How to Remove Black Streaks From Your Roof (Safely)

If your roof has dark streaks running down from the top, you are not alone. It is one of the most common sights in Florida neighborhoods, and it is more than a looks problem.

What causes black streaks on a roof?

The streaks are a type of blue-green algae that spreads by wind and thrives in warm, humid weather. It feeds on the limestone filler used in many asphalt shingles. The dark color is a protective coating the algae grows against the sun. North-facing and shaded parts of the roof usually get it first, because they stay damp longer.

Is it harmful?

Algae alone does not usually destroy a roof quickly, but it causes real problems over time:

• Dark streaks absorb heat, which can raise attic temperatures and cooling bills.
• Algae holds moisture, which helps moss and lichen take hold. Those can lift shingles.
• Some insurers and buyers see a stained roof as a sign of poor upkeep.

The wrong way: high pressure

Pressure washing a shingle roof blasts away the granules that protect the asphalt from the sun. Many shingle makers say high pressure voids the warranty. On tile roofs, high pressure can crack tiles and force water under them.

The right way: soft washing

1. Protect the area. Wet the plants and grass around the house, and cover anything delicate.
2. Apply a roof cleaning solution at low pressure. It kills the algae instead of scraping it off.
3. Let it work. Most streaks fade within minutes to a few days as the dead algae washes off.
4. Rinse gently and rinse the plants again.

Because the algae is killed, a soft washed roof often stays clean for a year or more.

Should you clean your own roof?

Roofs are slippery when wet, and cleaning solutions make them more so. Falls from roofs cause serious injuries every year. Unless you have the right safety gear and a low, gently sloped roof, roof cleaning is a job for an insured pro.

How to keep the streaks away

• Trim branches that shade the roof and drop leaves on it.
• Keep gutters clear so water does not sit at the roof edge.
• Ask about zinc or copper strips near the ridge: rain washes small amounts of metal down the roof, which slows algae.
• When you replace the roof, ask for algae resistant shingles.

Frequently asked questions

Does roof cleaning damage shingles?

Soft washing done correctly does not. High pressure washing does.

How much does roof cleaning cost?

Price depends on the roof size, pitch, height and material. The fastest way to know is to get a few bids for your home.

Post your roof cleaning job with a photo of the roof and local pros will send you prices, or find a roof cleaning pro with good reviews near you.