When is the best time to spray a shingled roof with a recommended cleaner to get rid of algae?
Although algae stains are unsightly, they do little to damage shingles. But when the time comes to replace the shingles, ask the roofer to install algae-resistant ones containing small granules of copper or zinc to prevent algae from forming. In the meantime, the stains can be cleaned with a simple home remedy by mixing 1 quart of bleach with 1/4 cup of trisodium phosphate in 1 gallon of water.
A word of warning: Any cleaner you apply to the shingles will make the roof very slippery and hazardous. Experts strongly advise that a qualified professional perform the cleaning. Make sure the contractor you hire is fully insured; any damage to the shingles should be covered under the contract.
If you decide to do the cleaning yourself, apply the cleaning solution using a garden sprayer and scrub the affected areas with a stiff, long-handled brush. After cleaning, spray the area with a garden hose, making sure all the shingles are fully rinsed. You also can buy oxygen bleach roof cleaners that do an excellent job of removing algae stains. Oxygen bleaches are materials that release oxygen for cleaning and bleaching stains when mixed with water.