How can I prevent moss and algae from growing on an asphalt shingle roof?

Short Answer

Prevent moss and algae on asphalt shingles by keeping the roof dry and clean: trim overhanging branches, clear leaves and needles, ensure good attic ventilation, and keep gutters flowing. Install copper or zinc strips near the ridge so rain releases ions that inhibit growth. If discoloration has started, apply a roof-safe bleach solution and rinse gently—never pressure wash. For long-term control, consider algae-resistant (AR) shingles during your next re-roof.

Why Moss and Algae Grow

• Shade + moisture: Overhanging trees, clogged gutters, and poor ventilation keep shingles damp.
• Nutrients: Organic debris (leaves, needles) feeds growth.
• North-facing slopes: These stay cooler and wetter, making them prime spots for algae and moss.

Tools and Materials

• Extension ladder with stabilizer/stand-off
• Fall arrest harness with roof anchor and lifeline
• Non-slip footwear, gloves, safety glasses
• Pump sprayer (garden type)
• Garden hose with gentle spray nozzle (no pressure washer)
• Soft brush or plastic scraper
• Plastic sheeting/tarps to protect plants
• Bleach (5–6% sodium hypochlorite) and non-ammonia surfactant, or oxygen-based cleaner (for light growth)
• Zinc or copper roof strips (2–3 in exposed), corrosion-resistant roofing nails, roofing sealant
• Pruning tools (pole saw, loppers)

Preventive Steps (Most Effective)

1. Increase sun and airflow
   • Trim branches back 6–10 ft from the roof to reduce shade and leaf litter.
   • Avoid letting vines or ivy touch roofing.
2. Keep the roof clean and dry
   • Blow off leaves/needles seasonally with a leaf blower from a stable ladder or while tied-in. Work from the ridge downward.
   • Clean gutters and downspouts twice a year so water drains quickly.
3. Improve attic ventilation
   • Balanced intake and exhaust keeps the roof deck drier and cooler. If you see condensation or hot attic temps, add soffit vents, ridge vent, or box vents as appropriate.
4. Install zinc or copper strips (proven long-term control)
   • Tuck strips under the ridge cap with 2–3 inches exposed. Rainwater carries metal ions down the roof, inhibiting growth.
   • Copper is generally more effective than zinc on algae; both deter moss.
   • For long roof runs, add a second row halfway down.
5. Choose algae-resistant shingles when re-roofing
   • AR shingles contain copper-infused granules that inhibit staining for years.

Cleaning and Treatment (If Growth Exists)

Follow manufacturer guidance (ARMA recommendations align closely with this approach). Work on a cool, overcast day with no wind.

Mix (for dark streaking/algae and light moss)

Per gallon of solution:
- 1/2 gallon household bleach (5–6% sodium hypochlorite)
- 1/2 gallon water
- 1/2 cup non-ammonia surfactant (TSP substitute or non-ammonia dish soap)

Never mix bleach with ammonia or acids. Pre-wet plants and soil; cover delicate landscaping with plastic and remove covers immediately after rinsing.

Application Steps

1. Safety set-up
   • Ladder at 4:1 angle, extending 3 ft above the eave; use a stabilizer.
   • Tie off to a properly installed roof anchor; wear a harness and non-slip shoes.
   • Avoid working on wet, icy, or very hot shingles.
2. Debris removal
   • Gently brush or blow off loose debris. Don’t pry up shingle edges.
3. Apply cleaner
   • Using a pump sprayer, apply the solution from the ridge down. Keep runoff controlled; work in small sections.
   • Dwell time: 15–20 minutes. Keep the surface damp with light mists—don’t let it dry.
4. Rinse
   • Rinse with a garden hose, low pressure, top-down. Avoid forcing water under shingles.
5. Stubborn moss
   • After treatment, dead moss will brown and loosen over a few weeks. Then lightly brush downward to remove. Don’t scrape aggressively.

For lighter, plant-friendly cleaning, oxygen-based cleaners (sodium percarbonate) can help with surface algae, but they’re weaker on established moss and stains.

Installing Zinc or Copper Strips

• Plan to expose 2–3 inches of metal below the ridge line.
• Slide the strip under the uppermost shingle course or ridge cap; fasten with corrosion-resistant roofing nails at manufacturer spacing.
• Seal nail heads with a dab of roofing sealant.
• Typical cost: $2–5 per linear foot for material; professional installation can run $8–15 per linear foot.

Safety Considerations

• Work with a helper and keep children/pets clear.
• Fall protection is non-negotiable on anything steeper than a walkable pitch. Roof pitch quick check: - ≤ 4:12 (low slope): still use caution - 5:12 to 6:12: harness strongly advised - > 6:12: hire a pro or use full fall protection with roof jacks/scaffolding
• Do not pressure wash asphalt shingles; it can strip granules and void warranties.
• Protect landscaping: pre-wet, cover, and post-rinse thoroughly.

Tips for Best Results

• Treat on a cool, cloudy day so the solution doesn’t evaporate quickly.
• Start near the ridge and work downslope to avoid stepping on treated, slippery areas.
• Rinse siding, gutters, and metal surfaces to minimize staining or corrosion from runoff, especially with copper-based methods.
• Schedule preventive maintenance each spring or fall to keep debris off the roof.

Common Mistakes to Avoid

• Pressure washing: Removes protective granules and shortens roof life.
• Harsh scraping: Lifting shingles can break seals and invite leaks.
• Mixing chemicals: Bleach + ammonia creates toxic fumes.
• Skipping plant protection: Bleach solutions can damage landscaping.
• Ignoring ventilation and gutters: Moisture issues will keep bringing growth back.

When to Call a Pro

• Steep or high roofs, multi-story access, or fragile/brittle shingles.
• Extensive moss that has lifted shingles or caused water intrusion.
• If you’re not equipped for fall protection or unsure about chemical handling.

Time and Cost Snapshot

• DIY cleaning on an average single-story roof: 2–4 hours plus setup; $40–$120 in materials (sprayer, cleaner, tarps).
• Pro cleaning: roughly $300–$800 depending on size and access.
• Zinc/copper strip retrofit: $2–5/lf materials; pro install $8–15/lf.

By combining shade reduction, dry roof practices, gentle cleaning, and long-term ion strips (or AR shingles), you’ll keep asphalt shingles looking good and performing well while avoiding damage.