Those black streaks on a Savannah roof are algae, not dirt, and pressure is the one thing that will not remove them safely. Here is the soft-wash method, what it costs you in plant damage if you skip the prep, and when to stay off the roof.
The black streaks running down a Savannah roof are a living organism, not dirt, and that single fact decides the whole job. They are almost always a blue-green algae called Gloeocapsa magma, and it feeds on the limestone filler in asphalt shingles. Because it is biological, it comes off with chemistry and dwell time, not with force. The method that works is a low-pressure soft wash: wet the plants below, apply a dilute sodium hypochlorite solution at garden-hose pressure, let it dwell fifteen to twenty minutes while the black visibly fades, then rinse gently. The method that ruins roofs is a pressure washer, which blasts off the ceramic granules that are the shingle's actual weather protection and can void the shingle warranty outright. If you take one thing from this page: on asphalt shingles, pressure is never the answer.
Homeowners usually describe it as dirt, soot, or "the roof is just old." It is none of those. Gloeocapsa magma is an airborne algae that lands on a roof, finds the powdered limestone that manufacturers use as filler in asphalt shingles, and settles in to eat. It protects itself from UV with a dark sheath, and that sheath is the black you see. The streaks run vertically because rain carries the colony downslope from wherever it established, which is why the pattern so often starts under a ridge line and fans downward.
Savannah is close to an ideal habitat for it. The algae wants warmth, humidity, and shade, and the coastal Georgia climate supplies all three for most of the year. That is also why the north-facing slope of a roof is usually filthy while the south-facing slope still looks acceptable - the north side dries slowest and holds moisture longest. If one plane of your roof looks a decade older than the rest, you are looking at a moisture map, not a materials defect.
An asphalt shingle is a mat saturated in asphalt with a layer of ceramic granules embedded on top. Those granules are not decoration. They are the UV shield that keeps the asphalt underneath from drying out and cracking, and they are held on by nothing more than the asphalt they were pressed into when the shingle was made. A pressure washer wand strips them off in sheets. You will see the evidence at the bottom of the downspout: a pile of what looks like coarse black sand. Every bit of that is roof life you just removed.
This is not a fringe opinion among cleaners. The Asphalt Roofing Manufacturers Association has long advised against pressure washing asphalt shingles for exactly this reason, and most shingle warranties are written so that pressure-cleaning damage is not covered. A crew that shows up to clean a shingle roof with a pressure washer is telling you something about the crew.
There is a second failure mode that is less obvious. Pressure drives water up under the shingle courses, past the lap where the roof sheds water by gravity. That water reaches the decking and the felt, and it does not come back out quickly. Leaks that appear a few weeks after a "roof cleaning" are frequently this, and by then nobody connects the two events.
Soft washing means applying a cleaning solution at roughly garden-hose pressure - typically under 100 PSI at the surface - and letting chemistry do what pressure cannot. For roof algae the working solution is sodium hypochlorite (the active ingredient in household bleach) diluted with water, usually with a surfactant added so it clings to the slope instead of running straight into the gutter.
Concentration matters more than people expect. Too weak and the algae survives and regrows within months; too strong and you are needlessly hard on landscaping, metal flashing, and anyone standing nearby. Professionals generally mix down to somewhere in the low single digits as a percentage of active hypochlorite, which for a homeowner working from standard household bleach lands near a one-to-one mix with water. Start weaker than you think you need and step up if the black is not fading, rather than starting hot.
The visual feedback is genuinely useful here: correctly mixed solution turns the black streaks pale within a few minutes. If nothing is happening after ten minutes, your mix is too weak or the surfactant has let it run off before it could work. You should not need to scrub, and you should not need a second pass on the same day.
Working from the ground with an extension wand is safer and usually sufficient on a single-story Savannah ranch. On anything steeper or taller, the honest answer is that this becomes a job for someone with roof-rated fall protection.
First, pick the weather. You want an overcast, still day with no rain forecast for several hours. Direct summer sun dries the solution before it can work, which is the single most common reason a DIY attempt underperforms. Wind is the other enemy, because it carries the spray onto plants and cars you were not aiming at.
Second, saturate every plant, shrub, and patch of lawn below the work area with plain water before you mix anything. Wet foliage absorbs far less of the solution than dry foliage. Keep them wet throughout, and rinse everything thoroughly again when you finish.
Third, apply the solution from the bottom of the slope upward, working in overlapping bands. Going bottom-up keeps you from running dry solution over already-treated shingles and gives you an even result without stripe marks.
Fourth, let it dwell. Fifteen to twenty minutes is typical. Watch the color change rather than the clock - when the streaks have gone from black to a pale grey-tan, the organism is dead.
Fifth, rinse gently from the top down, again at hose pressure. Some cleaners skip the rinse entirely and let rain handle it. That works on the shingles but is unkind to gutters and to whatever is planted below, so a light rinse is the better habit.
What you should not expect is a roof that looks brand new the same afternoon. The dead algae continues to weather off over the following weeks of rain. A roof that still looks slightly mottled two days later is normal and usually finishes clearing on its own.
This is where most self-inflicted damage happens, and Savannah yards give it plenty to work with. Azaleas, camellias, ferns, and established foundation plantings sit directly in the runoff path of a roof, and a hypochlorite solution that is perfectly safe for shingles will burn foliage that takes it undiluted.
Pre-wetting is the main defense, because water already in and on the leaf tissue dilutes what lands there. Tarping is the second. For anything you genuinely cannot replace - a mature camellia, an heirloom planting - cover it rather than trusting dilution alone. And rinse the whole bed generously when the job is done, not just the spots you think got hit.
Downspouts deserve a thought too. If yours discharges into a bed, a rain barrel, or a pond, redirect it for the day. Runoff from a roof wash concentrates in exactly that spot.
People often use "moss" as a catch-all for anything green or dark on a roof, but the three common colonizers behave differently and Savannah sees them in very unequal measure.
Algae is the flat black staining described above, and it is overwhelmingly the Savannah problem. Moss is the green three-dimensional growth that builds a mat and holds water against the shingle - it is far more common in cooler, wetter, heavily shaded climates than in coastal Georgia, though a deeply shaded north slope under dense canopy can grow it. Lichen is the toughest of the three: crusty, often pale grey-green, and it roots into the asphalt itself, which means even a correct soft wash may leave a faint ghost where it was.
The practical consequence is that lichen sets realistic expectations. Algae comes off cleanly. Lichen dies and releases its grip but can leave a mark, and no honest cleaner should promise otherwise. Anyone guaranteeing a lichen-scarred roof will look uniform afterward is overselling.
If your roof genuinely has a moss mat rather than algae staining, the treatment is the same chemistry with more patience, and the mat is left to weather off rather than pulled or brushed. Prying moss off a shingle takes granules with it.
The tree canopy that makes Savannah neighborhoods beautiful is also what keeps their roofs damp. A live oak arching over a roof does three things at once: it blocks the sun that would otherwise dry the surface, it drops organic debris that holds moisture and feeds growth, and it keeps humidity high in the air immediately above the shingles.
You can see this on a single street. Two identical houses built the same year will look a decade apart if one sits under canopy and the other does not. That is not a roofing failure - it is a drying-time difference, and it tells you which slopes will need attention first and most often.
Trimming branches back so the roof gets some direct sun and airflow is the highest-leverage prevention available, and it costs nothing on an ongoing basis. Clearing leaf litter out of valleys and off low-slope sections matters for the same reason.
A properly soft-washed roof in this climate typically stays visibly clean for a few years before the algae re-establishes, though shade and canopy shorten that considerably. Two things extend it.
The first is metal. Zinc and copper are toxic to algae in trace amounts, which is why you often see a clean band of shingle directly below a copper flashing or a galvanized vent - rain picks up trace metal and carries it downslope. Zinc strips installed near the ridge reproduce that effect deliberately. They are not dramatic and they will not clean an already-stained roof, but on a newly cleaned roof they meaningfully slow the return.
The second is simply keeping the roof dry and clear: trimmed branches, clean valleys, functioning gutters. Water that sits is water that grows something.
Algae-resistant shingles are worth knowing about if a replacement is already on your horizon. Most manufacturers now offer lines with copper-containing granules and a warranty against staining for a period of years. That is a decision for replacement time, not a reason to replace a sound roof.
Some of this job is genuinely a homeowner task and some of it is not, and the dividing line is honest to draw.
Stay off if the roof is steep enough that you would not walk it comfortably, if it is two stories or more, if the shingles are brittle or curling with age, or if there is any soft spot underfoot suggesting decking damage. Wet shingles treated with a slick surfactant are considerably more slippery than dry ones, which is a hazard people discover at the worst moment.
Age matters as well. A roof near the end of its service life may not be worth cleaning at all - if shingles are cupping, cracking, or shedding granules heavily on their own, cleaning buys appearance on an asset that needs replacing. A cleaner willing to tell you that is worth more than one who takes the job.
If you are weighing doing it yourself against hiring out, our Savannah roof cleaning cost guide lays out what the work actually runs here, and the moss-specific breakdown covers the heavier shaded-canopy cases.
Not every roof in the area is asphalt, and the rules shift by material.
Metal roofs, increasingly common on coastal and newer Savannah construction, tolerate cleaning well but are unforgiving about chemistry - hypochlorite can dull or corrode certain finishes and fasteners, so a gentler detergent and a thorough rinse are the safer route. Clay and concrete tile are structurally strong but individually brittle, and the real risk is cracking tiles by walking them wrong rather than anything chemical. Cedar shake needs a different approach again, since bleach-based solutions can be harsh on wood.
The common thread across all of them is that low pressure plus the right chemistry beats high pressure every time. The material only changes which chemistry.
Black streaks on a Savannah shingle roof are algae feeding on the shingle's limestone filler, encouraged by heat, humidity, and the shade of a magnificent tree canopy. They come off with a dilute hypochlorite solution applied at hose pressure and given time to work - not with a pressure washer, which strips the granules that keep the roof alive and can void the warranty in the process. Wet the plants first, work bottom-up, let it dwell until the black pales, rinse gently, and accept that the last of the discoloration weathers off over the following weeks. Keep branches trimmed and consider zinc at the ridge, and the job holds for years rather than months.
Free estimate
Tell us what needs cleaning in your area — we’ll reach out right away.