downspout extension
Credit: Allen-Douglas

Placing additional downspout extensions helps move the rainwater drainage away from the foundation or basement of a building.  One of the most common items on a home inspection report is improper roof drainage.  Many houses have decent gutter design, but when it comes to draining water away from the foundation, the water from the downspout is dumped directly against the base of the structure.  This causes settlement and foundation problems over time.  Also, basement water problems, may result from downspout water.  Downspout extensions work well to move the water away from the house.  They are particularly useful if the grade is very flat around the building.

Steps for placing the downspout extension


  •  First, clean the gutters and downspouts.
  • Place a hard plastic or concrete downspout tray near the building foundation at the location of each downspout.  Downspouts typically have a 65 to 75 degree bend near the ground elevation.  This is the location where the downspout extensions are attached. The downspout tray provides a  good impervious surface near the building foundation for when water leaks out.
  • Place the extension on the end of the downspout outlet.  There are 3 general types of downspout extensions including aluminum or plastic, pre-fabricated expandable extension, and flexible tubing with a converter piece from the downspout outlet to the 4" flexible tubing.  Use aluminum screws to attach aluminum extensions to metal downspouts.  Use any type of metal screw to attach plastic extensions to metal or plastic downspouts.
  • To promote proper drainage set the downspout extension to a minimum slope of 1 inch per foot (8.3% grade).  Do not allow any low spots in the outlet extension where water is trapped.  To avoid low spots, use wooden stakes driven into the ground or stones to support the downspout outlet.
  • Ideally, place the end of the extension at least as far away from the foundation as its depth.  For basement walls, this is a distance of 10 to 12 feet.  Use landscaping to blend the drainage in with the yard.  Some simple bushes or small trees can hide the outlet pipes.  If there is a landscape bed, place the pipe under the mulch, through to the front of the bedding.  Use a black or brown downspout extension or flexible tubing to blend it in with mulch.  If the building site is small, connect the downspout extensions to an underground drainage system draining away from the house.  Do not connect the downspout water to a system that connects to a sump pump.

Bad and Good Placement of Downspout Outlets

Poor downspout outlet placement with tray in the wrong location and outlet too close to building

bad downspout outlet
Credit: Allen-Douglas

Good design using a downspout to a 4 inch flexible non-perforated pipe. Screws should be used to attach the connector to the metal downspout.

metal downspout to 4" pipe connector
Credit: Allen-Douglas

Good design using a black 4 inch flexible outlet pipe placed along the back of the landscape bushes.

flexible downspout pipe in landscaping
Credit: Allen-Douglas

This good downspout drainage design uses the full width of a lanscape bed to move the water away from a basement wall.

downspout away from basement wall
Credit: Allen-Douglas