Sink it: Address Roof Runoff

Gutters and downspouts can promote infiltration when directed toward well-vegetated areas and away from impervious surfaces (i.e., driveways, compacted soils, walkways). In order to be effective during rain events, it is critical for gutters and downspouts to be appropriately sized. Calculating the correct size can be challenging- you need to know roof area, pitch, and a general sense of potential rainfall volumes both annually and during episodic and intense rain events.

Contact a local qualified professional to assist with calculating correct gutter and downspout sizes, and also consult with your local planning/building department to check for specific requirements. The guidelines listed here are general for sizing and installation of gutters and downspouts.



Shape and size matter. The shape determines the amount of water it can handle from your roof during a storm. Ogee shaped gutters, for example, can handle more water than rounded gutters. However the ogee gutter’s sharp edges and corners can collect sediment and debris.



Select gutters at least 5 inches wide. Use materials made from galvanized steel (29 gauge minimum) or aluminum (.025 inch minimum). To enhance flow, slope gutters according to the manufacturer’s recommendations (commonly 1/16 inch to 1/8 inch per 1 foot of sectional gutter; or 1/16 to 1/8 inch per 10 feet of seamless gutters). Tilt the gutter forward keeping the front 1/2 inch lower than the back. For straight runs exceeding 40 feet, use expansion joints at connections. Select elbows with 45, 60, 75 or 90 degree angles, as needed.



Reduce the volume and velocity of runoff by using downspouts to help slowing and spreading water out. If outlets cannot be directed to RAINGARDENS or SWALES, other options including outlet protection such as diverters can be used. Downspouts can also be plumbed to storage tanks (see RAINWATER BMPs).



Space downspouts from 20 to 50 feet apart. Do not exceed 45-degree angle bends. Where needed use 4-inch-diameter extensions (flexible or rigid) to convey water to infiltration areas such as rain gardens and swales or to other safe outlets away from structures and steep slopes. Do not direct downspout outlets to driveways or other impervious surfaces unless there are no safe alternatives.


  • Set a schedule

  • Clean gutters at the beginning of each rainy season and after severe storms

  • Trim vegetation away from gutters

  • Add gutter guards (reduces) debris buildup

  • Consider drip-line treatment below gutters that clog often

  • Check for leaks, damaged parts, rust, and evidence of past erosion



(‘kusari dio’ in Japanese) can be used instead of a downspout as an inexpensive option to direct water away from your foundation. They are visually appealing and provide some runoff reduction through evaporation and spillage.