If you manage commercial properties, you have seen it. The green film creeping up a north-facing facade. The dark streaking running down the walls from the roofline. The black patches spreading across the concrete at the building’s base. Most property managers assume it is dirt, and they assume a pressure washer will take care of it. The reality is more complicated. What you are looking at is almost always biological growth, not simple surface soiling, and understanding what is causing it changes both how it needs to be treated and what happens if you leave it alone.
This post breaks down the most common types of exterior staining on commercial buildings, what causes each one, why the mid-Atlantic climate makes properties in Southeast Pennsylvania particularly susceptible, and what the right treatment approach looks like for each.
Green Staining: Algae
The green film you see on commercial building exteriors is algae, a single-celled organism that grows wherever moisture, limited sunlight, and a surface to anchor to come together. On commercial buildings, algae most commonly appears on north and east-facing walls, under overhangs and soffits, in the shaded lower sections of facades, and on any surface that stays wet longer than the rest of the building after rain.
Algae spreads by releasing airborne spores that land on surfaces and colonize when conditions are right. It feeds on moisture and the minerals present in many building materials, including the calcium carbonate found in many masonry products and the organic compounds present in wood, paint, and sealants. Once established, a single algae colony can spread across an entire facade within one or two seasons.
In Southeast Pennsylvania’s humid climate, algae growth on commercial buildings is essentially inevitable without regular preventive maintenance. Properties near streams, retention ponds, tree lines, or in low-lying areas where ground moisture is elevated will see accelerated growth. Properties with EIFS (Exterior Insulation and Finish Systems), textured stucco, and porous masonry are particularly hospitable surfaces for algae establishment because their texture and porosity give spores more to anchor to.
Algae itself does not immediately damage most building materials, but it holds moisture against the facade, which accelerates deterioration of the substrate below it. On EIFS systems, moisture intrusion behind algae growth is a leading cause of the kind of systemic failure that requires costly remediation rather than routine cleaning.
Black Staining: Gloeocapsa Magma and Mold
Black staining on commercial building facades is most commonly caused by one of two things: Gloeocapsa magma or mold. They look similar from a distance, they both appear as dark streaking or patchy discoloration, and they are both frequently misidentified as dirt or soot. The distinction matters because they require different treatment approaches.
Gloeocapsa magma is a cyanobacteria, sometimes loosely called blue-green algae, that produces a dark, protective outer sheath as a UV shield. It appears as dark gray to black streaking, most commonly running vertically from the roofline downward along wall surfaces. On commercial buildings, it frequently appears below parapets, roof drains, window heads, and any horizontal surface from which water runs down the facade during rainfall. It is the same organism responsible for the black streaking seen on residential roofs, and it is just as common on commercial building envelopes.
Gloeocapsa magma spreads by the same spore mechanism as algae and is transported from building to building by wind. Once a colony establishes on a commercial property, it will grow progressively unless treated. Unlike ordinary surface dirt, it cannot be removed by standard pressure washing alone. The outer sheath that protects the bacteria also makes it resistant to physical removal without the application of appropriate cleaning chemistry at the right concentration and dwell time.
Mold on building exteriors presents as irregular black, gray, or greenish-black patches, often in areas with chronic moisture exposure such as around leaking gutters, window frames, HVAC condensation discharge points, and shaded areas near ground level. Unlike Gloeocapsa magma, which spreads downward from rooflines, mold typically grows outward from a moisture source. Addressing mold on a commercial building exterior requires both treating the visible growth and identifying the moisture source that is sustaining it, whether that is a gutter leak, an HVAC issue, or inadequate surface drainage.
Orange and Brown Staining: Rust and Mineral Deposits
Not all exterior staining on commercial buildings is biological. Orange and brown streaking typically indicates one of two non-biological causes that require different treatment than algae or mold:
- Rust staining. Orange or brown streaking that runs vertically from metal elements such as anchor bolts, fasteners, structural connections, window frames, or decorative metalwork indicates rust migration. Water carries iron oxide particles from corroding metal down the facade surface, leaving a stain that can penetrate deeply into porous materials. Rust staining requires a specialized chemical treatment to remove effectively and does not respond to standard pressure washing.
- Efflorescence and mineral deposits. White, gray, or light brown haze on masonry and concrete surfaces is typically efflorescence, which is the crystallization of water-soluble salts as they migrate through masonry and evaporate on the surface. It is particularly common on new construction, after significant rainfall, or in areas where water infiltration is occurring. Efflorescence is not harmful to the building but indicates water movement through the masonry that warrants attention. It is removed with acid-based cleaning solutions appropriate to the specific masonry type.
Why Southeast Pennsylvania Buildings Are Particularly Susceptible
The mid-Atlantic region creates near-ideal conditions for biological growth on building exteriors. The combination of warm, humid summers with significant rainfall, the partial shade created by mature tree canopies in suburban commercial areas, and the freeze-thaw cycling of Pennsylvania winters all contribute to conditions where algae, Gloeocapsa magma, and mold establish and spread faster than in drier or sunnier climates.
Properties in Montgomery, Chester, Bucks, Delaware, and Berks County face these conditions consistently. Buildings along shaded corridors in communities like Blue Bell, Malvern, Newtown, and Media, or properties near the Wissahickon, Brandywine, or Schuylkill watersheds where ambient humidity is higher, will see accelerated biological growth compared to properties in open, sunny locations. The geography of Southeast Pennsylvania, wooded, hilly, and humid, is genuinely one of the more challenging environments for commercial building exterior maintenance anywhere in the eastern United States.
Why Pressure Washing Alone Is Not the Right Solution
This is the most important practical point in this post. Property managers who schedule standard pressure washing for buildings with algae, Gloeocapsa magma, or mold growth will see a temporary improvement in appearance followed by rapid regrowth, often within a single season. High-pressure water removes surface discoloration but does not kill the biological organism underneath it. Without treatment that addresses the root system of the growth, the colony regenerates from the portions that survived the washing, often growing back faster than before because the process of pressure washing can disturb and spread spores.
The industry-standard approach for biological growth on commercial building facades is soft washing, which uses lower water pressure combined with professional-grade cleaning solutions that penetrate biological growth, kill the root structure, and treat the surface to inhibit regrowth. The visual result of soft washing is equivalent to or better than pressure washing on biological staining, and it holds significantly longer because it has addressed the source rather than just the symptom.
HydroPower uses a soft wash approach for all commercial building facade cleaning. We assess each facade type before selecting cleaning chemistry and pressure settings appropriate to the material, because the concentration of cleaning solutions appropriate for concrete is different from what should be applied to EIFS, painted brick, or composite metal panel. Getting that selection wrong can cause surface damage that is far more expensive to address than the original staining.
What Property Managers Should Do When They See Staining
The practical guidance for commercial property managers who notice facade staining is straightforward:
- Do not wait. Biological growth compounds quickly. A small algae colony in year one becomes a heavily stained facade by year three. The cost of treatment increases significantly as coverage expands and root systems deepen into porous surface materials.
- Document the location and pattern. Before cleaning, note where the staining is concentrated, whether it runs from roofline elements downward, whether it appears near moisture sources, and whether any staining appears to be rust or mineral-based rather than biological. This helps a professional cleaning company plan the right treatment approach.
- Request a soft wash service, not just pressure washing. When contacting exterior cleaning vendors, specifically ask about their approach to biological growth. A company whose standard commercial offering is high-pressure washing without cleaning chemistry is not equipped to produce a lasting result on biological staining.
- Ask about treatment that inhibits regrowth. Professional soft washing with the right chemistry includes a rinsed treatment that leaves a residual on the surface to inhibit immediate regrowth. Ask specifically whether the vendor’s process includes this step.
- Address underlying moisture issues. If staining is concentrated around specific building elements such as gutters, window frames, or HVAC discharge points, investigate whether a moisture source is sustaining the growth before cleaning. Treating the staining without addressing the moisture source will produce a shorter-lived result.
Get an Assessment for Your Property
HydroPower provides professional commercial building washing throughout Montgomery, Chester, Bucks, Delaware, Berks, and Philadelphia County. If you are seeing green or black staining on your building’s facade, we can assess the type and severity of growth, recommend the right treatment approach, and provide a detailed quote with no obligation. Early treatment is always less expensive than remediation. Contact us to schedule an assessment.