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Yes, moss can be dangerous for your roof if left in place to grow without treatment. The moss soaks up and holds moisture. Having a constantly damp roof could lead to problems with rot for the parts of the roof that consist of wood. Additionally, a patch of thick moss may draw insects and birds, which could damage the roof. Thick moss could block the roof’s drainage system. Water then may back up after a rainstorm and cause further damage that could eventually lead to roof repair expenses or the cost of roof replacement.

It may require stronger cleansers or chemicals, higher-pressure spray, or scrubbing to completely remove algae from a metal roof. Start with a mild cleanser; if that doesn’t work and you're comfortable doing so, use a stronger mix of ⅓ cup of laundry detergent, 1 quart of bleach, and 3 quarts of water to gently scrub the algae with a soft-bristled brush. If algae is present across large portions of your roof, doesn’t come off with gentle scrubbing and cleansers, or is in hard-to-reach areas, this may be a job better left to a pro.

You should avoid using a pressure washer on any surface or item that could be damaged by the high-pressure spray, as some materials are too delicate to withstand the force.

Key items and surfaces to avoid include:

• Electrical Components: Never pressure wash electrical systems, as water can cause short-circuits and create a safety hazard. This includes utility meters, breaker boxes, electrical outlets, generators, and HVAC or air conditioner units.

• Delicate and Fragile Materials: Fragile items can crack, shatter, or be stripped by the pressure. Avoid pressure washing windows, glass, light fixtures, shingled roofs (especially asphalt), and stucco.

• Painted, Stained, or Sealed Surfaces: Unless your goal is to strip the finish, do not pressure wash painted surfaces, stained wood (like decks), or anything with a protective seal that could be stripped away. Be especially cautious with older surfaces that may contain lead paint.

• Soft Woods and Certain Decking: Delicate surfaces like softwood, composite decking, and antique or wooden patio furniture can be etched or damaged. If you must wash wood, use a lower-powered pressure washer with a wide-angle nozzle to prevent damage.

• Living Things: Never use a pressure washer on people, pets, or plants. The force can cause serious injury and will destroy landscaping.

No, you should not pressure wash or power wash roof shingles, including asphalt shingles. The high water pressure from these machines is too strong and can damage the shingles by forcing water underneath, loosening their adhesive, and causing them to fall off. A safer and better alternative is soft washing with a pump sprayer. To ensure the job is done safely without causing damage, it is best to hire a local roof cleaner.