How to Remove Common Roof Stains in Portland, OR

Portland’s wet climate means stains on roofs come with the territory. Moss, algae, lichen, rust, and black streaks each require a different approach to remove safely. Here’s what causes each type of stain and how to address it.

Common Roof Stains and How to Remove Them

1. Algae Stains (Dark Streaks)

Dark streaks running down shingles are almost always algae — specifically Gloeocapsa magma. It feeds on the limestone filler in asphalt shingles. Solution: Soft wash with a biodegradable algaecide. Avoid high bleach concentrations that damage shingles.

2. Moss Growth (Green Patches)

Moss thrives in Portland’s shade and moisture. It physically lifts shingles and traps moisture underneath. Solution: Brush off heavy growth, then apply a soft wash moss treatment to kill it at the root. Follow up with a moss inhibitor to slow regrowth.

3. Lichen (Raised Crusty Growth)

Lichen is a combination of algae and fungus that bonds tightly to shingles. It’s harder to remove than moss and can cause permanent staining if left too long. Solution: Apply a lichen-specific cleaner and allow dwell time before rinsing. Multiple treatments may be needed for heavy lichen.

4. Rust Stains

Rust appears where metal components — flashing, vents, nails — oxidize and stain the surrounding shingles. Solution: Commercial rust remover or an oxalic acid-based cleaner applied carefully to affected areas. The source of the rust (failing flashing, exposed nails) should also be addressed.

5. Tree Sap and Tannin Staining

Portland’s tree canopy is beautiful but leaves and sap create staining on roofs. Tannin from leaves creates brownish staining over time. Solution: Mild detergent or biodegradable degreaser applied at low pressure. Trim overhanging branches to prevent recurrence.

6. Black Mold or Mildew

Black spots or spreading dark patches can indicate mold or mildew growth, particularly in areas that stay wet. Solution: Soft wash with a mildewcide solution. Also check attic ventilation — poor ventilation traps moisture and promotes mold growth from below.

When to Call a Professional

DIY stain removal works for small, accessible areas. For widespread staining, multi-story roofs, or any stain that has been present for more than a season, professional treatment is more effective and safer. Professionals can also identify whether staining is a symptom of underlying roof damage.

Serving Portland and Surrounding Communities

Premium PowerWash handles all types of roof stains throughout Portland and the surrounding metro area, including Lake Oswego, Beaverton, Tigard, Tualatin, Oregon City, Wilsonville, and Sherwood.

Call (503) 575-6575 or get a free estimate online.