Long black streaks running down a roof are one of the most common things homeowners ask roofers about. On most roofs, they aren’t dirt or mould. They’re algae, and they’re mostly a cosmetic problem. Moss is a different story, since it can actually damage shingles over time. Homeowners who want a roofer to check which one they’re dealing with can go here to arrange a visit.
What the Black Streaks Are
It favours shaded, damp slopes. North facing sides of a roof, and areas under trees, usually show it first. Roofs in humid regions get it faster.
On its own, algae doesn’t eat through a shingle. It can hold a little more heat on a dark roof, and many owners simply don’t like how it looks. Some home buyers read it as a sign of neglect.
Cleaning Without Damage
Pressure washing is the most common mistake. The jet strips off the granules that protect shingles from the sun, and some manufacturers say it can void the warranty.
- A low pressure spray of diluted bleach or a cleaner made for roofs kills algae without scrubbing.
- The solution sits for a short time and then gets rinsed off gently with a garden hose.
- Plants below are soaked with water before and after, so runoff doesn’t burn them.
- Moss is loosened with a soft brush, working downhill, never against the shingle edges.
- A moss killer finishes off what’s left, and the rest washes away with the next few rains.
A wet roof covered in cleaner is very slippery. Many owners leave this job to a roofer or a roof cleaning service for that reason alone.
Keeping It From Coming Back
Metal helps. Strips of zinc or copper fitted just below the ridge release small amounts of metal every time it rains, and that runoff discourages algae and moss on the slope below.
Many shingle lines now include copper granules mixed into the surface for the same effect. Details on shingles with built in algae protection are easy to find online, and readers comparing options can go here for one example.
When Cleaning Isn’t Enough
Old shingles that have lost most of their granules don’t clean up well. The streaks fade but the surface stays patchy. If the roof is near the end of its life anyway, replacement with algae resistant shingles often makes more sense than repeated cleaning.
