Roof Moss vs. Algae vs. Lichen: What’s Growing on Your House (and Why It Matters)

If you’ve noticed dark streaks, green patches, or crusty spots on your roof, you’re not alone — and you’re probably asking the same question we hear all the time:

“Is that moss… or something else?”

In Southern Oregon, roofs commonly host moss, algae, and lichen, and while they may look similar from the ground, they behave very differently. Knowing which one you’re dealing with matters because each requires different removal and prevention techniques.

Let’s break down how to tell them apart — and what Rogue Valley homeowners should do about each.

Roof Moss (Green and Spongy)

Before getting into why moss is a problem, let’s start with how to recognize it on your roof.

What It Looks Like

Roof moss is usually the easiest to identify:

• Bright or dark green
• Soft, thick, and sponge-like
• Grows in clumps or mats
• Often found in shaded areas, valleys, and near gutters

Why It Grows

Moss thrives in cool, damp, shaded environments, which makes the Rogue Valley an ideal habitat. North-facing roof slopes, tree-covered neighborhoods, and roofs that stay damp for long periods are especially vulnerable.

Why It’s a Problem

Moss holds moisture against your shingles, acting like a sponge. Over time, this can:

• Lift shingle edges
• Accelerate granule loss
• Allow water to penetrate beneath the roofing system

According to the National Roofing Contractors Association (NRCA), organic growth like moss can interfere with proper water shedding and contribute to premature roof deterioration if not addressed.

How It’s Treated

Effective moss removal typically requires:

• Gentle manual removal to eliminate heavy growth
• Follow-up roof-safe chemical treatment to prevent regrowth

Pressure washing or harsh chemicals often cause more harm than good.

Roof Algae (Dark Streaks)

Algae doesn’t grow in clumps like moss, so the best way to identify it is by its distinct streaking pattern.

What It Looks Like

Algae usually appear as:

• Black or dark brown streaks
• Thin, flat discoloration
• Often running downward from the roof peak

This is commonly caused by a type of algae called Gloeocapsa magma.

Why It Grows

Algae feed on moisture and airborne nutrients. In Southern Oregon, it’s common on roofs that:

• Stay damp for long periods
• Receive partial sun
• Have older asphalt shingles

The Asphalt Roofing Manufacturers Association (ARMA) explains that algae doesn’t physically damage shingles the way moss can, but it does stain them and absorb heat, which may accelerate aging.

Why It’s a Problem

While algae is mostly cosmetic, it can:

• Reduce curb appeal
• Increase roof surface temperatures
• Mask underlying roof issues

How It’s Treated

Algae is usually addressed with:

• Low-pressure soft washing
• Algae-specific cleaning solutions
• Preventive treatments or algae-resistant shingles

Roof Lichen (Crusty Combination)

Lichen is often the trickiest to spot, so let’s break down the visual cues that set it apart from moss and algae.

What It Looks Like

Lichen is often mistaken for moss, but it’s different:

• Gray, green, or white patches
• Crusty, flat, or slightly raised
• Hard to scrape off
• Often circular or irregular in shape

What Makes Lichen Unique

Lichen is actually a symbiotic organism, combining algae and fungi. This makes it more stubborn and more damaging than algae — and sometimes even moss.

Why It’s a Problem

Lichen forms strong root-like structures that bond tightly to shingles. Over time, it can:

• Etch into shingle surfaces
• Remove protective granules
• Permanently damage roofing materials

The U.S. Environmental Protection Agency (EPA) notes that aggressive removal methods used on lichen can cause runoff and surface damage if not handled carefully.

How It’s Treated

Lichen typically requires:

• Specialized treatments
• Longer dwell times
• Extreme care to avoid shingle damage

In many cases, professional removal is strongly recommended.

Why Identification Matters in the Rogue Valley

Because moss, algae, and lichen behave differently, treating them the same way often leads to poor results — or roof damage.

For example:

• Bleach may kill algae, but damage shingles
• Pressure washing may remove moss, but strip granules
• Scraping lichen can permanently scar the roof

Southern Oregon’s climate makes regrowth more likely if the root cause isn’t addressed through proper treatment and prevention.

Which One Is Most Common Locally?

In the Rogue Valley:

• Moss is the most damaging and most common on shaded roofs
• Algae is widespread but primarily cosmetic
• Lichen is less common, but the hardest to remove safely

Many roofs actually have more than one type of growth at the same time, which is why a one-size-fits-all approach rarely works.

What to Do If You’re Not Sure What’s on Your Roof

From the ground, it’s often difficult to tell exactly what type of growth you’re seeing — and misidentification can lead to the wrong treatment.

A professional inspection can:

• Identify the type (or combination) of growth
• Recommend the safest moss removal removal method
• Determine whether preventive measures are needed

The Takeaway

Not all roof growth is the same — and in Southern Oregon, knowing the difference between moss, algae, and lichen can save you time, money, and unnecessary roof damage.

The right treatment depends on what’s growing, where it’s growing, and why it’s growing.

Addressing the issue early and correctly helps protect your roof and extend its lifespan.

If you’re unsure what’s on your roof or want expert guidance, BUX Exterior Cleaning provides safe, effective roof cleaning, as well as moss removal and treatment solutions designed specifically for Rogue Valley homes.