Black streaks on the roof: what they are, whether they matter, and how they come off

Short answer: it is algae, and it is fixable

Black or dark brown streaks running down a shingle roof are almost always algae, not dirt, soot or a manufacturing defect. The Asphalt Roofing Manufacturers Association says natural pigments produced by these algae can gradually turn light-colored roofs dark brown or black, in a uniform shade or in streaks. It is not an emergency, but it should be removed with a low-pressure soft wash, never a pressure washer, and the sooner it is treated the simpler the job.

At Park Cities Power Wash, roof cleaning is one of the most common add-ons to a house wash, because once the stone is bright again the roof suddenly looks ten years older than the walls. This guide explains what is growing up there, how to tell algae from other roof problems, what you can safely do yourself and how a professional soft wash removes it without harming the shingles.

What is actually growing on the shingles

According to the same ARMA guidance, Gloeocapsa magma is probably the most prevalent of several algae species that discolor roofs, and all of them travel through the air and settle on roofs and other outdoor surfaces. Once spores land somewhere that stays damp and shaded, they grow and darken. That is why the streaks so often start at the ridge or under a tree and spread downhill in stripes that follow the water.

Our roof cleaning cost article notes that this algae feeds on the limestone filler used in many asphalt shingles, which helps explain why it shows up on so many otherwise sound roofs. Shade, moisture and overhanging branches speed it up, and the north-facing slopes that never get afternoon sun usually look worst first.

• Dark stripes or a uniform darkening that starts high and runs down the slope.
• Worse on slopes facing away from the sun and under overhanging trees.
• Often paired with green film or growth on nearby stone, gutters and fascia.
• Sometimes accompanied by moss or lichen in the most shaded corners.

Algae, hail or wear? Telling them apart from the ground

Not every dark patch on a roof is algae, and it matters which one you have. Algae streaks are soft-edged and follow the water path. They look the same before and after a rain. Damage looks different: missing shingles, curled or lifted edges, bare spots where the granules are gone, or dents in metal vents and gutters after a storm. Rust-colored stains are another thing entirely; they often come from metal flashing, vents or fasteners, and they call for a different treatment than algae.

If a hailstorm came through recently and you are seeing bruises, missing granules or dented metal rather than streaks, the next call is a roofer, not a cleaning company. A Dallas roofer explains that process well in how to tell if hail damaged your roof and how the claim works. Cleaning a damaged roof will not fix it, and it is better to know what you are dealing with first. When our technicians clean a roof, our roof cleaning page notes that they check for loose shingles, damaged flashing and leaky windows and tell you what they find.

Does it hurt the roof, or only the look?

Algae is first a curb appeal problem. From the street, a streaked roof makes a well-kept house look neglected, and it is one of the first things buyers notice in listing photos. It is not going to cause a leak next week.

Over time, though, growth that is left alone holds moisture against the surface and spreads, and the moss and lichen that can join it in the shadiest spots are harder to remove. Our roof cleaning cost article also points out that a dark, algae-covered roof absorbs more heat, which adds to the load on the air conditioner during a long Dallas summer. The longer you wait, the more treatment the roof needs and the more the cleaning costs.

There is a practical timing question too. If you are planning to sell in the next year, a roof cleaning is one of the most visible improvements you can make for the money, because the roof takes up so much of the front photo. If you are planning to replace the roof soon anyway, cleaning it may not be worth it, and I would tell you so. Somewhere in between, the right answer depends on how far the growth has spread and how much it bothers you.

Why a pressure washer is the wrong tool

This is the one rule I would ask every homeowner to remember. ARMA's guidance is blunt: do not use a power washer or any type of brush or broom to clean algae from the roof, because high-pressure washing systems are likely to damage asphalt roofing. The force strips the protective granules that shield the shingle from the sun, and once they are gone they do not come back.

Our roof cleaning page states that we do not offer high-pressure washing for roofs at all. Every roof we clean is soft washed. That is not a sales line. It is the reason a roof cleaning should help a roof last rather than shorten its life. If anyone offers to pressure wash your shingles, thank them and find someone else.

What a homeowner can safely do from the ground

ARMA notes that discoloration may be lightened with a solution of household bleach and water, left on for a limited time and rinsed gently, but it also encourages using professionals and recommends that only trained people work on roofs. The bleach solution makes shingles slick, and a fall from a roofline is a far bigger problem than algae. I would not walk a roof to clean it, and I would never walk a tile, slate or metal one.

What you can do safely is slow the growth down and keep an eye on it. These steps cost little, need no climbing beyond a stable ladder, and make any professional cleaning last longer.

• Trim branches that hang over the roof so the slope gets sun and air.
• Keep gutters clear so water and debris do not sit at the roof edge.
• Clear leaves and pecan or acorn debris from valleys if you can do it from a ladder safely.
• Photograph each slope once a year so you notice when streaks start spreading.
• Ask about algae-resistant shingles the next time the roof is replaced.

How we soft wash a roof without walking on the wrong one

Our roof cleaning page lays out the process. We start with an inspection and prep: windows closed, pets inside, outdoor items moved and plants pre-wet. The roof is pre-wetted, then a solution of sodium hypochlorite and surfactant is applied, adjusted for the roof material, the severity of the growth and how sensitive the surrounding property is. After the dwell time, the roof is rinsed, the treatment is repeated if growth remains, and runoff onto plants and painted surfaces is neutralized before a final walkthrough.

The material decides how we reach it. Asphalt shingle roofs are usually walkable once the technician has assessed them. Clay tile, slate, metal and concrete tile are cleaned without walking on them, from ladders or lifts, because they crack, dent or break underfoot. If you want windows washed or the house painted after a roof cleaning, our roof page recommends waiting 24 to 48 hours so the roof can dry and the runoff can settle.

• Inspection for loose shingles, flashing problems and leaky windows.
• Plants pre-wet, outdoor items moved, windows closed.
• Low-pressure application of the cleaning solution and a dwell time.
• Gentle rinse, with a repeat application if growth remains.
• Neutralizer on vegetation and painted surfaces where runoff lands.
• Final walkthrough with you, or photos if you are away.

What roof cleaning costs and how long it lasts

Our roof cleaning page gives a range of $500 to $3,000, depending on material, how heavy the staining is, how steep and tall the roof is, and whether a lift is needed. A single-story shingle roof with moderate algae sits at the low end; tile, slate or standing-seam metal on a large, steep house sits at the high end. Roof work bundled with a house wash, gutter cleaning or windows is discounted. I go through all the price factors in what a house wash costs and what moves the price.

As for how long it lasts, ARMA is honest that cleaning is temporary and discoloration may return. Our roof cleaning page says results often last several years before regrowth appears, depending on trees, shade and moisture. Heavily shaded roofs come back sooner, which is why trimming branches is the best thing you can do after a cleaning.

Planning the roof alongside the rest of the house

A roof cleaning makes the most sense as part of a plan for the whole exterior. The roof runoff carries growth and grime down onto gutters, fascia and walls, so we usually clean the roof first and the house after. If you are deciding which surfaces need a soft wash and which can take pressure, soft washing versus pressure washing, surface by surface covers each one. For a realistic calendar, how often to wash the house, roof, gutters and windows puts the roof in context with everything else.

If you would like us to look, send a few photos of each slope from the ground, or let us view it from satellite imagery. We serve homes within about 30 miles of Dallas, and we will tell you honestly whether the roof needs cleaning now or can wait another season.

Questions people ask me