Those Black Streaks on Summit, NJ Roofs: What the Algae Is and Whether It Hurts the Roof
The dark streaks running down so many Summit roofs are a specific algae, not dirt or shingle failure. Here is what it actually is, what it does, and what to do about it.
What those black streaks actually are
Drive through Summit's tree-shaded neighborhoods and you will see it on roof after roof: dark, vertical streaks running down the slopes, almost always heaviest on the north and shaded sides of the house. Homeowners frequently assume it is dirt, soot, mold, or a sign the shingles are finally wearing out, but it is none of those things. It is a hardy blue-green algae called gloeocapsa magma, and it is extremely common on asphalt roofs across New Jersey and much of the humid eastern half of the country.
The algae feeds on the limestone filler that manufacturers blend into asphalt shingles, and it spreads its dark, streaky stain downward as it grows and as rain washes spores down the slope. It thrives on moisture and shade, which is exactly why it shows up worst on the slopes that stay damp the longest, the north faces and the spots tucked under Summit's heavy tree canopy that the sun never quite reaches to dry out after a rain.
It also tends to spread from house to house along a street, since the spores travel on the wind and in runoff, which is why you will often notice whole blocks of Summit homes showing the same streaking at the same time. That neighborhood-wide pattern is one more clue that what you are looking at is airborne algae doing what algae does, not a defect in any one roof or a sign that a particular roof is failing ahead of its neighbors.
Does it actually damage the roof?
The honest answer is that it is mostly cosmetic, but not entirely, and the distinction matters. In the near term the algae is primarily an eyesore that prematurely ages the look of an otherwise sound roof and can noticeably drag down curb appeal, which becomes a real concern when a home is about to go on the market and a buyer's first impression is a roof that looks tired and dirty. It is not, in the short run, eating through your shingles.
Over the long run, though, the algae and the moisture it traps and holds against the shingle surface can shorten roof life at the margins, keeping the surface damp for longer and contributing to gradual granule loss over many years of growth. So while a streaked roof is genuinely not an emergency and nobody should be scared into a replacement over it, it is also not quite nothing, and it is worth addressing thoughtfully rather than simply ignoring it for the entire remaining life of the roof.
How to clean it without wrecking the roof
The wrong way to clean roof algae is unfortunately the way many homeowners are first tempted to: a pressure washer. High-pressure water blasts the protective mineral granules right off asphalt shingles and does far more harm than the algae ever would, effectively stripping years of life off the roof in a single afternoon to make it look clean for a season. You should never, under any circumstances, pressure-wash an asphalt shingle roof.
The correct method is a low-pressure soft wash, using a cleaning solution specifically formulated for roof algae, applied gently and then rinsed off. Done properly, a soft wash kills the algae and lifts the dark stain without stripping the granules or damaging the shingle surface underneath. If you are not properly equipped to do that safely from a ladder, and most homeowners are not, it is a job genuinely worth handing to someone who is, rather than risking a serious fall or a damaged roof for the sake of saving a service call.
Keeping it from coming back
Cleaning removes the existing stain, but in Summit's persistent shade and moisture the algae tends to return over time unless something about the conditions changes. The most durable preventive measure is a strip of zinc or copper installed near the ridge of the roof: as rain washes down over the metal it releases trace amounts of ions that inhibit algae growth on the slope below, which is exactly why you sometimes notice a clean streak running down from a metal chimney cap or a galvanized vent on an otherwise stained roof.
Trimming back overhanging tree branches helps as well, by letting more sun and moving air reach the roof so the slopes dry out faster after each rain instead of staying damp for days. And when a roof is genuinely due for replacement anyway, algae-resistant shingles, which build copper or zinc directly into the granules at the factory, are well worth specifying in a town as shaded and tree-covered as Summit. Prevention designed into the roof beats cleaning it every few years.
What the streaks can tell you about the rest of the roof
While the algae itself is mostly cosmetic, a roof heavily covered in it is often a roof that is not getting much attention generally, and the inspection we do when a homeowner calls about streaking frequently turns up other things worth knowing: gutters clogged enough to keep the eaves damp, branches that should have been trimmed long ago, or shaded valleys holding debris and moisture against the shingles.
So a call about black streaks is often a useful occasion for an honest overall look at the roof, not just the cosmetic complaint that prompted it. We will tell you straight whether the streaking is purely a looks issue or a sign that the roof needs broader attention. Call 908-279-1073 for a free inspection and a clear, honest assessment of what is really going on up there.
If dark streaks are aging the look of your Summit roof, or you just want to know whether it is purely cosmetic or something more, call 908-279-1073 for a free inspection and an honest assessment.
A quick call to 908-279-1073 starts the free inspection, no obligation.