The short answer
Roofs should always be soft-washed, not pressure-washed. Soft washing is the method recommended by the Asphalt Roofing Manufacturers Association (ARMA) and by every major shingle manufacturer. Pressure washing strips granules, voids most warranties, and shortens the life of the roof.
If a contractor proposes hitting your roof with high pressure, that is the moment to look for another contractor.
What soft washing actually is
Soft washing is a cleaning process built around chemistry, not force. A roof-safe cleaning solution is applied at low pressure — typically less than 100 PSI, about the force of a garden hose. The solution does the work: it kills the algae, lichen, and moss that cause black streaks and stains. A gentle rinse follows.
Results are visible the same day on most roofs, and continue to improve as the next few rains finish lifting the dead organic growth.
Why high pressure damages a roof
Asphalt shingles are protected by a layer of mineral granules. Those granules block UV, shed water, and give the shingle its color. Pressure washing tears them off. Once the granules are gone, the asphalt beneath ages much faster — sometimes by years per cleaning.
High pressure also forces water under shingles and into seams it was never designed to handle. The result is leaks, soft decking, and shingle lift that nobody connects back to the cleaning months later when the problem appears.
What causes those black streaks?
The dark streaks running down most asphalt roofs are not dirt. They are a hardy algae called Gloeocapsa magma that feeds on the limestone filler in modern asphalt shingles. East Tennessee's humid summers are an ideal climate for it.
Soft washing kills the algae at the root, which is why the results last 4–7 years. Pressure washing only removes what is on the surface, so the algae returns quickly — and now you have a roof with fewer granules to protect it.
What about moss and lichen?
Moss and lichen are common on shaded roofs and under tree cover. Both lift more slowly than algae but respond well to a soft wash. The cleaning solution kills them at the root; they release over the next several rains as the dead growth weakens.
Trying to scrape or pressure-wash moss off shingles is one of the fastest ways to damage a roof. The roots have already gripped between the granules — pulling them out by force takes the granules with them.
How often should a roof be soft washed?
Every 4–7 years for most East Tennessee roofs. Heavily shaded or wooded lots may need it sooner. A roof that has never been cleaned and is showing visible streaking is overdue — and every additional year of growth makes the asphalt underneath more vulnerable.
Bottom line
If you want a roof that looks clean and reaches its full expected service life, soft washing is the only correct method. Ask any roofing contractor — they will tell you the same thing.