How to Remove Moss and Algae from Your Roof
How to Remove Moss and Algae from Your Roof
Moss, algae, and lichen are more than cosmetic problems. Moss roots (technically rhizoids) pry up shingle edges, allowing water underneath. Algae streaks trap moisture and accelerate granule loss. Left untreated, biological growth can shorten your roof's lifespan by years. This is an especially common issue in the Pacific Northwest, Southeast, and any humid climate with shade and moisture.
Estimated time: 2-3 hours (plus drying and rinsing time)
Estimated cost: $20-$60 DIY vs $250-$600 professional treatment
Identifying the Problem
Three types of biological growth commonly appear on roofs. Each requires a slightly different approach:
Moss
- Appearance: Thick, green, spongy growth, usually in raised clumps or mats
- Location: Shaded, north-facing slopes; areas under tree canopy; anywhere moisture lingers
- Damage: Rhizoids work under shingle edges and lift them, allowing wind-driven rain underneath. Thick moss holds moisture against the roof surface, accelerating decay of the decking and underlayment
- Severity: Moderate to high -- active damage to shingles
Algae (Black Streaks)
- Appearance: Dark black or dark green streaks running vertically down the roof, usually starting near the ridge
- Cause: A cyanobacterium called Gloeocapsa magma that feeds on the limestone filler in asphalt shingles
- Location: North-facing and shaded slopes, but can appear anywhere in humid climates
- Damage: Primarily cosmetic, but the dark color absorbs heat (increasing cooling costs) and the organism's acids slowly degrade shingle granules
- Severity: Low to moderate -- slow damage, mainly aesthetic
Lichen
- Appearance: Crusty, light green, gray, or yellowish patches that are tightly bonded to the shingle surface
- Location: Similar to moss -- shaded, moist areas
- Damage: Very difficult to remove without damaging shingles. Lichen bonds aggressively to granules and pulls them off when removed
- Severity: Moderate -- removal itself can cause damage, so prevention is key
What You'll Need
Tools
- Pump sprayer (1-2 gallon garden sprayer) or hose-end sprayer
- Garden hose with standard nozzle (not a pressure washer)
- Soft-bristle push broom or long-handled brush
- Ladder with stabilizer
- Safety glasses or goggles
- Rubber gloves (chemical-resistant if using bleach)
Materials
Choose one of the following cleaning solutions:
- Bleach solution (most effective): 50% household bleach (sodium hypochlorite) mixed with 50% water. Add 1/4 cup of trisodium phosphate (TSP) per gallon for extra cleaning power.
- Commercial moss killer: Products like Wet & Forget, Spray & Forget, or Moss Out are formulated for roofs. Follow the label instructions.
- Vinegar solution (gentler alternative): Full-strength white vinegar in a pump sprayer. Less effective than bleach but safer for surrounding vegetation.
For Prevention (after cleaning)
- Zinc strips or copper strips (6-inch rolls)
- Roofing nails or screws to attach strips
- Roofing sealant to seal nail holes
Best Timing
- Season: Spring or fall, when temperatures are moderate (50-70°F / 10-21°C). Avoid hot summer days -- bleach solution evaporates before it can work, and hot shingles scuff easily underfoot.
- Weather: Choose an overcast day with no rain expected for at least 4-6 hours after application. Direct sun degrades the bleach too quickly. Rain washes the solution off before it kills the growth.
- Time of day: Early morning or late afternoon is ideal to avoid rapid evaporation.
Step-by-Step Instructions
Step 1: Protect Landscaping Below
Bleach solution running off the roof will harm or kill plants, grass, and shrubs below.
- Soak all plants and ground vegetation within 10 feet of the roof edge thoroughly with plain water from a garden hose. Saturated plants absorb less bleach.
- Cover garden beds, shrubs, and sensitive plants with plastic tarps or drop cloths.
- If you have a rain barrel connected to your downspouts, disconnect it before spraying.
If using a commercial product or vinegar solution, check the label for vegetation safety. Most commercial moss killers are less harmful to plants than bleach but still warrant precaution.
Step 2: Prepare the Cleaning Solution
Mix your chosen solution and fill your pump sprayer:
- Bleach: Combine equal parts bleach and water in the sprayer. For a 2-gallon sprayer, use 1 gallon of bleach and 1 gallon of water. Add TSP if desired and stir gently.
- Vinegar: Pour full-strength white vinegar directly into the sprayer.
- Commercial product: Follow the manufacturer's mixing instructions.
Step 3: Apply the Solution
Working from a ladder positioned at the roof edge, or from a low section of roof near the eave (only on gentle pitches below 6:12), spray the solution onto the affected areas. Apply from the bottom up to prevent the solution from running under shingles and to ensure even coverage. Saturate the moss or algae thoroughly -- the surface should be visibly wet.
Do not walk on moss-covered areas. They are dangerously slippery. Use your sprayer's reach to cover as much area as possible from a safe position. Reposition the ladder as needed.
Step 4: Let the Solution Work
- Bleach solution: Wait 15-20 minutes. Do not let it dry completely -- if it starts drying in spots, lightly re-mist those areas.
- Vinegar: Wait 30 minutes to 1 hour. Vinegar is a weaker acid and needs more contact time.
- Commercial products: Follow the label. Some products (like Wet & Forget) are spray-and-leave formulas that work over several weeks with rain exposure -- no rinsing required.
Step 5: Rinse Gently
Using a garden hose with a standard nozzle, rinse the treated area from the top down, letting water flow in the natural direction of the roof slope. Use moderate pressure -- enough to wash away dead moss and solution but not enough to blast granules off shingles.
NEVER use a pressure washer on an asphalt shingle roof. The high-pressure stream strips granules, damages shingle surfaces, forces water under shingles, and can void your roof warranty.
Step 6: Remove Remaining Moss Debris
For thick moss, the bleach treatment kills the moss but does not always remove the bulk. After rinsing, use a soft-bristle push broom to gently sweep dead moss downward off the roof, following the direction of the shingle overlap. Do not brush upward -- this lifts shingle edges. Collect debris from the gutters and ground below.
Step 7: Rinse Landscaping
Immediately after the roof rinse, thoroughly hose down all vegetation, tarps, and ground around the house to dilute any bleach runoff. Remove tarps from plants. Water the surrounding soil generously.
Step 8: Inspect the Shingles
Once the roof has dried, inspect the cleaned areas for shingle damage. Moss that was growing for years may have already lifted or cracked shingles. Damaged shingles should be replaced -- see How to Replace Damaged Shingles. Look for areas of significant granule loss, which may indicate the shingles are nearing end of life (see Signs You Need a New Roof).
Prevention Methods
Cleaning removes existing growth, but without prevention, moss and algae will return within 1-3 years.
Zinc or Copper Strips
Metal strips installed along the roof ridge are the most effective long-term prevention. When rain washes over the strips, it carries trace metal ions down the roof surface, creating an environment hostile to moss, algae, and lichen.
- Cut 6-inch-wide zinc or copper strip to the length of the ridge.
- Lift the ridge cap shingles (see How to Install Ridge Cap Shingles) and slide the strip underneath so that 2-3 inches of metal are exposed below the ridge cap.
- Nail or screw the strip in place and seal fastener holes with roofing sealant.
- Reinstall the ridge cap over the top portion of the strip.
Copper is more effective than zinc but costs more. Both last 20+ years.
Other Prevention Strategies
- Trim overhanging tree branches to increase sunlight and airflow on the roof surface. Sunlight and dry conditions discourage biological growth.
- Keep gutters clean (How to Clean Gutters Safely) -- clogged gutters cause water to back up under the eave, creating persistent moisture.
- Improve attic ventilation to reduce moisture buildup on the roof surface.
- Consider algae-resistant shingles when it is time for replacement. These shingles contain copper granules that inhibit Gloeocapsa magma growth. See Asphalt Shingles.
What About Lichen?
Lichen is the most difficult growth to remove and often causes shingle damage during removal. If lichen is present:
- Apply the bleach solution as described above. Lichen will die but remain attached.
- Do not scrape or scrub lichen aggressively. It will pull granules off the shingle surface.
- Let dead lichen weather off naturally over several months, or accept the cosmetic issue.
- If lichen coverage is extensive, consult a professional to evaluate whether shingle replacement is more cost-effective than treatment. See Roof Repair vs Replacement.
