The answer to 'downspouts algae growth' is that algae growth in downspouts is a common issue caused by moisture, lack of sunlight, and organic debris. Symptoms include green or black slimy buildup that can clog the downspout and prevent proper drainage. To address this, you'll want to regularly clean and maintain your downspouts to prevent algae from accumulating.
For minor algae growth, you can often handle this as a DIY project by flushing the downspouts with a garden hose or using a soft brush to scrub off the buildup. However, if the algae growth is extensive or the downspouts are difficult to access, it's best to call in a professional roofing contractor to properly clean and inspect the system.