Extend an existing downspout by cutting away enough of the spout to accommodate a joining guttering fitting, then attach the desired length of the downspout addition to the fitting, and connect two to the existing spout. It is important to ensure both the fitting and extension match the existing downspout in both size and design.
The type of downspout extension system you install depends largely on how you want the extension to perform. A rigid piece of downspout pipe is particularly useful in areas of low traffic. To keep a traveled path free of a pipe extension, install a swing-up elbow fitting, which allows you to manually swing the pipe upward when not in use. A roll-up sleeve can be attached at the bottom of the existing spout, and it extends and inflates to accommodate increased water flow; afterward, it recoils.
If you want water to drain away from a building, consider installing a drainage pipe, which connects to the opening of the downspout and carries the water to a second location via underground pipes.
Downspout extensions are most often needed when the existing guttering allows water runoff to pool at the base of a structure. This could cause structural damage to a building as well as allow for mold or rot. By funnelling the runoff further away, you can aid in proper drainage after inclement weather.