Black Streaks on the Roof: Algae, Not Dirt, and What to Do About It

Algae streaks show up most on the lighter-coloured laminate roofs that became popular on 2000s and 2010s homes in Langford, Westhills and Royal Bay, where builder-grade shingles often lacked algae-resistant granules, and on the shake-look laminates of Oak Bay and the Uplands, whose east and north slopes sit in shade behind mature trees. On a dark charcoal roof in Gordon Head the same algae is there; it simply does not show.

The streaks are a different organism from moss but they like the same slopes. Victoria's mild winters and months of humid air, even in the rain shadow with only about 608 mm a year, keep shaded shingles damp long enough for spores carried on the wind to take hold. Home inspectors here note streaking when a house sells, which is usually when owners first ask whether it matters. Salt air on the waterfront streets of Esquimalt and Sidney does not stop it.

The three are easy to confuse from the ground. Algae is a flat stain that follows the flow of water down the slope, darkest where the roof stays wet longest, and it does not stand up from the surface. Moss is three-dimensional: green cushions you could pick off, thickest in valleys, along tab edges and under trees. Dirt and needle debris sit on top and wash off in a hard rain. The Asphalt Roofing Manufacturers Association describes algae discolouration as an airborne organism settling on shingles, which is why it appears on roofs with no trees at all.

Where the streaks and the moss appear together, the moss is the one doing damage, and the streaks are simply confirming that the slope is a damp one.

For most of a shingle's life, no. Algae feeds on the limestone filler in the shingle and shades the granules, and over many years some roofers believe it holds a little extra moisture, but the visible effect is cosmetic. The practical risk is indirect: an owner who sees streaks may have the roof pressure-washed, which strips granules and does far more harm than the algae. The second risk is at resale, where a streaked roof photographs badly and prompts a buyer's inspector to look harder.

A professional clean for algae is gentle by design. What to expect and what to ask for:

• A low-pressure application of a diluted cleaning solution, left to work, then a low-pressure rinse.
• Plants, gutters and downspout outlets protected or flushed, because runoff can harm gardens.
• No pressure washer on asphalt shingles, the same rule Oregon State University Extension gives for moss.
• Streaks fading over weeks as the dead algae weathers off rather than disappearing on the day.
• A zinc or copper strip at the ridge to slow regrowth, which works on algae as it does on moss for roughly 4.5 m below the strip.
• A written note of the method if you are on an annual moss and algae contract.

If the roof is due anyway, the lasting answer is at replacement. Algae-resistant laminate shingles blend copper-bearing granules into the surface so the copper ions suppress growth for years, and most major lines offer an AR version at a small premium. Metal roofing sheds both algae and moss and suits heavily shaded lots. In our pricing model a 2,000 sq ft two-storey home in laminate asphalt is about $6,900 to $12,600 before GST, and the AR upgrade is a line item to ask for rather than a separate price tier.