Greene County
Pressure Washing

Roof

Those black streaks on a shingle roof are usually algae. Pressure takes off the granules that protect the shingles, so the roofing industry is blunt about it: don’t.

Never pressure wash

Method
A bleach and water solution, a short dwell and a gentle rinse. No scrubbing.
Pressure
None. Do not use a power washer on asphalt shingles.
Cleaners
Household bleach and water, or only what the shingle maker approves.

Pressure or soft washing

Soft wash Cleaner goes on first, a soft brush does the scrubbing, and a garden hose does the rinsing.
distance
Pressure wash A machine pushes water through a nozzle. The tip, the distance and the angle set how hard it hits. Never pressure wash

The Asphalt Roofing Manufacturers Association says not to use a power washer, or any brush or broom, to clean algae from a roof. It says high-pressure washing is likely to damage asphalt roofing and should not be used for any purpose1. CertainTeed adds that the spray can easily dislodge the granules that protect the roof from UV rays and water2.

ARMA says discoloration may be lightened with a solution of household bleach and water, left on at least 15 minutes and no more than 20. It warns the result is only temporary and the streaks may come back. Other chemicals or methods should not be used without the shingle maker’s approval, and ARMA encourages using professionals for roof cleaning1.

Nozzle and pressure

40° 25° 15° 0°
Spray angle by nozzle tip. A narrow angle puts the same water on one small spot; a wide fan spreads it out. No tip to pick. These sources give no safe pressure setting for this surface.

There is no safe pressure setting for asphalt shingles in these sources. The answer is not to use a pressure washer at all12.

Cleaners to use and avoid

Use

  • A solution of household bleach and water, as ARMA describes, with a 15 to 20 minute dwell1.
  • Any other product only with your shingle maker’s approval1.

Avoid

  • Power washers, brushes and brooms1.
  • Scrubbing of any kind, which can loosen and remove granules1.

Protect plants and keep runoff out of drains

Cleaning chemicals can damage plants and grass. The Brick Industry Association’s advice holds for any surface: protect nearby plants, and where needed keep the cleaning solution and runoff off plants and the soil around them3. Soaking beds with plain water before and after, and covering what you can, is the practical version of that.

Street drains are not sewers. EPA reminds people that storm drains lead directly to local waterways, not to treatment facilities. Its tips for washing outdoors: use biodegradable, phosphate-free, water-based detergents, use as little as you can, wash on a driveway or yard rather than in the street, and send the wash water onto grass or gravel4. In Greene County, polluted runoff that enters the storm sewers is often discharged into local rivers and streams without treatment5, and Beavercreek asks residents not to dump anything down a storm sewer or drain6.

If you see suds, a rainbow sheen or an odd color in a ditch, stream or storm drain, Greene County asks you to report it to the County Engineer’s Office at 937-562-75007.

Common damage

  • Roof granule lossPressure and scrubbing knock loose the granules that protect shingles from UV and water21.
  • Damaged shinglesARMA says high-pressure washing is likely to damage asphalt roofing1.

When not to pressure wash at all

  • Ever, on asphalt shingles1.
  • From a ladder or on a roof without fall protection. Our safety page covers why this is a job many people leave to someone else.
  • Where bleach runoff will drain into beds and shrubs under the gutters without the plants being protected.

Step by step

  1. Decide who goes upARMA encourages using professionals for roof cleaning. Read the safety page before you get on a ladder.
  2. Protect what’s belowSoak and cover plants under the eaves and gutters.
  3. Apply the solutionApply the bleach and water solution without scrubbing.
  4. Wait 15 to 20 minutesLet it work at least 15 minutes and no more than 20.
  5. Rinse gentlyRinse with low-pressure water from a garden hose. Never a power washer.

Before you start, read the safety page. If the job looks bigger than you want to take on, DIY or hire lays out what each way involves.

Sources

Every superscript number on this page points to one of these. Each was read on the date shown.

  1. Algae Discoloration of Roofs Asphalt Roofing Manufacturers Association. Checked 2026-10-26.
  2. What type of maintenance does my roof require? CertainTeed. Checked 2026-10-26.
  3. Technical Note 20: Cleaning Brickwork (August 2018) Brick Industry Association. Checked 2026-10-26.
  4. Stormwater Best Management Practice: Vehicle Maintenance and Washing (EPA-832-F-21-032F) U.S. Environmental Protection Agency. Checked 2026-10-26.
  5. Stormwater Basics Greene County, Ohio. Checked 2026-10-26.
  6. FAQs: Stormwater Program City of Beavercreek, Ohio. Checked 2026-10-26.
  7. Illicit Discharge & Illegal Dumping Greene County, Ohio. Checked 2026-10-26.