The longer answer

The capacity difference:

  • 5" K-style handles about 1.2 gallons per minute per foot. Adequate for ~600 sq ft of roof per downspout at average rainfall.
  • 6" K-style handles about 2.0 gallons per minute per foot. Handles ~1,200 sq ft of roof per downspout.

When to upgrade from 5" to 6" in KC:

  1. Roof area exceeds 1,500 sq ft. KC storms can overwhelm 5" on larger roofs.
  2. Steep pitch (8/12 or steeper). Steep roofs move water faster — bigger gutters handle the surge.
  3. Multiple valleys feeding into one gutter section. Valleys concentrate flow.
  4. Current gutters overflow in heavy storms. Direct evidence you're undersized.
  5. You're paying for a new install anyway. The $1.50/ft upgrade cost is worth it for the headroom.

When 5" is fine: Single-story ranches under 1,500 sq ft, gable roofs with no valleys, no history of overflow, budget-conscious projects.