For general routine cleaning, you can get the job accomplished using only a pressure washer filled with water. Keep the spray nozzle at least 12 inches away from the roof tiles, and blast away the dirt and grime.
Avoid pouring harsh cleaning chemicals into your pressure washer as this may permanently damage the mechanism. You can add a small amount of household bleach, but only a solution of less than 1 percent, heavily diluted with water.
Chlorine bleach is possibly the most effective weapon against mildew, mold and algae, but take caution. Chlorine bleach is an extremely powerful oxidizer that can harm the eyes and skin, so wear protective goggles and rubber gloves.
Furthermore, you will need to dilute the bleach heavily before applying it to your tiles. Pour 3/4 to 1 cup of chlorine bleach into a gallon of water before applying.
As long as you exercise these precautions, you can safely use chlorine bleach as an effective tile roof cleaner.
Oxygen bleach (or per-oxygen bleach) is a natural and nontoxic alternative to chlorine bleach. Often found in powdered forms, you can get the most out if it by mixing it with water according to the manufacturer's instructions.
Unlike chlorine bleach, oxygen bleach will not harm your skin. Oxygen bleach will not give you as quick or dramatic a result as chlorine bleach, but it can still effectively kill mold and algae and clean your roof tiles, according to home improvement expert Danny Lipford.
Commercial Roof Cleaner
Home improvement stores and general hardware stores sell cleaners specially formulated for tile roofs, such as OxiOut, Spray & Forget, Roof Reviver and Rust-Oleum Roof Cleaner. Always follow the manufacturer's instructions when applying a commercial cleaner, and always read the label and verify that the cleaning solution is safe for your particular type of tile roof.
For example, if you have synthetic roofing tiles, you should not use a solution specially formulated for concrete tiles.