Gutter Troubleshooting: Overflow, Foundation Splash, and Sizing vs Clog vs Slope

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Overflow During Normal Rain vs Design Storms

Water coming over the front lip is the symptom everyone notices. Timing tells you more than the splash:

• Every rain, including drizzle. Usually a clog, a crushed downspout, or a sag that has reversed the pitch. Capacity math is not the first stop. Scoop the trough, run a hose from the high end, and watch whether water moves to the outlet.
• Only in heavy, short storms. That is the IPC design-hour pattern. A 5" K-style that is fine at 2 in/hr can exceed 74 gpm at 5 in/hr on a big eave. Run plan area and a local intensity through the calculator before you blame the weather.
• Only at inside corners or long runs. Two slopes dumping into one inside miter, or 50 ft of trough to a single 2×3, is a count-and-layout problem even if the total gpm looked acceptable on paper. Practical max run is about 35–40 ft per downspout.

Water Damage at the Foundation

Gutters that hold on the roof but dump at the corner still fail the job. A 2×3 that is sized for 30 gpm and then ends 6 inches off grade is a splash factory. Look for: mulch washed away at the downspout, staining on the foundation, a basement wall that weeps in the same storm the eave survived.

Fix the discharge, not only the trough. An extender that fits 2×3 and 3×4, pitched away from the wall, is how the calculated gpm leaves the building. Buried flex pipe that kinks is a clog with extra steps. If the trough itself is overflowing, foundation splash is a second symptom of the same undersized or blocked system — size it, then extend it.

Ice Dams and Gutters in Winter

Ice in the trough is often blamed on “too-small gutters.” The dam itself usually starts on the deck: a warm attic melts snow, water runs to the cold eave, and refreezes. A gutter full of ice then has nowhere to put the next melt, so water backs under the shingles. Upsizing from 5" to 6" does not heat the attic.

Ventilation, air sealing, and insulation are the dam controls — start with the attic ventilation calculator and the attic troubleshooting page. What gutters can do in ice country: keep the trough empty going into a freeze (clogs become ice plugs), keep downspouts clear so melt has an exit, and hang the front edge below the roof plane so sliding snow is less likely to tear the trough off. Heat cable in a clogged 5" is not a sizing upgrade. The code ice-barrier band is also not a gutter size: ice & water shield coverage calculator.

When It Is Sizing vs Slope vs Clog

Walk it in this order:

1. Look in the trough. Leaves, shingle grit, and a bird nest are a clog. Clean it. A guard does not add capacity; it only tries to keep the capacity you already hung.
2. Check slope. IPC Table 1106.6 capacities assume a pitched gutter (the 1/8 in/ft column is what this cluster uses). A dead-level decorative hang holds water and overflows at the middle sag. A string line from outlet to high end should show fall toward the downspout, not away from it.
3. Count outlets and run length. One 2×3 at the corner of a 60-ft eave is a layout miss even if total Q looked small. Add a downspout before you jump a size, unless per-section flow already exceeds the 5" line.
4. Then run the numbers. Plan area (footprint of this slope — get it from the square calculator if you need it), SMACNA pitch factor, local 100-year hourly rate. If Q per section is over 74 gpm, 5" K-style at 1/8 in/ft is past Table 1106.6. That is a sizing issue. If Q is 30 gpm and it still overflows, it is not.

Attic moisture and ice on the deck are a different system: attic ventilation troubleshooting.

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