Roof flashing is a thin layer of metal or other material that is installed on the roof's edges, seams, and valleys to prevent water from seeping into the building. Flashing is typically installed around ventilation, air conditioning units, chimneys and skylights. Over time roof flashing can deteriorate, causing leaks and water damage to the building's interior. That's why it's crucial to replace damaged or worn-out flashing as soon as possible. Roof flashing replacement is a job best left to professionals.