water-soluble enough to rinse away. On stucco and limestone, the concentration needs to be appropriate for the surface to avoid damage — this is not a DIY application.
Rust staining on Austin driveways and siding most commonly comes from one of two sources: irrigation systems drawing from well water with elevated iron content, or aging metal irrigation components (heads, pipes, clamps) corroding over time. Either way, repeated exposure deposits iron oxide on surrounding surfaces — concrete, stucco, painted wood, and limestone.
Pressure washing alone moves rust staining around but doesn’t remove it. Iron oxide needs a chemical conversion before it’s water-soluble. Oxalic acid-based rust removers (or sodium bisulfate formulations) convert the iron oxide into a form that can be rinsed away. Applied correctly, they lift significant staining in 5 to 10 minutes of dwell time.
One important note: if your irrigation system is the source, the staining will return unless the source is addressed. A water softener or iron filter on the irrigation supply, or replacement of corroded components, is the long-term solution. The cleaning removes what’s there; it can’t prevent what’s still spraying rust-colored water three times a week.
Austin has some of the hardest municipal water in Texas. The Edwards Aquifer water supply dissolves calcium and magnesium from the limestone it passes through. When that water evaporates on your windows, stone, or outdoor fixtures, those minerals stay behind as white chalky deposits.
Light hard water deposits on glass respond to mild acidic solutions — diluted white vinegar or commercial glass restorers. More established mineral etching, where the calcium has bonded to the glass surface over time, requires professional polishing compounds. At that stage, it’s no longer just a stain; it’s surface damage.
The correct prevention is finishing any window cleaning with deionized (DI) or reverse-osmosis (RO) water. With the minerals removed, there’s nothing left behind as the water evaporates. This is the standard technique for professional window cleaning and the reason professional results last longer than garden hose rinse-downs.
Austin’s cedar and live oak pollen seasons are not subtle. Mountain cedar (Ashe juniper) pollinates heavily from December through February. Live oak follows in March and April. During peak events, surfaces across the city are coated in yellow-green pollen that is both highly allergenic and, if left to accumulate, a nutrient base for algae growth.
Pollen itself washes off relatively easily with a house wash and surfactant — it’s an organic particle, not a penetrating stain. The concern is what happens when pollen accumulates in surface texture over multiple seasons without cleaning. It traps moisture, provides nutrients for algae, and degrades certain surface coatings over time.
Timing matters here: cleaning immediately after cedar season (late February to March) removes the heavy accumulation before it can drive algae growth. A second wash after live oak pollen in May addresses the spring drop.
The mistake that compounds every stain: Pressure washing without a chemical pre-treatment. For algae, mold, tannin, and rust, pressure alone moves the staining around without removing it — and on delicate surfaces like limestone, stucco, and shingles, high pressure causes surface damage that makes future staining worse. The chemical does the work. The pressure just rinses.
Surface Compatibility: Which Methods Work Where
This is where most DIY attempts and inexperienced contractors cause lasting damage. The same stain requires different treatment depending on what surface it’s on.
Limestone and stucco: Both are porous and both are vulnerable to high-pressure washing. Maximum 500 PSI for any cleaning on these surfaces. Chemistry does the heavy lifting — pressure only rinses. If a contractor proposes high-pressure washing on your limestone facade or stucco exterior, that’s a red flag.
Asphalt shingles: Maximum 100 to 150 PSI. Anything higher strips granules. Only soft washing should be applied to a shingle roof — there is no situation where a pressure washer wand belongs on an asphalt shingle.
Concrete driveways and sidewalks: The one surface that benefits from high pressure — 2,000 to 3,000 PSI with a surface cleaner for even results. Chemical pre-treatment still matters for organic staining and tannin, but the pressure can be high.
Wood decks and fences: 600 to 800 PSI maximum, with grain-specific technique. Wood-safe detergents, not bleach. Allow complete drying before staining or sealing.
Frequently Asked Questions
What are the black streaks on my roof in Austin?
Black streaks on roofs in Austin and the Hill Country are almost always Gloeocapsa magma — a blue-green algae that feeds on the limestone filler in asphalt shingles. It spreads from the ridge down in streaks that darken over time. The correct removal method is a controlled soft wash at low pressure. High-pressure washing removes the granules that protect your shingles and voids most manufacturer warranties.
What causes green stains on siding and concrete in Austin?
Green staining on siding and concrete in Austin is typically algae growth — accelerated by humidity, shade from live oaks and cedars, and pollen debris that provides a nutrient base. North-facing walls and shaded concrete surfaces are most commonly affected. Soft washing with a sodium hypochlorite solution kills the algae at the root, which is why results last 12 to 24 months rather than a season.
What are the brown stains under my gutters and on my driveway?
Brown staining under gutters and on driveways near trees in Austin is almost always tannin staining — organic compounds released by live oak, cedar elm, and pecan leaves as they decompose. Tannins absorb quickly into porous surfaces. They require a specific alkaline pre-treatment before pressure washing — standard pressure alone will not fully remove established tannin staining.
How do I remove hard water stains from windows in Austin?
Hard water stains on Austin windows form because the local water supply runs through limestone, depositing calcium and magnesium on glass as water evaporates. Light deposits respond to a mild acid solution. Established mineral etching requires professional polishing. Prevention is finishing window cleaning with deionized or reverse-osmosis water, which leaves no minerals behind as it dries.
Can rust stains be removed from concrete driveways?
Yes. Rust staining on concrete requires an oxalic acid-based rust remover applied before pressure washing. Standard pressure washing without chemical pre-treatment will not remove rust — it lightens it temporarily at best. For irrigation rust staining, the source also needs to be addressed or the staining returns. Elevated iron content in well water or corroded irrigation components are the most common causes in Austin.
Not Sure What You’re Looking At?
Send us a photo or describe what you’re seeing. We’ll tell you what it is and what it takes to remove it.
Call or Text 512-883-5420