What Is Causing Those Dark Streaks on Your Shingled Roof?

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Shingled Roof

From the ground looking up at your roof, you have seen a lot of dark streaks. Asking what those streaks are leads you to answers you may wish you hadn’t asked. However, because the answers may provide some relief regarding roof damage, they are easily researched. 


Streaks can be the result of particulates. Small dirt particles blowing in on the wind, or loose, tiny bits of the shingles themselves depositing in streaks after a rainstorm are common. As the rain comes down the slanted slopes of your roof, the particulates become lodged in areas where rivulets of water trickle down. A simple roof cleaning with pressurized water can fix this.

Animal Waste 

Birds, squirrels, and other flying or scampering critters that flitter and scurry across rooftops frequently pause to urinate and defecate. The urine alone can cause streaking, but combined with the more liquid feces of birds, you get streaks. A powerful cleanser and pressure wash treatment might help remove some of this and restore the shingles to their original appearance. Finding ways to prevent animals from being on your roof in the first place reduces the likelihood of repeating this type of cleansing process.


Organic material deposited on roofs breeds algae. The algae finds the perfect home in the dirt, decaying leaves, animal waste, etc. Then it begins to grow and leave a streaky appearance on your roof. An algaecide will kill the algae completely, but you should hire a professional roof cleaner to do this job. It’s generally not recommended for homeowners to haul algaecide up to their roofs and try to kill the algae themselves while avoiding a very dangerous sort of accident.

Just an Old Roof

It’s also likely that your roof is just a really old roof. If it’s older than 20 years, it’s time to replace it. The streaks are just a side effect of aging and damaged shingles.

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