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Avoid cleaning your roof in direct sunlight. Keep in mind that metal and light-colored paint will reflect sunlight and create glare, which may drastically reduce your vision. If the layout of your roof requires you to climb onto it, do so when it’s dry out to reduce the risk of slipping. Of course, once you start washing your roof, it will get wet and pose a risk. However, it’s still a smart idea to reduce that risk as much as possible. Before you haul all of your gear up, set up your ladder. Climb onto the roof and judge your ability to reach all of its parts. If you feel that certain parts are too precarious for you to safely reach, err on the side of caution and do not attempt to clean it. Keep in mind that your power washer or hose will extend your reach considerably. If any out-of-reach areas look like they need scrubbing afterward, hire a professional to do so. Do not attempt to wash your roof alone. Whether you need to mount the roof itself or can do the job from a ladder, play it safe. Ask someone to spot you so they can:  Assist you in bringing gear up and down. Alert you to any hazards that you may not be aware of. Call for help in the event of an accident. Protect yourself from a fall. Wear a safety harness. Attach a safety line between that and a sturdy feature of your roof (like a chimney). If none are available, use either a:  Bolt-strap anchor for corrugated roofs. Ridge clamp for standing-seam roofs.
Wait for dry weather and overcast skies. Clean only those areas that are easily reached. Have a partner. Secure yourself.