5-Inch vs. 6-Inch Gutters in South Dakota: Which You Actually Need

Standing in a homeowner's driveway in late April with a notebook, this is the single most common question we get: "5 or 6?" The honest answer takes 30 seconds. Most Sioux Falls homes are fine on 5". Steep roofs, long unbroken slopes, and the storm side of any two-story benefit from 6". Here's the math behind it.

What the inch actually means

A 5-inch K-style gutter has roughly 5.1 square inches of cross-section. A 6-inch K-style has 7.5 square inches — about 47% more capacity. Combined with the bigger 3×4 downspout (vs 2×3), a 6-inch system moves nearly double the water of a standard 5-inch system per minute.

Sioux Falls climate inputs

• Annual rainfall: 27 inches (mostly between April and September)
• Peak storm intensity (100-yr): ~1.7 inches per hour
• Annual snowfall: 38 inches
• Hail-day frequency: ~3 hail-producing storms per year

Translate that to a 2,000 square foot roof during a peak storm and you're moving roughly 2,100 gallons per hour off your roof. A 5-inch system at standard downspout spacing tops out at ~1,800 gal/hr. The math is why we see overflow at corners on steep two-stories every May.

Quick decision rule

Go 6-inch if any of the following is true:

1. 1Roof pitch is 8/12 or steeper.
2. 2You have a single roof slope longer than 40 feet draining to one corner.
3. 3You've had overflow before during a normal Sioux Falls May storm.
4. 4Your roof catchment area is over 1,800 square feet.
5. 5You're on the storm-side of any two-story home in Brandon, Hartford, or Garretson where straight-line wind pushes rain hard against the eave.

Otherwise 5-inch is fine. The upgrade is ~$1.50/ft — about $200 on a typical ranch. Worth it on the storm side, not always worth it for the back of a single-story.

Mixing sizes

We do this often. The front of a home might be a small entry porch that's fine on 5-inch. The back catches 80% of the rain off a long slope and needs 6-inch. Mixing on a single house is fine — the color and profile match between the two sizes so it looks intentional, not patched.

Bottom line

Most Sioux Falls homes do well on 5-inch K-style with 3×4 downspouts. If you've ever stood in your kitchen watching water sheet off a corner, you needed 6-inch. We can usually tell which side of the line your home falls on within a minute of pulling up the driveway.