Detailed Answer
Overflow during a storm is the gutter system telling you something specific. Identifying which failure mode is happening helps determine the correct fix.
Debris blockage is the most common cause. Even gutters that appear clear from ground level can have partial downspout clogs or compressed debris mats that restrict flow significantly. A gutter 30% blocked with wet leaf debris and a single clogged downspout can cause overflow during moderate rain that an unobstructed system handles without issue. Overflow that resolves itself between storms and returns with the next heavy event almost always points to debris accumulation.
Undersized gutters can't be fixed by cleaning alone. A 5-inch gutter on a home with a steep roof pitch and a large uninterrupted drainage plane can be physically overwhelmed during atmospheric river rain rates even when perfectly clean. If gutters overflow despite being recently cleaned, the system is undersized. The solution is upgrading to 6-inch gutters or adding additional downspout outlets to reduce the drainage load per section.
Downspout quantity and placement matter more than most homeowners realize. The general guideline is one downspout per 30–40 linear feet of gutter on flat-to-moderate roofs, and one per 20–30 feet on steep pitches. Homes with long gutter runs that have only one downspout at each end often overflow in the middle during heavy rain because the slope is sufficient to move water in dry conditions but the volume exceeds what two downspouts can discharge during a surge.
Improper slope is often a deferred issue. Gutters installed level — or with reverse pitch, sloping away from the downspout — hold water and overflow at the low end. This is particularly common on sections of gutter that have been rehanged multiple times, or where hanger failure has allowed a section to settle flat. A slope adjustment is a relatively inexpensive repair.