TL;DR
Pressure washing an asphalt roof strips the mineral granules off the shingles, and those granules are the shingle's only shield against ultraviolet light. Without them the asphalt below dries, cracks, and curls years ahead of schedule. Soft washing at low pressure is the industry method, and walking a roof at all is a real fall risk best left to a contractor.
Why do granules matter so much?
An asphalt shingle is a fiberglass or organic mat saturated with asphalt and surfaced with ceramic-coated mineral granules. Those granules block ultraviolet light, add fire resistance, and provide the color.
Asphalt exposed to sunlight hardens and becomes brittle. That is the entire failure mode of an aging roof, and granule loss is the first step in it. A roof that loses a significant share of its granules early loses years of service life.
Source: roofing industry material and maintenance guidance
Can you pressure wash an asphalt shingle roof?
No. A pressure washer blasts the mineral granules off the shingle surface, and manufacturers generally warn against it, with some voiding warranties outright. It also drives water up under the shingle laps into the deck below. Low-pressure soft washing with a cleaning solution is the accepted method.
The granule loss is easy to confirm. After a pressure wash, gutters and downspout splash blocks fill with what looks like coarse black sand. That is the roof.
Water forced under the shingles is the quieter half of the damage. Shingles shed water downward by overlap, exactly like siding, and a jet aimed up the slope defeats it and wets the roof deck.
What soft washing actually is
Soft washing applies a dilute cleaning solution at garden-hose pressure, lets it dwell, and rinses gently. The chemistry kills and releases the growth, and gravity plus low-pressure water carries it away over the following days and rains.
The trade-off is patience. A soft-washed roof often keeps improving for a couple of weeks rather than looking perfect the moment the work stops. That is normal and is the point.
Most exterior cleaning solutions for roofs are dilute sodium hypochlorite with a surfactant. That means plants below the roofline need to be soaked before, protected during, and rinsed after, and it means the same absolute rule about mixing applies.
Never combine a chlorine-based cleaner with ammonia, with any acid, or with alcohol. Those combinations produce toxic gas. Use one product at a time and rinse thoroughly in between. The rules are covered in our guide to cleaning mistakes that damage surfaces.
What are the black streaks on a roof?
They are algae, a hardy species that feeds on the limestone filler in shingles and holds moisture against the surface. It is cosmetic in the short term but keeps the roof damp, which encourages moss, and moss is genuinely damaging.
The streaks run downward from the ridge because spores wash down with the rain. North-facing and shaded slopes are worst because they dry slowest.
Zinc or copper strips installed near the ridge release trace metal ions as rain runs over them and suppress regrowth on the slope below. That is a prevention measure rather than a cleaning one, and it costs far less than repeated washing.
Why scraping moss off is a mistake
Moss builds a mat that holds water directly against the shingle and lifts the shingle edges as it grows, which is a real path for leaks. It has to be dealt with.
Scraping, wire-brushing, or power-brushing takes the granules with the moss and tears at the shingle edges. Pulling a thick mat off often lifts a shingle tab loose too.
The industry approach is to treat the moss, let it die and dry, and let the weather remove the residue over the following months. Gentle removal of the dead material with a soft brush, working down the slope, is acceptable. Forcing anything is not.
Source: industry exterior cleaning and safety guidance
The safety issue nobody should ignore
Roof work is one of the more dangerous things a homeowner can attempt. A wet roof coated in cleaning solution is far more slippery than a dry one, and the slope, the height, and the ladder transition are all serious hazards.
Contractors doing this work generally clean from a ladder or from the ground with extension equipment, precisely so they do not have to stand on a wet slope. That is a good model to copy or a good reason to hire out.
If the roof is steep, tall, tile, slate, or metal, hire a contractor. Tile and slate crack under foot weight and are expensive to repair, and metal roofs have coatings that the wrong cleaner will strip.
What to check before and after
Before cleaning, walk the perimeter and look up. Missing tabs, lifted edges, exposed nail heads, damaged flashing, and sagging gutters are repair items, and washing a roof with those problems drives water into them.
Clear the gutters first so the runoff has somewhere to go. A blocked gutter overflowing down the wall during a roof wash is how the siding gets streaked, a problem covered in our article on siding washing mistakes that drive water inside.
Afterward, check the gutters and splash blocks for granules. Some loss is normal on any roof, especially a new one. A gutter full of granules after a wash means the method was too aggressive.
What roof cleaning generally costs in the market
Industry Baseline Range
| Service | Typical published range |
|---|---|
| Soft wash, single-story asphalt roof | $250 to $600+ |
| Soft wash, two-story or complex roof | $450 to $1,200+ |
| Moss treatment and removal | $400 to $1,500+ |
These figures reflect published industry averages. Current market pricing varies significantly and actual quotes may fall well outside these ranges based on site-specific conditions, material costs, and project complexity.
Current Market Reality
Real quotes often land above published averages, because insurance for roof work is expensive, labor and fuel costs have risen, and roof jobs vary more than almost any other exterior service. Pitch, height, roof material, the amount of moss, the number of valleys and dormers, and access for equipment all move the number. A steep, shaded, moss-covered roof with limited access is a completely different job from a low-slope ranch.
Our exterior surface cleaning guide covers the wider category, and our pressure washing services page covers the exterior side of the business.
FAQ
Can I pressure wash my roof?
No. High pressure strips the mineral granules that shield the asphalt from ultraviolet light, and it drives water up under the shingle laps into the roof deck. Manufacturers generally warn against it and some void warranties. Low-pressure soft washing is the accepted method.
What are the black streaks on my roof?
They are algae that feed on the limestone filler in asphalt shingles. The streaks run downward because spores wash down with the rain, and shaded slopes are worst because they dry slowest. It is cosmetic at first, but it keeps the roof damp and encourages moss.
Should I scrape moss off my roof?
No. Scraping or brushing takes granules with the moss and tears at shingle edges, and pulling a thick mat can lift a tab loose. Treat the moss, let it die and dry, and let the weather clear the residue over the following months.
Is roof cleaning safe to do myself?
It is one of the riskiest household jobs, and a roof wet with cleaning solution is dangerously slippery. Contractors usually work from ladders or from the ground with extension equipment for that reason. Steep, tall, tile, slate, and metal roofs should always be left to a contractor.
How can I stop algae from coming back?
Zinc or copper strips installed near the ridge release trace metal ions as rain runs down the slope and suppress regrowth. Trimming overhanging branches to let more sun and airflow reach the roof helps too. Prevention costs far less than repeated washing.
Is some granule loss normal?
Yes. New roofs shed loose granules for a while, and all roofs lose some slowly with weather. What is not normal is a gutter full of granules right after a cleaning, which means the method was too aggressive and the roof lost years of protection.
Roofs are the clearest case in the house where the visible result and the correct method point in opposite directions, and where patience preserves the asset. If you would rather hand off the inside of the house, our house cleaning services cover the whole home. Every home is different, so the real number depends on size, condition, and what you want covered. Get a quote and we will walk you through it.