Black streaks on a Bellevue roof are algae, and the safe way to remove them is a soft wash: a cleaning solution applied at low pressure, left to work, then rinsed off gently. Pressure washing is the wrong tool for asphalt shingles. The Asphalt Roofing Manufacturers Association says high pressure washers are likely to damage asphalt roofing and should not be used on it at all.
Here is what the streaks are, why the two methods are so different, and what a roof wash costs.
What the black streaks are
The dark stripes running down a shingle roof are not dirt, soot or mold from the attic. According to ARMA, the most common cause is an algae called Gloeocapsa magma. It travels through the air, settles on roofs and grows best where conditions stay warm and humid. As it grows it turns light shingles brown to black, either evenly or in streaks.
A few things help you tell algae from other problems:
- Algae covers large areas or runs in vertical streaks, and shows most on light gray, tan and white shingles.
- Moss is green and grows in raised clumps in one area, mostly in shade.
- Tree droppings and leaf stains show up in spots under overhanging branches, not across a whole slope.
Why Bellevue roofs streak
Bellevue has more mature trees than most of the metro. Fontenelle Forest runs along the Missouri River on the east side of town, and the neighborhoods near it, Haworth Park and the bluffs sit under big oaks, maples and cottonwoods. Shade keeps a roof damp for longer after rain and morning dew, and damp shingles are where algae and moss get started.
We are based in Bellevue, and that matches what we see on the job: the roofs and siding with the most streaking are the ones under tree cover and on the north side of the house.
Soft washing vs pressure washing
| Soft washing | Pressure washing | |
|---|---|---|
| What does the work | A cleaning solution that kills the algae | Force from a high pressure stream |
| Water pressure | Low | High |
| Effect on shingles | Granules stay on the shingle | Likely to damage asphalt roofing (ARMA) |
| Where we use it | Roofs, siding, painted surfaces, wood | Concrete and stone |
Asphalt shingles are surfaced with small mineral granules. ARMA warns against power washers and against brushes or brooms on a roof, because force and friction loosen and remove those granules. A pressure washer can make a roof look clean for a day and leave it with less protection than it had before.
Soft washing gets the same visual result with chemistry instead of force. ARMA's own guidance for lightening algae stains is a solution of household bleach (sodium hypochlorite) and water, applied gently, left on for 15 to 20 minutes, then rinsed with a gentle spray. It also points out that the process makes a roof slippery and recommends hiring trained professionals instead of walking the roof yourself.
How a roof wash works
Our pressure washing and soft washing service follows three steps on a roof:
- Treat. We apply a cleaning solution matched to the surface that kills the algae and loosens the staining.
- Wash. Low pressure only. The solution does the cleaning, with no scrubbing and no high pressure on the shingles.
- Rinse. A low pressure rinse carries it off, and we rinse plants, walkways and windows below so nothing is left behind.
Moss is a separate case. ARMA advises checking with your shingle manufacturer before removing moss, so if your roof has thick moss as well as streaks, tell us when you ask for a quote.
How long a clean roof stays clean
ARMA is clear that cleaning is temporary: the algae is in the air, so it comes back over time. How fast depends on shade and moisture. A roof under heavy trees streaks again sooner than an open roof in full sun.
If your roof is near the end of its life anyway, ARMA notes that algae resistant shingles are available and are designed to hold off algae growth for extended periods. For roof replacement or gutter replacement we recommend our partners at ProGrade Exteriors.
What a roof wash costs
Roof washing with Fontenelle runs $500 to $1,200+. The price depends on the size of the roof and how much buildup there is. A house wash for the siding runs $300 to $800+, and it often makes sense to do both at once, since the same algae grows on north facing siding.
Under all those trees, the gutters are often due at the same time. Ask for a gutter cleaning in Bellevue on the same quote, and add a window cleaning if the glass needs it after the wash. Members of our membership program save 15% to 30% on every cleaning.
Getting a quote
Send us your address and a photo of the roof if you have one, and we will text you a price. Request a free quote and we will tell you whether your roof needs a soft wash, a moss check or just a closer look.
Want the crew to handle it?
Fontenelle Window Cleaning serves Omaha, Bellevue, Papillion and the whole metro. Free quotes by text, usually within the hour.
