The Hidden Hazards of Algae, Mold, Moss & Mildew in Western Washington

While many homeowners in the Pacific Northwest grow used to the greenish tints and dark streaks on their roofs, driveways, and siding, these organic growths are more than just an eyesore. They pose serious safety, health, and structural risks if left unchecked—especially in our damp Western Washington climate.

Let’s break down what these growths are, where they thrive, and what risks they bring to your home and family.

1. Moss: Soft, Green—and Surprisingly Destructive

Where it Grows:

Shady, damp areas on asphalt shingles, wood decks, brick walkways, and driveways, especially on north-facing surfaces.

Why It’s a Problem:

• Moss retains moisture like a sponge, which can seep into roofing materials and cause rot or undermine structural integrity over time.

• Its root-like structures (rhizoids) can lift shingles, exposing your roof to water damage and leaks.

• Moss buildup on hard surfaces like walkways becomes extremely slippery when wet, leading to slip-and-fall accidents, especially for children or elderly residents.

🧠 Reference: The Washington State University Extension notes that moss not only shortens roof lifespan but can void roof warranties if left untreated. 🔗 WSU Guide: Managing Roof Moss

2. Algae: Staining and Surface Degradation

Where it Grows:

Often seen as black or green streaks on siding, roofs, and concrete. It thrives in moist, shaded areas.

Why It’s a Problem:

• Algae feeds on limestone filler in asphalt shingles, leading to shingle decay and early roof failure.

• On sidewalks and decks, it becomes slimy and hazardous, especially during morning dew or light rain.

• Aesthetic degradation aside, certain strains of blue-green algae (cyanobacteria) can produce toxins that are harmful when aerosolized or touched by pets.

🧠 Reference: The Asphalt Roofing Manufacturers Association (ARMA) confirms algae as a major contributor to roof damage and discoloration. 🔗 ARMA on Algae Stains

3. Mold: The Health Risk You Can’t Ignore

Where it Grows:

On siding, under eaves, around windows, in shaded corners, and near organic debris or moisture accumulation.

Why It’s a Problem:

• Mold can release spores into the air, causing respiratory irritation, asthma flare-ups, and allergic reactions—especially in children, seniors, and immunocompromised individuals.

• Outdoor mold can find its way indoors through windows, HVAC systems, or on shoes and clothing.

• It also eats away at wood and paint, accelerating surface degradation and requiring costly repairs.

🧠 Reference: The Centers for Disease Control and Prevention (CDC) warns about the health impacts of mold exposure, even outdoors, especially when disturbed during yard work or home maintenance. 🔗 CDC Mold Fact Sheet

4. Mildew: The Slippery Surface Invader

Where it Grows:

Frequently seen as white, powdery patches on siding, painted wood, fences, and around plant beds where airflow is limited.

Why It’s a Problem:

• Mildew can weaken paint finishes, requiring premature repainting.

• It leaves surfaces slick and greasy, increasing the risk of accidental falls.

• Mildew can also be a precursor to more aggressive mold growth if not addressed early.

🧠 Reference: According to the EPA, mildew is often a sign of broader moisture problems that can evolve into structural or indoor air quality concerns. 🔗 EPA Moisture Control Guide

What Starts as a Stain Can Become a Liability

In Western Washington’s lush, damp environment, these organic growths can appear in as little as a few weeks during rainy seasons. Left untreated, they:

• Lower curb appeal and home value

• Compromise material integrity (wood rot, roof deterioration)

• Increase liability risk from falls and injuries

• Threaten the health of your family and pets

• Void roofing and siding warranties

The Solution? Safe, Professional Removal

PacWest Pressure Washing uses non-toxic, biodegradable, and surface-safe cleaning methods tailored for Pacific Northwest homes. Our soft-wash and pressure-wash systems:

• Remove algae, mold, moss, and mildew without damaging your home’s materials

• Restore color and safety to walkways, decks, and siding

• Prevent regrowth with eco-safe treatments

• Keep your family, pets, and garden safe in the process

☀️ Don’t let your summer plans be interrupted by a slip, a leak, or an unexpected repair bill.Let us safely eliminate these hazards—so you can get back to enjoying the season.

📞 Contact us today or request a free estimate and let PacWest handle the grime—so you don't have to.