How to Extend a Downspout

If you're going to run flexible pipe, first glue a 3x4-inch reducer coupling to the end of the rigid pipe in the trench. Then, slip the flexible pipe over the reducer and secure it with a stainless-steel pipe clamp. At the very end of the trench, dig a wide, shallow hole (approximately 16-inches-deep x 36 inch-diameter) and line it with flat rocks or a 4-inch layer of gravel. Lay down the pipe end on this rocky bed and cover it with more rocks or several inches of gravel.

In Step 3, you put down wide pieces of cardboard beside the proposed trench. Finish by backfilling the trenches, using a garden hoe to pull dirt off the cardboard and into the trench.