It isn’t dirt. And the longer it goes unaddressed, the more it’s costing you—in shingle life, in energy efficiency, and eventually in repair bills that could have been avoided entirely.
We see algae on roofs across Tacoma, Pierce County, and the South Sound every single week. In this climate, it’s not an if—it’s a when. Knowing what it actually is and when to act can make the difference between a cleaning job and a full replacement.
Quick Answer: The dark streaks on your roof are Gloeocapsa magma, a cyanobacteria that feeds on limestone filler in asphalt shingles. It’s not cosmetic. Algae holds moisture against your shingles, accelerates granule loss, degrades asphalt, and can shorten a roof’s lifespan by years. In Tacoma’s wet, overcast climate, it spreads faster and does more damage than in most other regions.
What Is the Black Stuff Growing on My Roof?
The dark streaks running from the ridgeline to the gutter are caused by a cyanobacterium called Gloeocapsa magma. It’s a microscopic organism that travels through the air as spores and produces a dark pigmented coating as a defense against UV rays—that coating is what you’re seeing from the ground.
It’s not soot. It’s not pollution. It’s a living organism actively degrading your shingles. According to the Asphalt Roofing Manufacturers Association (ARMA), Gloeocapsa magma is the most common cause of dark staining on asphalt roofs in North America, and its prevalence is highest in humid, overcast climates exactly like ours.
Algae appears first where roofs stay wet longest—north-facing slopes, shaded sections, valleys. In Tacoma neighborhoods like the North End, Proctor, and Fern Hill, where mature trees create extended shade, it establishes significantly faster than on open rooftops.
Algae vs. Moss vs. Lichen: What You’re Actually Looking At
| Organism | Appearance | Root Structure | Primary Damage | PNW Prevalence |
| Algae | Dark brown/black streaks | None—surface only | Granule loss, moisture retention | Extremely high |
| Moss | Green, spongy mats | Yes—penetrates shingle | Physical lifting, water infiltration | Very high |
| Lichen | Gray-green, crusty patches | Deep—etches surface | Permanent granule damage | Moderate–high |
Algae is the gateway organism. It has no root structure but retains moisture and breaks down granules, creating the damp surface conditions that moss needs to establish. Moss has physical roots that penetrate shingles, lift their edges, and allow water to infiltrate. Lichen bonds directly to the shingle surface and can permanently damage it—and is the hardest to remove safely.
What Causes Algae to Grow on a Roof in Tacoma?
Tacoma averages 35 to 50 inches of rainfall annually and roughly 200 cloudy days per year. UV radiation—one of the primary natural inhibitors of algae growth—is reduced for months at a stretch. Roof surfaces stay damp longer. There are fewer dry windows to interrupt the growth cycle. Add in the Douglas firs, western red cedars, and big-leaf maples throughout Hilltop, the North End, and Old Town, and you have a near-perfect environment for algae to establish and spread continuously.
Older homes in Tacoma and Pierce County are especially vulnerable—asphalt shingles manufactured prior to approximately 2000 contain higher concentrations of limestone filler, which Gloeocapsa magma feeds on directly.
Does Algae Damage Asphalt Shingles? Yes—Here’s How
Stage 1: Algae establishes as a thin biofilm. Dark streaking appears. Shingles still largely intact but beginning to absorb more moisture.
Stage 2: The biofilm continuously traps moisture against the shingles, accelerating granule loss. Check your gutters—a substantial deposit of dark, sand-like material after rain is a clear signal.
Stage 3: With granules reduced, the asphalt layer oxidizes under UV exposure, becomes brittle, and cracks—creating pathways for water infiltration.
Stage 4: Water reaches the underlayment and roof deck. Deck rot, insulation damage, and interior leaks follow. At this stage, you’re no longer talking about a cleaning problem.
Dark algae also absorbs significantly more solar heat than clean shingles, raising attic temperatures and increasing cooling loads—an energy cost most homeowners don’t connect back to their roof.
What Happens Year by Year if Algae Goes Untreated
Years 1–2: Surface streaking. Granule loss beginning. Lowest-cost intervention window—a professional soft wash stops the progression at minimal expense.
Years 3–5: Moss establishes in algae-affected areas. Shingle edges show early lifting. Cleaning still effective but more intensive.
Years 5–8: Moss rhizoids have penetrated shingle material. Edges curling. Water likely infiltrating in moss-heavy areas during heavy rain.
Years 8+: Lichen may be establishing. Underlayment damage probable. The conversation shifts from cleaning to planning for repair or replacement.
The Warranty Issue Worth Knowing
Most shingle manufacturers include maintenance provisions in warranty terms. Allowing algae and moss to go unaddressed can be classified as a maintenance failure—potentially voiding coverage on a future claim. Documented regular cleaning is part of protecting the warranty you paid for.
Algae-Resistant Shingles: Worth Asking About
If you’re approaching a replacement, algae-resistant shingles embed copper or zinc granules that inhibit Gloeocapsa magma. In Tacoma’s climate, the upgrade is almost always worth the modest cost difference.
GAF StainGuard Plus carries a 25-year algae resistance warranty. CertainTeed Landmark IR incorporates StreakFighter technology designed for high-humidity climates. Owens Corning Duration also includes algae-resistant granules and is a common choice for re-roofing in Pierce County. Ask your contractor whether any of these are included in their bid before signing.
The Right Treatment—and What to Avoid
Soft washing is the professional standard: low-pressure water with a biocidal treatment that kills algae, moss, and lichen at the surface level without stripping granules. It’s what ARMA recommends and what roofing professionals throughout Pierce County use.
Pressure washing is the most common DIY mistake—it strips granules in a single pass, effectively aging a roof by several years in one afternoon. Avoid it.
Zinc and copper ridge strips release trace metals with each rain, creating a surface environment inhospitable to regrowth. They don’t replace cleaning for an affected roof, but are effective for ongoing prevention on shaded slopes.
Don’t Wait Until It Costs More Than It Has To
Algae isn’t an emergency today. It won’t cause a leak by next week. But it’s working right now—on the shingles you’re counting on for the next fifteen or twenty years.
The homeowners who get the most out of their roofs in the Pacific Northwest are the ones who treat maintenance as a habit rather than a reaction. A professional soft wash every few years is inexpensive. Replacing a roof ten years early because algae was left to do its work is not.
If you’re not sure what’s on your roof or how far along it is, a professional inspection is the most useful—and least expensive—first step available to you.
About The Roof Doctor
The Roof Doctor is a family-owned and operated roofing company with more than 60 years of experience serving Tacoma, Pierce County, and the Pacific Northwest. Licensed, bonded, and insured, available 24/7, with most jobs completed in one to two days. Whether you need a cleaning, an inspection, or a frank conversation about what you’re looking at, they’ll give you an honest assessment and get it handled right. Call us anytime—we’re available 24/7 and happy to help.
