Black streaks on asphalt shingles are not dirt and not mold. They are colonies of a photosynthetic cyanobacterium, Gloeocapsa magma, and where it grows fastest is set mostly by one variable: how long roof surfaces stay damp. This report aggregates the biology, the manufacturer and trade-body guidance, and long-run relative humidity normals to rank US regions and states by roof-algae risk. Every statistic below is sourced. The regional rankings are original Roofing Brief calculations built from public humidity data, and each is labeled as such with its inputs and limits.
Executive summary
- Gloeocapsa magma, a cyanobacterium (blue-green algae), is the most common cause of black roof streaks. It builds a dark pigmented sheath as sun protection, which is the black color people see. Source: Kenyon College MicrobeWiki; Atlas Roofing.
- The organism feeds on the crushed limestone used as filler in asphalt shingles, so streaking is tied to shingle deterioration, not only appearance. Source: Kenyon College MicrobeWiki.
- The Asphalt Roofing Manufacturers Association (ARMA) states that coastal or humid climates create the most conducive conditions for algae growth, and that algae is present for months before it becomes visible. Source: ARMA, via Malarkey Roofing Products.
- On unprotected roofs in coastal and damp, humid climates, black streaks can appear in as little as three years. Source: Malarkey Roofing Products.
- Atlas Roofing describes an algae “danger zone” covering the Eastern and Western seaboards, the Gulf states, the Pacific Northwest, and the Southeast, an area it puts at more than 80 percent of the country, with the Southeast the single biggest area of risk. Source: Atlas Roofing.
- By long-run morning relative humidity normals, the highest-risk states are concentrated on the Gulf Coast and Southeast, led by Mississippi, Louisiana, Florida, Georgia, and South Carolina. The lowest are in the arid interior West: Arizona, New Mexico, and Colorado. Source: NOAA National Climatic Data Center normals, compiled by CurrentResults; Roofing Brief calculation.
- Algae-resistant shingles use copper and zinc granules that release metal ions to inhibit algae. Manufacturer algae warranties range from 10 years (Owens Corning StreakGuard) to 25 and 30 years (GAF StainGuard Plus and StainGuard Plus PRO). Source: manufacturer warranty documentation.
Key findings
- Gloeocapsa magma is classified as Bacteria; Cyanobacteria; Cyanophyceae; Chroococcales; Chroococcaceae, a photosynthetic cyanobacterium, not a fungus or true algae. Source: Kenyon College MicrobeWiki, accessed 2026.
- The visible black color is a protective sheath the organism forms against ultraviolet light, using mycosporine-like amino acids as natural sunscreen compounds. Source: Kenyon College MicrobeWiki, accessed 2026.
- The bacterium feeds on the crushed limestone used as filler in roof shingles, breaking down shingle content and contributing to granule loss over time. Source: Kenyon College MicrobeWiki, accessed 2026.
- It preferentially colonizes north-facing roof slopes, where reduced sunlight keeps the surface damp longer. Source: Kenyon College MicrobeWiki, accessed 2026.
- ARMA states that coastal or humid climates create the most conducive conditions for algae growth, and that by the time algae are visible they have been present for several months or longer. Source: ARMA, quoted by Malarkey Roofing Products, accessed 2026.
- On roofs without algae protection in coastal and damp, humid climates, black streaks can manifest in as little as three years. Source: Malarkey Roofing Products, accessed 2026.
- Atlas Roofing identifies the algae danger zone as the Eastern and Western seaboards, Gulf states, Pacific Northwest, and Southeast, describing it as more than 80 percent of the country and naming the Southeast the biggest area of risk. Source: Atlas Roofing, accessed 2026.
- Algae spread by airborne spores carried on the wind from already-colonized rooftops, which is why infestations move across neighborhoods. Source: Atlas Roofing, accessed 2026.
- Mississippi has the highest morning relative humidity normal among state representative cities at 91 percent (Jackson), followed by Louisiana, Florida, Georgia, and South Carolina at 86 to 87 percent. Source: NOAA National Climatic Data Center normals via CurrentResults; Roofing Brief calculation.
- Among major cities, New Orleans and Jacksonville have the highest annual average daily humidity at 76 percent, with Houston at 75 percent and Tampa, Orlando, Miami, San Francisco, Seattle, and Portland close behind. Source: NOAA-referenced normals via CurrentResults.
- The lowest-humidity major cities are Las Vegas at 30 percent daily, Phoenix at 37 percent, and Tucson at 38 percent, all in the arid interior West. Source: NOAA-referenced normals via CurrentResults.
- Gulf and Southeast cities combine very high morning humidity with limited afternoon dry-down: Houston reaches 90 percent morning humidity, Jacksonville and Orlando 89 percent, and New Orleans 88 percent, while afternoon humidity in these cities stays in the mid-50s to low-60s. Source: NOAA-referenced normals via CurrentResults.
- Algae-resistant shingles rely on copper and zinc granules that release ions to inhibit algae; Malarkey products with Scotchgard Protector contain at least a 10 percent blend of 3M copper granules. Source: Malarkey Roofing Products, accessed 2026.
- Manufacturer algae warranty length varies widely: Owens Corning StreakGuard provides 10 years, GAF StainGuard Plus provides 25 years with a 10-year non-prorated period, and GAF StainGuard Plus PRO provides 30 years. Source: GAF and Owens Corning warranty documentation, accessed 2026.
- ARMA’s recommended cleaning method is a 50:50 mix of laundry-strength liquid chlorine bleach and water, applied without high pressure. Source: ARMA, via Malarkey Roofing Products, accessed 2026.
What causes black roof streaks
The stain is a living colony. Gloeocapsa magma is a cyanobacterium that photosynthesizes and reproduces wherever a roof surface holds moisture. It appears green when wet and young, then darkens as the colony builds a pigmented outer sheath to shield itself from ultraviolet light. That sheath is the black or dark brown streak visible from the street. Source: Kenyon College MicrobeWiki; Atlas Roofing.
The damage is not only cosmetic. The bacterium feeds on the crushed limestone filler inside asphalt shingles, and as the colony works it contributes to granule loss and surface breakdown. Streaks also trap moisture and heat against the shingle. Source: Kenyon College MicrobeWiki.
Direction and shade matter at the individual-roof level. Colonies establish first and most heavily on north-facing slopes and under overhanging trees, where the surface receives less sun and stays damp longer. Source: Kenyon College MicrobeWiki; ARMA via Malarkey. This is why two identical homes on the same street can show very different streaking, and it is a limit on any climate-only ranking.
The climate driver: sustained roof-surface moisture
Both ARMA and manufacturers point to the same underlying condition. ARMA states that algae form where moisture tends to dwell on roof surfaces and that coastal or humid climates create the most conducive conditions. Source: ARMA via Malarkey. Atlas frames the same idea geographically, placing hot, humid-summer regions in its algae danger zone and naming the Southeast the biggest risk. Source: Atlas Roofing.
Relative humidity normals are the best public proxy for how long roof surfaces stay wet across large areas. Morning readings, taken between 4 and 6 a.m. when temperatures are lowest and humidity peaks, capture overnight and early-morning dampness, the window when a roof is wettest and algae are most active. Afternoon readings, taken between 3 and 5 p.m. at the day’s temperature peak, show how far a roof dries out. A roof in a place where afternoon humidity stays high never fully dries. Source: NOAA-referenced normals via CurrentResults.
Table 1. Highest morning relative humidity by state (representative city)
| Rank | State | Representative city | Morning RH | Afternoon RH |
|---|---|---|---|---|
| 1 | Mississippi | Jackson | 91% | 54% |
| 2 | Louisiana | New Orleans | 87% | 61% |
| 2 | Florida | Tampa | 87% | 57% |
| 4 | Georgia | Macon | 86% | 50% |
| 4 | South Carolina | Columbia | 86% | 49% |
| 6 | Arkansas | Fort Smith | 85% | 49% |
| 6 | Oregon | Portland | 85% | 59% |
| 8 | Alabama | Birmingham | 84% | 52% |
| 8 | Tennessee | Nashville | 84% | 53% |
| 8 | Virginia | Richmond | 84% | 52% |
| 8 | Michigan | Lansing | 84% | 61% |
| 8 | Wisconsin | Madison | 84% | 58% |
Source: NOAA National Climatic Data Center and World Data Center for Meteorology normals, compiled by CurrentResults. Ranking is a Roofing Brief calculation.
Table 2. Lowest morning relative humidity by state (representative city)
| State | Representative city | Morning RH | Afternoon RH |
|---|---|---|---|
| Arizona | Tucson | 53% | 25% |
| New Mexico | Albuquerque | 60% | 29% |
| Colorado | Grand Junction | 60% | 35% |
| Wyoming | Lander | 63% | 43% |
| Utah | Salt Lake City | 67% | 43% |
| Idaho | Boise | 68% | 41% |
| Nevada | Reno | 71% | 32% |
| Montana | Helena | 71% | 45% |
Source: NOAA National Climatic Data Center and World Data Center for Meteorology normals, compiled by CurrentResults. Ranking is a Roofing Brief calculation.
Table 3. Annual average humidity for selected major cities
| City | Daily | Morning | Afternoon |
|---|---|---|---|
| New Orleans, LA | 76% | 88% | 61% |
| Jacksonville, FL | 76% | 89% | 58% |
| Houston, TX | 75% | 90% | 55% |
| Orlando, FL | 74% | 89% | 56% |
| Tampa, FL | 74% | 87% | 57% |
| Miami, FL | 73% | 84% | 62% |
| Seattle, WA | 73% | 83% | 62% |
| Portland, OR | 73% | 85% | 59% |
| Atlanta, GA | 68% | 81% | 52% |
| Dallas, TX | 65% | 82% | 49% |
| Denver, CO | 52% | 67% | 40% |
| Phoenix, AZ | 37% | 51% | 23% |
| Las Vegas, NV | 30% | 40% | 20% |
Source: NOAA-referenced normals (1961 to 1990 base period) compiled by CurrentResults.
Regional differences
The humidity data separates the country into distinct roof-algae zones. The Gulf Coast and Southeast combine the highest morning humidity in the country with warm summers, so roofs stay damp overnight and rarely dry fully in the afternoon. This matches Atlas’s identification of the Southeast as the biggest area of risk. Source: Atlas Roofing; NOAA-referenced normals.
The Pacific Northwest is the second pattern. Portland and Seattle post morning humidity of 83 to 85 percent and daily averages near 73 percent, as high as parts of the Southeast, but with cooler temperatures. Algae still grows there, and Atlas lists the Pacific Northwest in the danger zone, but cooler conditions generally slow colony development compared with the Gulf. Source: Atlas Roofing; NOAA-referenced normals.
The arid interior West is the low-risk pattern. Las Vegas, Phoenix, and Tucson sit at 30 to 38 percent daily humidity with afternoon readings in the low 20s, so roof surfaces dry quickly and algae struggle to establish. Source: NOAA-referenced normals.
The cool, humid North (Great Lakes and interior Northeast) is a mixed case. Cities such as Grand Rapids, Buffalo, and Rochester show high daily humidity, but colder winters push algae into dormancy for months, which slows the pace of visible streaking relative to the warm South. Source: Atlas Roofing (winter dormancy); NOAA-referenced normals.
Original synthesis: Roofing Brief calculations
The following rankings are original Roofing Brief calculations derived from public humidity normals and trade-body statements. They are analytical constructs, not published statistics, and each carries the limits noted.
Roofing Brief calculation 1: state roof-algae climate risk ranking
Logic: ARMA identifies moisture that dwells on roof surfaces as the driver of algae. In the available public dataset, morning relative humidity (measured 4 to 6 a.m.) is the closest proxy for how long a roof stays damp overnight. We ranked state representative cities by morning relative humidity normal. Inputs: NOAA National Climatic Data Center and World Data Center for Meteorology normals compiled by CurrentResults; ARMA guidance via Malarkey. Result: the top band is Mississippi (91 percent), Louisiana and Florida (87 percent), Georgia and South Carolina (86 percent), Arkansas and Oregon (85 percent). The bottom band is Arizona (53 percent), New Mexico and Colorado (60 percent), Wyoming (63 percent). Limitations: one representative city per state, which can understate or overstate a large state; 1961 to 1990 base period; humidity is a proxy for surface wetness, not a direct measurement; the ranking does not account for tree shade, roof slope orientation, or rainfall frequency.
Roofing Brief calculation 2: persistence-adjusted risk using the afternoon humidity floor
Logic: two metros can share a morning humidity value yet differ in how dry the roof gets at midday. Afternoon relative humidity acts as a dry-down floor. Metros where afternoon humidity stays high never allow the roof to dry, which extends the daily window algae can grow. Inputs: NOAA-referenced afternoon relative humidity normals via CurrentResults. Result: Gulf and Southeast metros combine high morning humidity with high afternoon floors, keeping roofs wet through the day (New Orleans 61 percent afternoon, Miami 62 percent, Jacksonville 58 percent, Tampa 57 percent, Orlando 56 percent, Houston 55 percent). By contrast, interior metros with similar morning humidity dry down much further by afternoon (Dallas 49 percent, Nashville 53 percent, Sacramento 46 percent), which shortens the daily growth window. Limitations: afternoon humidity does not capture rainfall or dew duration; marine-cooled California cities (Los Angeles 64 percent, San Diego 62 percent afternoon) show high floors but low rainfall and cool temperatures, so a high afternoon floor alone does not guarantee heavy algae. This calculation refines, and should be read alongside, calculation 1.
Roofing Brief calculation 3: the warm-humid algae belt
Logic: algae grow fastest where high sustained humidity meets warm temperatures, since Atlas notes the organism goes dormant in winter cold. We intersected the highest morning-humidity states with the warm-summer Gulf and Southeast region. Inputs: NOAA-referenced humidity normals via CurrentResults; Atlas Roofing regional and dormancy statements. Result: a three-band map. The core belt is the warm, very humid Gulf and Southeast (Mississippi, Louisiana, Florida, Georgia, South Carolina, Alabama, Arkansas, and the humid Carolinas and Tennessee). The secondary band is the cool, very humid Pacific Northwest (Oregon, Washington), where humidity is high but temperatures are lower. The slow-growth band is the cool, humid North (Great Lakes and interior Northeast), humid but subject to winter dormancy. Limitations: bands are qualitative groupings from state-level proxies, not a validated growth model; local microclimate can move any individual roof between bands.
Roofing Brief calculation 4: the algae warranty coverage gap
Logic: manufacturers price algae risk into warranty length, so the spread between brands quantifies how differently they treat the same hazard. Inputs: GAF and Owens Corning warranty documentation. Result: Owens Corning StreakGuard covers algae for 10 years, GAF StainGuard Plus for 25 years (10 years non-prorated), and GAF StainGuard Plus PRO for 30 years, a spread of up to 20 years between the shortest and longest stated coverage. Set against Malarkey’s finding that unprotected shingles in humid coastal climates can streak in as little as 3 years, the practical implication is that in a core-belt climate the algae warranty term is a more meaningful differentiator than in the arid West, where streaking is slow regardless. Limitations: warranty terms are the manufacturers’ own published figures and are subject to conditions, exclusions, and proration that vary by product and are not modeled here.
Prevention technology and cleaning
Algae-resistant shingles work by embedding copper or zinc granules alongside standard granules. When it rains, the metal granules release ions that inhibit algae spores. Malarkey uses 3M copper granules, and its Scotchgard Protector products contain at least a 10 percent blend of those granules. Source: Malarkey Roofing Products; CertainTeed. For existing streaks, ARMA recommends a 50:50 mix of laundry-strength liquid chlorine bleach and water applied without high pressure, since pressure washing strips protective granules. Source: ARMA via Malarkey.
Warranty coverage for algae is separate from the main shingle warranty and varies by brand and product label, from 10 years for Owens Corning StreakGuard to 25 and 30 years for GAF StainGuard Plus and StainGuard Plus PRO. Source: manufacturer warranty documentation. For a full brand-by-brand parse, see the Roofing Brief warranty database linked below.
Chart recommendations
- US map, state roof-algae risk bands. Data needed: state morning relative humidity normals plus warm/cool classification. Source: NOAA-referenced normals; Roofing Brief calculation 3. Insight: visualizes the core Gulf and Southeast belt versus the cool-humid Pacific Northwest and North. Citation-worthy because it is the first roof-specific algae risk map built from humidity normals.
- Scatter plot, morning versus afternoon humidity by city. Data needed: morning and afternoon RH for major cities. Source: NOAA-referenced normals. Insight: separates all-day-wet Gulf cities from morning-wet, afternoon-dry interior cities. Citation-worthy because it explains why two humid cities can differ in algae speed.
- Bar chart, algae warranty length by brand. Data needed: stated algae warranty years by product. Source: manufacturer warranty documentation. Insight: shows the 10-to-30-year spread. Citation-worthy as a quick buyer reference.
Methodology
Source selection prioritized primary and institutional sources: an academic microbiology reference for organism biology, the Asphalt Roofing Manufacturers Association for trade guidance, manufacturer technical and warranty documentation, and NOAA-referenced climate normals for humidity. Inclusion required that a statistic be traceable to a named source with a date or base period. We excluded unsourced roundup pages, contractor marketing claims without primary backing, and any figure we could not trace to a fetched source. Where a widely repeated claim (such as a specific decade of first notoriety) could not be verified against a primary source in this pass, it was omitted. The regional rankings are Roofing Brief calculations built from the humidity normals and are labeled as such throughout, with inputs and limitations stated. Humidity normals are from the 1961 to 1990 base period as compiled by CurrentResults from NOAA National Climatic Data Center and World Data Center for Meteorology data, and they use single representative cities per state. This last update was July 2026.
Source quality ranking
- Tier 1, primary and institutional: NOAA National Climatic Data Center and World Data Center for Meteorology humidity normals; Kenyon College MicrobeWiki (academic microbiology reference); Asphalt Roofing Manufacturers Association guidance.
- Tier 2, industry and manufacturer: Malarkey Roofing Products; GAF and Owens Corning warranty documentation; CertainTeed; Atlas Roofing.
- Tier 3, aggregation of primary data: CurrentResults compilation of NOAA humidity normals, used because it presents the underlying NOAA figures in comparable table form.
- Excluded: unsourced blog roundups, contractor pages without primary citations, AI-generated summaries, and unverifiable date claims.
Journalist-friendly additions
Most quotable statistics
- On unprotected roofs in humid coastal climates, black streaks can appear in as little as three years. Source: Malarkey Roofing Products.
- Atlas Roofing puts the roof-algae danger zone at more than 80 percent of the country, with the Southeast the biggest risk. Source: Atlas Roofing.
- Mississippi records the highest morning humidity normal among state representative cities at 91 percent, versus 53 percent in Tucson, Arizona. Source: NOAA-referenced normals; Roofing Brief calculation.
- Manufacturer algae warranties range from 10 years to 30 years for the same hazard. Source: manufacturer warranty documentation.
Data limitations
Humidity normals are a proxy for roof-surface wetness, not a direct measurement of it, and the compiled series uses a single representative city per state from the 1961 to 1990 base period. The rankings do not include rainfall frequency, dew duration, tree shade, or roof orientation, all of which move individual-roof risk. Warranty figures are manufacturers’ published terms and are subject to conditions and proration not modeled here. Algae growth speed also depends on temperature, so two equally humid regions can differ in how fast streaks appear.
Recommended dataset fields
State; representative city; morning relative humidity normal; afternoon relative humidity normal; daily average humidity; warm or cool summer classification; assigned risk band (core, secondary, slow-growth); base period; source.
Press summary (about 150 words)
Black roof streaks are colonies of a cyanobacterium, Gloeocapsa magma, that feeds on the limestone filler in asphalt shingles and shields itself with a dark pigment. Where it grows fastest depends on how long roofs stay damp. The Asphalt Roofing Manufacturers Association says coastal and humid climates are the most conducive, and Atlas Roofing places the biggest risk in the Southeast, within a danger zone it estimates at more than 80 percent of the country. Using NOAA-referenced humidity normals, The Roofing Brief ranks the Gulf Coast and Southeast as the core algae belt, led by Mississippi at 91 percent morning humidity, and the arid interior West as lowest risk, with Tucson at 53 percent. On unprotected roofs in humid climates, streaks can appear within three years. Manufacturer algae warranties for copper-granule shingles range from 10 to 30 years. Source list and Roofing Brief calculations included.
Five suggested headlines
- The US Roof-Algae Map: Where Black Streaks Hit Fastest in 2026
- Mississippi to Arizona: Ranking Every State by Roof-Algae Risk
- Why Gulf Coast Roofs Streak in Three Years and Desert Roofs Do Not
- The Roof-Algae Belt Covers 80 Percent of the Country. Here Is the Map
- Copper Granules and 30-Year Warranties: The Roof-Algae Defense Data
Frequently asked questions
What causes black streaks on roofs? Black streaks are colonies of Gloeocapsa magma, a cyanobacterium that forms a dark protective sheath. It is the most common cause of roof streaking. Source: Kenyon College MicrobeWiki; Atlas Roofing.
Is roof algae the same as mold? No. The black streaking organism is a cyanobacterium (blue-green algae), not mold. Moss is a separate plant that grows in thicker damp patches. Source: Kenyon College MicrobeWiki; Malarkey Roofing Products.
Which US region has the worst roof algae? The Gulf Coast and Southeast, which combine the highest sustained humidity with warm summers. Atlas Roofing names the Southeast the biggest area of risk. Source: Atlas Roofing; NOAA-referenced normals.
Which state has the highest roof-algae climate risk? By morning humidity normal, Mississippi ranks highest at 91 percent for its representative city, Jackson. This is a Roofing Brief calculation. Source: NOAA-referenced normals.
Do dry states get roof algae? Rarely and slowly. Arid interior cities such as Las Vegas (30 percent daily humidity), Phoenix (37 percent), and Tucson (38 percent) let roofs dry quickly, so algae struggle to establish. Source: NOAA-referenced normals.
How fast do black streaks appear? On unprotected roofs in coastal and humid climates, streaks can appear in as little as three years. Source: Malarkey Roofing Products.
Why do streaks show up on the north side first? North-facing slopes get less sun and stay damp longer, which favors colonization. Source: Kenyon College MicrobeWiki.
Do algae-resistant shingles work? They use copper or zinc granules that release ions to inhibit algae. Malarkey Scotchgard Protector products use at least a 10 percent blend of 3M copper granules. Source: Malarkey Roofing Products; CertainTeed.
How long is an algae warranty? It varies by brand: Owens Corning StreakGuard covers 10 years, GAF StainGuard Plus covers 25 years (10 non-prorated), and GAF StainGuard Plus PRO covers 30 years. Source: manufacturer warranty documentation.
How do you clean roof algae safely? ARMA recommends a 50:50 mix of laundry-strength liquid chlorine bleach and water, applied without high pressure, since pressure washing strips granules. Source: ARMA via Malarkey Roofing Products.
Related Roofing Brief reports and guides
- Algae Streaks on a Roof: Why They Form, How to Clean, and How to Prevent Return
- Best Roof Moss Killer (2026): Top Picks Ranked and Compared
- Soft Wash Roof Cleaning Cost in 2026: Price Per Square Foot, by Roof Type
- Asphalt Shingle Roof Runoff Pollution (2026): Granules, PAHs, and Metals in Stormwater
- The Roofing Warranty Comparison Database 2026
Reviewed by The Roofing Brief Team. Last reviewed July 2026.