Look at your roof from the street on a bright afternoon and you may see long dark streaks running down the slopes, usually on the side that never gets much sun. Many homeowners assume it is soot or a stain from the trees. In most Minnesota neighborhoods it is a living colony of algae, and it keeps spreading as long as the roof stays damp and shaded. Roof cleaning, sometimes called roof soft washing, is the careful removal of that growth without damaging the shingles underneath.
This page explains what is growing on your roof, why blasting it with a pressure washer is a bad idea, how a low-pressure soft wash approach works, and what cleaning can and cannot do. To skip ahead, get a free quote by calling or texting (651) 858-2232 or by using the form on the contact page. We will look at the roof, talk through the method, and confirm the plan with you before any work starts.
What the dark streaks on your roof really are
The black or dark gray streaks on asphalt shingle roofs across the Twin Cities are most often caused by a cyanobacterium commonly known as Gloeocapsa magma, often loosely called roof algae. It produces a dark protective sheath, which is why it looks like a stain rather than a plant. It arrives as airborne spores, lands on a shingle, and colonizes wherever there is enough moisture.
Algae streaks usually start as faint gray patches and then run downhill in lines, because rain carries cells down the slope and seeds new growth below. That is why the pattern looks like someone dripped dark paint from the ridge toward the gutter.
If the shingles still keep the rain out, does it matter? Two reasons say yes. Dark streaks absorb more heat than clean shingles, and a roof that looks tired drags down the curb appeal of the whole house, which counts when you are selling or refinancing. The growth also holds moisture against the shingle surface, and persistent dampness is where moss and lichen move in. Algae is mostly cosmetic at the start, but it is a sign the roof is staying wet.
Why the north side goes first
Walk around your home and compare the slopes. The north-facing side nearly always looks worst. In our latitude the sun sits in the southern half of the sky, so north slopes get far less direct light and heat. After rain, dew or melting snow they stay wet longer, and algae and moss thrive on a slow-drying surface.
Other patterns follow the same logic. Slopes under large oaks, maples or spruce stay damp even on the south side. The lower edge of a roof, where water slows near the gutter line, often shows heavier growth. Valleys, where two slopes shed water together, hold moisture and debris. Homes near the river bluffs and lakes, such as many around Hastings and the Mississippi and St. Croix valleys, see more morning humidity and fog, which keeps shingles damp longer.
Moss and lichen are a different problem
Algae sits on the surface. Moss and lichen deserve more attention. Moss grows in thick green or brown cushions, usually in shady areas, along shingle tab edges and in valleys where debris collects. Its root-like structures grip the granule surface and the gaps between shingles, and as it grows it holds water against the roof and can lift a shingle tab slightly. Lichen shows up as crusty gray, green or orange patches that look painted on, and it can be stubborn to remove completely.
Resist the urge to scrape moss off with a stiff brush or putty knife. Dry scraping pulls granules away with the moss. A proper approach treats the growth first so that it releases, then rinses it off gently. A heavy moss mat may need more than one visit, because treatment works over days and weeks and the dead moss loosens gradually with rain and wind.
Why pressure washing shingles is a bad idea
This is the most important point on this page. A pressure washer is the wrong tool for an asphalt shingle roof. Shingles are a fiberglass mat coated with asphalt and covered with small ceramic granules. Those granules protect the asphalt from sunlight and give the shingle its color. A high-pressure stream blasts granules away, and once they are gone they do not come back.
Pressure can also force water upward under shingle tabs, toward the underlayment and roof deck, which is the opposite of how a roof is built to shed water. A blasted roof can look clean for a few weeks and then wear out sooner than it should. And a wet shingle roof is slippery, so a person holding a powerful wand on a slope has very little margin for error.
For these reasons, many shingle manufacturers and roofing trade groups advise against pressure washing a shingle roof. If a company offers to pressure wash your shingles, ask how they will protect the granules. If the answer is vague, keep looking. Pressure has its place on hard surfaces, which you can read about on our pressure washing page, but a shingle roof is not one of them.
How a low-pressure soft wash approach works
Soft washing flips the logic. Instead of using force to scrub growth off, it uses a cleaning solution to loosen and kill algae, moss and lichen, then rinses with gentle, low-pressure water, closer to a garden hose than a pressure washer. The solution needs time to do the work, not the water stream.
In general, a proper soft wash roof visit looks like this:
- Look over the roof. Slope, pitch, shingle condition, nearby plants and how heavy the growth is all get considered before anything is mixed.
- Protect what needs protecting. Plants, shrubs, outdoor furniture and anything near the roofline are wet down and covered or moved as needed, since runoff can reach the garden.
- Apply the solution at low pressure. It goes on evenly over the affected slopes and is allowed to sit.
- Rinse gently. Low-pressure water carries the loosened growth down, taking care not to push water under the shingles.
- Check and clean up. The ground, gutters and downspouts are rinsed and checked for debris.
The exact method is confirmed when you get the quote, because it depends on what is growing and what surrounds the house. Results can keep improving for days or weeks after a visit as treated growth breaks down and rain washes it off, so a roof may not look perfect the moment the work ends. A good provider tells you that up front. A visit often pairs with gutter cleaning and house washing.
Shade, tree cover and zinc or copper strips
Cleaning removes the growth that is there today. It does not change the conditions that grew it. Here is what slows the return:
- Trim overhanging branches. Letting sun reach the roof is the most effective step.
- Clear debris from valleys and edges. Leaves, maple seeds, cottonwood fluff and pine needles hold moisture and feed moss.
- Keep gutters flowing. Overflowing gutters keep the eave damp.
Mature trees shading north slopes are common in older parts of Woodbury and Cottage Grove, so regrowth is a real consideration there. How long a cleaning lasts depends on how much light and airflow the roof gets, and an honest provider will say so.
Another idea you may have heard about is that metal strips near the ridge can slow algae and moss. When rain falls on a zinc or copper strip, tiny amounts of metal wash down the slope below it, and those ions discourage growth. Roofing suppliers sell strips made for this purpose.
The effect only reaches the area below the strip and may not stop growth on very shaded roofs. Strips are also a roofing task, because they are fastened near the ridge cap and the fasteners must be sealed correctly. Raise the idea with a roofer and ask whether it suits your pitch and shading.
What winter does to a roof
Snow that sits on a shaded north slope for weeks keeps shingles cold and wet, which suits moss. When snow melts in the afternoon and refreezes overnight, meltwater freezes at the eaves, and ice dams can push water back under the shingles. Water trapped in moss expands as it freezes, which can loosen granules over time.
Roof cleaning belongs to the warmer part of the year, when shingles are dry enough to work on safely and temperatures are above freezing. A responsible provider will not work on a roof with snow, ice or frost on it.
Ladders, slopes and roof safety
Wet algae is one of the slipperiest surfaces you will ever stand on. We strongly recommend homeowners do not climb onto a streaked or mossy roof.
- Steep roofs are harder to work on, and some need specialized fall protection.
- Never go onto a damp, mossy or frosty roof.
- Power lines, skylights, solar panels and fragile vents all need attention before anyone works near them.
Ask any provider how they plan to work safely on your roof pitch. If the answer does not include a plan for fall protection, that is a warning sign.
Shingle manufacturer terms and what to ask
Asphalt shingles come with manufacturer documentation, and the terms differ between brands and product lines. Some mention algae resistance and some say nothing about cleaning methods. We cannot speak for any manufacturer, so check the source. A short list of questions before cleaning a roof, especially a newer one:
- Does the manufacturer say anything about cleaning methods, solutions or pressure?
- Is there a recommended approach for algae, moss or lichen?
- Does the roofer who installed it have a view on cleaning?
- Can you find the shingle brand and product name in your closing papers or installer paperwork?
If you have the documents, share them when you ask for a quote and we will talk through the plan with you.
What roof cleaning cannot fix
Here is what cleaning will not do:
- Restore lost granules. Bare patches, shiny spots or granules piling up in the gutters are wear, and cleaning cannot replace them.
- Repair damage. Cracked, curled or missing shingles, failed flashing and loose fasteners need a roofer.
- Stop a leak. If water is getting into the house, call a roofer promptly.
- Make an old roof look new. Heavy, long-term growth may leave a faint shadow.
- Control how long the roof stays clean. Shade, moisture and nearby spores decide how soon growth returns.
If we see signs of damage while looking at the roof, we will tell you, even if it means the job is not the right fit.
How to get a free quote
Call or text (651) 858-2232, or fill out the quote form on the contact page. Tell us which side of the house is affected and roughly how old the roof is. We will look at the roof, talk through the method and confirm the plan before work begins.
We serve homes across the south and east metro, including Hastings, Eagan, Lakeville and the rest of our service areas. If you are not sure your roof is a candidate, ask. Telling you a roof needs a roofer first is part of the service.