Black streaks are usually organic growth
In humid parts of Houston, dark streaks are commonly caused by algae and organic buildup, especially on shaded roof slopes.
Cleaning is not the same as pressure washing
A roof should not be blasted with high pressure. Roof-safe cleaning methods are selected based on roof type, age, condition, and staining level.
When cleaning makes sense
If the roof still has useful life left, cleaning may improve curb appeal and help manage buildup before replacement is necessary.
Related Highmark pages
For next steps, compare roof repair, residential roofing, storm damage roofing, and roof washing.