What Causes Those Black Streaks on Your Roof?

The streaking is caused by Gloeocapsa magma, a type of airborne algae that lands on a roof and feeds on the limestone filler in asphalt shingles. It spreads gradually, usually starting on the north-facing slope of a roof where shade keeps things damper for longer, and shows up as dark, streaky discoloration rather than an even coating. Pressure-washing the roof once won't fully solve it if the underlying conditions that let it grow haven't changed.

Algae needs moisture to establish and spread, and Houston's humidity provides exactly that for most of the year. Roofs here develop visible streaking faster than they would in a drier climate, which is why it's one of the most common cosmetic questions homeowners ask about an aging roof, regardless of how well the roof is otherwise performing.

Mostly cosmetic, but not entirely harmless. Algae growth itself doesn't typically cause a leak, but heavy, sustained growth can trap moisture against the shingle surface and contribute to somewhat shortened shingle life over time. For most homeowners the bigger concern is how it looks, especially on visible street-facing slopes, rather than any structural risk.

Algae-resistant shingles, which use specially formulated granules, resist staining better than untreated shingles, though no product is permanently immune in a climate this humid. Gentle, low-pressure cleaning methods designed for roofs can improve appearance without damaging the shingle surface. High-pressure washing, on the other hand, can strip protective granules and cause more harm than the algae itself — it's one of the more common ways homeowners accidentally shorten their own roof's life trying to fix a cosmetic issue.

If streaking is heavy, widespread, and the roof is already showing other signs of age — granule loss, curling, a roof approaching the upper end of its service life — algae discoloration is often just one symptom of a roof that's due for replacement rather than an isolated issue to treat on its own.

Algae needs shade and moisture more than it needs a specific surface, which is why the north-facing slope of a roof, or any area shaded by nearby trees, tends to show streaking well before sun-exposed slopes do. If you notice discoloration on one side of the roof but not the others, that pattern itself is a strong sign of algae rather than a different kind of staining.

Heavy streaking is one of the more common things a buyer notices during a walkthrough or a pre-listing inspection, even when the roof is otherwise structurally sound. It doesn't necessarily mean a roof needs replacing before selling, but it's worth knowing it's likely to come up in conversation, and addressing it — whether through cleaning or, on an older roof, replacement — can be worth factoring into a pre-sale plan.

A cleaned roof isn't permanently protected unless the shingles themselves resist algae growth or the underlying moisture and shade conditions change. Since Houston's humidity isn't going anywhere, streaking can gradually return over the following seasons even after a thorough cleaning, which is normal and not a sign the cleaning was done incorrectly — it's simply the same conditions producing the same result over time.

Algae streaking typically follows the direction water runs off the roof, appearing as vertical or diagonal streaks rather than an even coating. Dirt or debris buildup tends to look more uniform and often concentrates near valleys or low points instead. If the discoloration you're seeing doesn't follow that streaked pattern, it may be worth a closer look to confirm what's actually causing it before assuming it's algae.