Moss or dark streaks on the shingles
Black streaks are algae and ARMA says there is no scientific evidence they damage shingles. Moss is different: it lifts shingle edges and raises the risk of blow-off, so it goes on the list before winter.
What is probably happening
- Dark streaks, usually on the north or shaded side, are algae. Cosmetic, per ARMA.
- Green cushions are moss. ARMA: moss can cause the leading edges of shingles to lift or curl, which increases the risk of blow-off in wind, and severe cases can lead to leaks or deck damage.
- Both mean shade, damp and debris. Overhanging branches and full gutters feed them.
Do this in the next five minutes
- Photograph the roof from the ground. Note whether it is streaks (algae) or cushions (moss).
- Clear the gutters and cut back branches that shade the roof. ARMA lists this as the prevention that works.
- For a cleaning, get quotes for a soft wash. ARMA's method is a 50:50 mix of laundry-strength liquid chlorine bleach and water, left 15 to 20 minutes, then rinsed at low pressure.
- If the roof is already at the end of its life (see the repair-or-replace page), skip the cleaning and put the money toward the roof.
Do not
- Do not pressure wash. ARMA: it will cause granule loss and very likely premature failure of the roof system.
- Do not climb up with a garden sprayer. Wet, mossy shingles are the slickest surface on the house.
- Do not let anyone install zinc or copper strips on an existing roof. ARMA does not recommend it because of the leak and wind-damage risk from the fasteners.
- Do not rinse bleach into plant beds without wetting them down first.
Who to call
A roof-cleaning company that soft washes, or a roofer
Cleaning is not licensed roofing work in most states, but a roofer sees lifted and cracked shingles under the moss that a cleaner will not.
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What it should cost
$296 to $626
Roof cleaning, typical national range, average $461. Soft wash on a 1,600 square foot roof $560 to $900.
Source: Angi, roof cleaning cost, updated July 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the roofing page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- What is in your cleaning solution, and what pressure do you rinse at?
- How will you protect my plants, gutters and downspouts from the runoff?
- Will you tell me if you find lifted or cracked shingles under the moss?
- Does cleaning affect my shingle manufacturer's warranty?
End the call if
- A pressure washer on the truck.
- Zinc strips as part of the quote.
- A new-roof pitch for a roof that only has algae streaks.