What Are the Black Streaks on My Roof? A Massachusetts Homeowner's Guide

The streaks are a cyanobacterium, not dirt

The organism behind almost all dark roof streaking is Gloeocapsa magma. It is usually called an algae, and we will call it that here because that is the word everyone uses, but strictly it is a cyanobacterium, a photosynthesizing bacterium rather than a true alga.

The dark color is not the organism itself. It is a protective sheath the colony produces to shield itself from ultraviolet light. That is why the staining deepens the longer a colony is left alone, and why it looks like something has run down the roof rather than settled on it.

It is airborne. It gained wide notice in the southeastern United States in the 1990s and has since spread through most of the United States and Canada, which is why roofs in this part of Massachusetts now show it routinely when they did not a generation ago.

What the industry actually says about damage

This is the part where a lot of roof cleaning marketing overreaches, so we will be straight about it.

The Asphalt Roofing Manufacturers Association, the trade body for the companies that make the shingles, describes algae discoloration as a surface stain and not structural damage. That is their position, not ours, and it is worth knowing before anyone tells you your roof is being eaten.

You will also read that the organism feeds on limestone filler in modern shingles and shortens roof life. That claim is common online, but it is not well supported. The widely cited reference for it carries a citation-needed flag, and we are not going to repeat it as fact in order to sell you a wash.

So the honest summary is this: on current industry guidance, algae streaking is a cosmetic problem. It affects how your house looks, what a buyer thinks when they pull up, and what your neighbors notice. Those are real reasons to deal with it. Structural panic is not one.

Moss is the different case

Moss is not the same organism and it does not behave the same way. Where algae sits on the surface, moss builds up a mat that holds water against the roof and can physically lift the edges of shingles, letting water reach the layers underneath. Oregon State University Extension describes exactly that mechanism.

Worth noting even here: the same Extension source says the moss itself does not directly damage roof materials, and describes the problem as what the mat traps and holds rather than the plant attacking the shingle. The mechanism is real. It is slower and more indirect than most roof cleaning adverts suggest.

The practical point stands: if you have moss as well as streaking, the moss is the more urgent of the two, and it tends to show up on the same shaded slopes for the same reasons.

Why one slope goes first, and why your neighbors are next

Streaking almost always appears first on the slope that stays damp longest. In practice that is the north facing side, and any aspect shaded by trees for most of the day. Sun dries a roof surface and slows colonization. Shade does the opposite.

That is also why trimming back branches that overhang a roof does more for the problem than any single cleaning will.

Because the organism is airborne, a heavily colonized roof is a source. If one house on a street is streaked and the houses around it are starting to show the same shadow on the same slope, that is the ordinary pattern rather than a coincidence.

Why pressure washing is the wrong tool

This is the one point where the manufacturers are emphatic, and it is worth quoting plainly. ARMA instructs homeowners not to use a power washer, or any brush or broom, to clean algae from a roof surface. They state that high pressure washing is likely to damage asphalt roofing, and that the friction of scrubbing may loosen and remove granules.

Those granules are the roof. They are the ceramic coated layer that takes the ultraviolet hit so the asphalt underneath does not. Strip them and you have traded a cosmetic problem for a real one, which is the exact inversion of what you paid for.

A roof that has been pressure washed often looks clean for a season and then ages visibly faster than its neighbors. By then the connection is not obvious to the homeowner.

What a correct clean looks like

The method the manufacturers describe is chemical rather than mechanical. A solution of liquid household bleach, sodium hypochlorite, and water is applied to the roof surface, left to work for at least fifteen minutes but no more than twenty, and then rinsed off gently with water. The solution does the work. The pressure does not.

ARMA does not publish one universal mixing ratio, and says directions vary between shingle manufacturers and with how heavy the discoloration is. Anyone quoting you a single fixed recipe for every roof is being more confident than the manufacturers are.

That low pressure, solution led approach is what the trade calls soft washing, and it is what we use. It is also why a soft wash and a pressure wash are not two prices for the same job. They are different jobs, and only one of them is the method the shingle makers approve.

It will come back, and anyone who says otherwise is selling

ARMA is explicit that the effectiveness of cleaning is only temporary and that the discoloration may recur. Spores keep landing. The slope that stayed damp before stays damp after.

What changes the timeline is the conditions, not the cleaning: sunlight reaching the roof, branches cut back, and on a re-roof, algae resistant shingles, which are manufactured specifically to inhibit growth for an extended period.

If you want someone to look at a roof in Waltham, Watertown, Newton, Belmont, Lexington or Weston and tell you honestly whether it needs cleaning now or can wait until spring, that is a free written estimate and we are happy to say wait.