Roof cleaning removes the black streaking, moss and lichen from asphalt shingles using a low-pressure chemical treatment rather than physical force. Done correctly it kills the organism at the root, which is why a properly treated roof stays clean for years rather than months.
What the black streaks actually are
The staining is Gloeocapsa magma, a hardy cyanobacteria that feeds on the crushed limestone used as filler in asphalt shingles. It arrives airborne, takes hold on the north and shaded slopes first, and spreads downward in the characteristic streak pattern as rain carries spores with it.
Two consequences follow from it being alive rather than merely dirty. It will not wash off with rain or with pressure, because you are removing the visible colony while leaving the root structure embedded in the shingle mat. And it keeps growing, holding moisture against the shingle and gradually consuming the granules that protect the asphalt underneath from UV.
Moss and lichen are a further stage of the same problem. Lichen in particular anchors into the shingle surface and pulls granules away when removed mechanically — which is precisely why it must be killed chemically and left to weather off rather than scrubbed.
Why pressure washing a roof is a mistake
A pressure washer will make an asphalt roof look clean for about a season, and it will take years off its life doing it. High pressure strips the granule layer — the sacrificial surface that shields the asphalt from sunlight. Once those granules are in your gutters, the shingle ages fast and no amount of later care reverses it.
It also drives water up under the shingle courses, which is how pressure washing a roof turns into a leak six months later.
The Asphalt Roofing Manufacturers Association's guidance on the subject is unambiguous: staining should be treated with a low-pressure chemical application, not mechanical force. Several shingle warranties are explicitly void if the roof has been pressure washed. This is the rare case where the correct method and the cheap-to-perform method are the same one — there is no reason to do it wrong except not knowing.
If a contractor quotes you a roof cleaning and shows up planning to use a pressure wand on your shingles, send them home.
How we do it
The treatment is applied at roughly garden-hose pressure using a dedicated soft wash system. The solution kills the colony outright — you can watch the staining lift within minutes of application on most roofs.
Before anything reaches the roof, every plant bed below the drip line is saturated with clean water. Pre-wetting is the step that actually protects landscaping; foliage already full of water does not take up solution. Everything gets a second rinse once we are done.
Where lichen has established, expect residual shadowing after treatment. It is dead and it releases over the following weeks of weather. Anyone promising an instantly spotless result on a lichen-heavy roof is either going to scrub it or is overselling.
What roof cleaning costs
Roof work is priced on the roof's footprint, its pitch, and how much of it we can safely reach and treat from a ladder versus needing to be on the surface. Heavy lichen coverage adds solution and time.
It is worth comparing against the alternative. A roof replacement is one of the largest single expenses a house incurs, and the organism causing the staining shortens the interval to that expense. Treating it is a small fraction of the cost of accelerating it.
We quote a flat rate in writing before we start, and we will tell you plainly if we think your roof is too far gone for cleaning to be the right spend.
How long it lasts
Because the treatment kills the organism rather than removing its visible surface, a cleaned roof typically stays clear for several years — considerably longer than a house wash. Shaded roofs, heavy tree cover and north-facing slopes come back sooner.
If you want to stretch the interval, the highest-leverage thing you can do is trim back branches that overhang the roof. Shade and trapped moisture are what make a roof hospitable in the first place.