ROOF CLEANING & MAINTENANCE
How to Clean Roof Shingles: Safe Moss & Algae Removal
Black streaks, green moss, and lichen on asphalt shingles are caused by biological growth that feeds on moisture and limestone filler in shingles. Cleaning your roof using Asphalt Roofing Manufacturers Association (ARMA) approved methods restores appearance without damaging protective granules.
CRITICAL WARNING: Never Use a Pressure Washer
Never use a high-pressure power washer or stiff wire brush on asphalt shingles. High-pressure water instantly dislodges the ceramic protective granules, exposing the underlying asphalt to accelerated UV breakdown and immediately voiding manufacturer warranties.
Understanding Roof Stains: Algae vs. Moss vs. Lichen
1. Black Algae (Gloeocapsa magma)
Vertical black streaks on north/shaded slopes. Superficial bacterial growth feeding on limestone granules. Responds rapidly to 50:50 bleach solution.
2. Green Clumped Moss
Thick vegetative pads with root-like rhizoids. Traps moisture and lifts shingle edges during winter freeze-thaw. Requires gentle brushing and chemical treatment.
3. Crustose Lichen
Circular symbiotic crusts anchored into granule matrix. Do NOT scrape forcefully when dry; treat with solution and let natural rain wash loose.
- Black Algae (Gloeocapsa magma): Airborne cyanobacteria that forms dark vertical streaks across north-facing or shaded roof slopes. Primarily cosmetic in early stages.
- Green Moss: Rooted vegetative growth that holds moisture against shingles and lifts shingle edges during winter freeze-thaw cycles, creating severe leak paths.
- Lichen: Symbiotic fungus/algae organism that anchors firmly into shingle granules and is difficult to remove without chemical treatment.
The ARMA-Approved Roof Cleaning Method
- Protect Surrounding Landscaping: Thoroughly wet all shrubs, flowers, and grass beneath the eaves with clean water before and after application. Cover delicate plants with plastic sheeting.
- Mix the Cleaning Solution: In a standard low-pressure pump sprayer, mix:
- 50% Household Laundry Bleach (Sodium Hypochlorite 5.25%–6%)
- 50% Clean Water
- Optional: 1 tablespoon of gentle, non-ammonia dish soap as a surfactant to help the solution cling to sloped shingles.
- Apply from Bottom to Top: Spray the cleaning solution onto dry shingles on an overcast, calm day (avoid hot direct sunlight which evaporates the solution before it sanitizes).
- Allow 15 to 20 Minutes Dwell Time: Let the solution work to eradicate the algae and moss spores. You will observe dark streaks lightening.
- Rinse with Gentle Garden Hose Pressure: Rinse the roof thoroughly with clean water using a standard garden hose nozzle from the top of the roof down.
Preventing Future Moss & Algae Growth
- Zinc or Copper Strips: Install a 2-inch strip of galvanized zinc or copper flashing along the roof ridge. Rainwater reacting with the metal releases microscopic metallic ions that inhibit biological growth down the slope.
- Trim Overhanging Branches: Trim tree limbs to allow sunlight and airflow to dry roof surfaces quickly after rain.
- Keep Gutters Clear: Clean out gutter troughs regularly to prevent standing moisture along eaves.
Related Maintenance Guides
Learn more about shingle lifespans in our How Long Does a Shingle Roof Last Guide or check Types of Roof Shingles.
Targeted Treatment by Growth Type: Algae vs. Moss vs. Lichen
Different organic roof growths require distinct chemical and physical handling methods:
- Blue-Green Algae (Gloeocapsa magma): Manifests as dark vertical streaks. Per the Asphalt Roofing Manufacturers Association (ARMA) Technical Bulletin on Algae Discoloration, treat specifically with a 1:1 solution of household laundry bleach (5.25% to 6% sodium hypochlorite) and clean water. Apply gently with a low-pressure garden pump sprayer, allow a 15- to 20-minute dwell time, and rinse thoroughly with a low-pressure garden hose.
- Moss (Bryophyta): Forms thick vegetative mats with root-like rhizoids that anchor into granules. Apply the ARMA bleach solution or a manufacturer-approved gentle moss remover to kill the root system. Do not scrape aggressively; allow dead moss to weather off naturally or brush lightly with a soft-bristle broom after complete drying.
- Lichen: Symbiotic fungal/algal organisms with deep crustose attachment. Requires repeated gentle soaking with ARMA chemical solution; never pry or pick dry lichen as it will tear away protective shingle mineral surfacing.
Universal Precautions: Never use a high-pressure washer (which immediately strips mineral granules and voids shingle warranties). Always pre-wet and post-rinse surrounding plants and grass with fresh water, flush aluminum gutters and metal flashings immediately to prevent chemical spotting, and review your specific shingle manufacturer’s warranty maintenance guidelines before chemical application.