A roofing contractor’s stain guide lists rust as orange-brown spots or streaks from metal flashing, nails or vents, often right under rusty flashing. Algae shows up as vertical black or gray lines instead, and that is what a roof cleaning price usually covers.

Bleach is the wrong chemical

The same remover maker says customers who tried bleach on concrete rust stains had no success, and that bleach can even turn the stain a brighter orange. The asphalt roofing industry group’s bleach-and-water method is for lightening algae, not rust, which is why rust needs its own line on the quote.

That group sets two limits for any roof cleaning: chemicals other than its bleach-and-water method shouldn’t be used without the shingle maker’s approval, and high-pressure washing is likely to damage the shingles.

Before the rust line goes on the quote Why
Ask which product they’ll use The roofing group wants the shingle maker’s approval for other chemicals
Ask to see a test spot first Even the concrete remover’s directions say to test a small area
Ask how plants will be protected An extension answer warns that oxalic acid will burn plants
Rule out the pressure washer The roofing group says high pressure is likely to damage shingles

Fix the flashing first

Another roofing contractor says the rust comes from parts such as the flashings along chimneys, vents and other penetrations, and that rusting metal could mean water is seeping through the roof. Have a roofer look before anyone cleans; a cleaned streak still sits under the metal that made it.

If algae is part of the job too, see whether roof cleaning is worth it and what to expect after a soft wash. Rust lower down has its own fix in rust spots on a driveway. Our methodology explains how we check sources.

Sources

Based on 6 sources · How we price