Gresham roof algae streaks we cleaned before fall moss season

The homeowner in Gresham’s Powell Valley area called in late August. He had noticed black streaks running down the north slope of his composition shingle roof — not thick moss yet, just dark ribbons that showed up after the July dry spell. His neighbor told him to wait until spring. We told him the opposite.

August is the window.

The house

Single-story ranch off SE Powell, built in 1992, standard three-tab composition shingle, decent pitch on the north run, heavy Douglas fir cover from the lot line. Gutters had been cleaned in May but the roof itself had not been touched in four years.

From the ground the streaks looked like cosmetic staining. On the ladder they were Gloeocapsa magma — black algae holding moisture in the shingle granules. Not moss yet, but moss follows algae on north slopes every time in Gresham.

What we found on the roof

• Black algae streaks on the north slope — roughly 60% coverage, heaviest below the fir drip line
• Early moss tufts in two valleys where needles had packed — thumb-sized, not a mat, but established
• Fir needle debris in both valleys and behind the chimney cricket
• Granule loss — minor, age-appropriate for a 33-year-old roof, nothing catastrophic

This was roof cleaning with soft-wash chemistry, not pressure. Our full Portland roof moss guide covers the method: low PSI, treated solution, hand-brush in valleys, rinse from the ridge down. High pressure would have stripped the remaining granules off a roof that still had useful life.

Why August beats waiting until spring

Gresham sits wetter than inner Portland from October through April. Once fall rains start, algae spreads, moss spores land on the damp surface, and needle debris packs in the valleys. By March you are not cleaning streaks — you are treating a moss mat and hand-brushing thick growth.

Cleaning in August:

• Dry weather lets chemistry dwell and rinse properly
• Algae is active but moss has not colonized thick yet
• You enter fall with a clean surface instead of a wet biology layer feeding new growth
• Gutter cleaning after roof work catches the debris we rinse down

We also applied a moss treatment on the north slope and valleys — the same salmon-safe chemistry we describe in our post-treatment guide. That buys two to four years before moss re-establishes if gutters stay clear.

What we did not recommend

No tear-off. No “emergency” re-roof pitch. The shingles were aging but functional. The homeowner needed cleaning and treatment, not a $25,000 sales conversation.

We cleared the valleys by hand, soft-washed the north and east slopes, treated for moss prevention, and flushed the gutters before we left. About four hours on a single-story ranch.

What we sent the homeowner

North slope before and after from the driveway angle, a valley close-up showing the early moss tufts, and a note to schedule gutter cleaning again in November after fir needle drop. He booked that on the spot.

If you see black streaks on your Gresham roof this summer, that is not just ugly — it is the stage before moss. August and September bookings are easier to get than the March panic. 503-995-1947 for a written roof walk and quote.