Why Gutters Overflow in Heavy Rain (Even When Clean)
Updated
Short answer
Usually a clog at the downspout outlet. If the gutter is clean, the cause is too few or too small downspouts, a run that has lost its slope, or an inside corner collecting water from two roof planes. One inch of rain on 1,000 square feet of roof is about 623 gallons.
Diagnose it
- Overflow right at the downspout: the outlet or first elbow is plugged. See unclogging a downspout.
- Overflow at the far end of a long run: the run may be too long for one downspout. HomeGuide uses about one downspout per 30 to 40 feet.
- Standing water mid-run: a low spot; the gutter needs re-pitching ($75 to $200).
- Overflow at an inside corner: two roof planes dump there; a splash guard or a corner downspout helps.
- Water behind the gutter: the gutter has pulled away from the fascia or the drip edge is missing.
Rockford storms
Summer is the test: NOAA's normals put May through August at 17.41 inches combined, mostly in storms. A gutter that handles a drizzle can fail in a thunderstorm.
Quick answers
Why do my gutters overflow when they are clean?
Too few downspouts, a run that has lost its slope, or an inside corner taking water from two roof planes.
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