A downspout extension has a low-profile to direct rainwater away from a building. A transition adapter can be used to transition from a generally circular or square cross-section of a downspout to a low-profile cross-section of a corrugated duct of the extension. The extension can include internal supports to withstand being stepped on and can have a generally rectangular or trapezoidal cross-section with a longer width than height. The corrugated duct can also be comprised of integrated accordion corrugation sections and transverse non-collapsible ribs to give the corrugated duct extension greater flexibility. The extension can be connected to a downspout using a flexible elbow duct, a downspout connector, and a downspout adapter. The transition adapter of the extension can also be connected directly to a downspout.