Why Are There Black Streaks on My Roof? (And What Actually Removes Them)

If you've noticed dark streaks running down the north-facing slope of your roof, you're not imagining it and you're not alone. Drive through almost any neighbourhood in Ajax, Whitby, or Oshawa and you'll see the same thing on house after house. It's one of the most common questions we get, so here's the straight answer.

Those streaks are alive

The black staining is a blue-green algae called Gloeocapsa magma. It travels on the wind, lands on your roof, and feeds on the limestone filler baked into asphalt shingles. It thrives in shade and moisture, which is why the streaks almost always start on the north side and under tree cover — the parts of the roof that stay damp longest after Durham's humid summers and wet springs.

Left alone, it does more than look bad. The algae holds moisture against the shingle and slowly eats the granules that protect the asphalt from UV. A roof covered in algae also absorbs more heat, which pushes up cooling costs in July and August. It is a maintenance problem disguised as a cosmetic one.

Why you should never pressure wash a roof

This is the part that costs people money. A pressure washer blasts the protective granules straight off your shingles and can force water up under them, into the roof deck. Most shingle manufacturers will void your warranty if the roof has been pressure washed. You might get a clean-looking roof for one season and take years off its life doing it.

The rule of thumb: pressure removes the symptom and the shingle. Soft washing removes the cause and leaves the shingle alone.

What soft washing actually does

Soft washing uses low pressure — about the force of a garden hose — paired with a biodegradable cleaning solution. The solution kills the algae at the root rather than scraping it off the surface. Because the organism is dead rather than just rinsed away, the roof stays clean far longer than a pressure wash ever could. It's the method recommended by asphalt shingle manufacturers, and it's what keeps your warranty intact.

• Low pressure — no granule loss, no water forced under the shingles
• Kills algae at the root, so results last years instead of months
• Manufacturer-approved approach that keeps shingle warranties valid
• Quoted to your roof's size and pitch — no guesswork

How long until they come back?

A proper soft wash typically keeps a roof clear for several years, depending on how much shade and tree cover the roof gets. Homes tucked under mature maples in older Durham neighbourhoods will see algae return sooner than a house on an open lot. If your roof is heavily shaded, a periodic maintenance wash is far cheaper than replacing shingles early.

If you've got streaks and you're not sure how far gone they are, send us a photo. We'll tell you honestly whether it needs cleaning now or whether it can wait.