Mix a phosphoric acid–based cleaner with water in a bucket. Use a ratio of 10 percent cleaner to 90 percent water for small, surface rust problems. For a stronger solution, raise the ratio to 50 percent cleaner and 50 percent water mixture. When mixing the water and acid, add the acid to the water, not the other way around.
Set a steady ladder against the building or house that has the rusty roof. Have someone hold the ladder as you climb it. This will help steady and support the ladder, reducing the chance of injury.
Saturate a brush broom in the mixture and scrub the rust for several minutes until well saturated.
Leave the mixture on the rust for an hour for the highest phosphoric acid–based cleaner ratio and longer for the more diluted mixture.
Rinse the painted metal roof thoroughly with a water hose to remove all traces of the acid. Repeat the entire process if oxidization remains.