You look up at your roof one day and see dark streaks, green patches, or a general layer of grime. Your first instinct is to clean it. Hold on. Knowing what you are looking at and how to address it correctly is the difference between a clean roof and one that is years shorter-lived.
What those dark streaks are
The black or dark gray streaks on roofs are almost always algae, specifically a type called Gloeocapsa magma. It feeds on the limestone filler in asphalt shingles and thrives on north-facing slopes and shaded areas where moisture lingers. It is extremely common. Look around your neighborhood and you will probably see it on several roofs.
The algae itself does not damage shingles in the short term. It is primarily a cosmetic issue. Over many years, heavy algae growth can retain moisture against the shingle surface, which may accelerate granule loss, but the timeline is long.
What about moss?
Moss is a different story. Unlike algae, which sits on the surface, moss grows roots that work their way under shingle edges and lift them. Heavy moss growth creates gaps where water can penetrate. It also retains significant moisture against the shingle surface.
If you have moss on your roof, it should be addressed. But how you address it matters.
What NOT to do
Do not pressure wash your roof. This is the most common mistake homeowners make. A pressure washer will strip granules off your shingles faster than 10 years of UV exposure. Those granules are the protective coating that keeps your shingles alive. Once they are gone, the asphalt underneath is exposed to the sun and deteriorates rapidly. Pressure washing a roof to make it look better actually shortens its life significantly.
Do not scrub the shingles. Walking on the roof and scrubbing with a stiff brush has the same effect as pressure washing. You are removing granules and damaging the shingle surface.
Do not use bleach at full strength. Concentrated bleach can discolor shingles, corrode metal flashing, and damage landscaping below.
"Pressure washing a roof to make it look better actually shortens its life. The cure is worse than the condition."
What you can do
If the algae or moss bothers you enough to address it, the safest approach is a gentle application of a diluted cleaning solution designed for roofs. A mixture of water and oxygen bleach, or a commercial roof cleaning product, applied with a low-pressure garden sprayer and allowed to sit can kill algae and loosen moss without damaging the shingles.
For moss, after the cleaning solution has done its work, the dead moss can be gently brushed off with a soft broom, working downward with the shingle grain, never against it.
For most Utah homeowners, cleaning is not necessary unless the moss is heavy enough to be causing problems. Algae streaks are cosmetic. The risk of damaging your shingles during cleaning often outweighs the cosmetic benefit.
Prevention is easier than cleaning
Trim back tree branches to increase sunlight and airflow on the roof surface. Zinc or copper strips installed along the ridge can also help. When it rains, trace amounts of metal wash down the roof surface and inhibit algae and moss growth.
Some shingle products, including several CertainTeed lines, include algae-resistant granules in the manufacturing process. If algae is a persistent problem and you are approaching a replacement, it is worth asking about algae-resistant options.
Not sure if your roof needs cleaning or just an inspection? We will tell you honestly whether those dark streaks are a cosmetic issue or something that needs attention. Contact us for a free assessment.
"Did a great job. Friendly and fast. Fair price."Lee Rackham , Google Review