Gutter Overflowing at the Corner or Below a Valley? Why It Happens and How to Fix It
| # | Table of Contents |
|---|---|
| 1 | Why Corners and Valleys Overflow |
| 2 | Clog, Slope or Volume? How to Tell |
| 3 | Fixes, From Simplest to Biggest |
| 4 | Gutter Guards and Corners |
| 5 | What Overflow Does Below |
| 6 | FAQ |
Why Corners and Valleys Overflow
A roof valley is the inside angle where two roof planes meet. All the rain that lands on both planes near that valley runs into it and travels down the valley as a concentrated stream. At the bottom, that stream lands in a short section of gutter, usually right at an inside corner of the house.
Three things go wrong at that spot:
- Volume. The gutter at the base of a valley carries water from two roof areas instead of one. If it was sized like the rest of the system, it can fill faster than it drains.
- Speed. Water picks up speed as it runs down a valley, especially on steeper roofs. Instead of dropping into the gutter, it can overshoot the front edge in a sheet.
- Corner joints and turbulence. A mitered corner slows and churns the flow, and corner seams are a common spot for debris to snag and for small leaks to start.
Gutter sizing is based on short bursts of heavy rain, not on daily totals. SMACNA, the sheet metal contractors' association, bases its gutter sizing calculator on rainfall intensity over a 5-minute duration. That is why a gutter can handle a steady all-day rain and still overflow at the valley in a short, hard storm.
Clog, Slope or Volume? How to Tell
| What you see | Likely cause |
|---|---|
| Overflow at one corner, gutter clean, only in heavy rain | Volume or speed from the valley above |
| Water shooting past the front edge in a sheet | Valley overshoot; splash guard needed |
| Overflow at the corner in light rain too | Debris at the corner or a blocked downspout |
| Standing water at the corner after the rain stops | Gutter sloped away from the downspout, or a sag |
| Dripping from the corner seam, not over the edge | Leaking corner joint, not overflow |
| Overflow along the whole gutter | Clog, blocked downspout or undersized gutters throughout |
The easiest test is to watch during a heavy rain, safely from the ground. Note whether the water spills over the front edge, pours over the back edge toward the fascia, or drips from the seam. Each points to a different fix.
Fixes, From Simplest to Biggest
- Clear the corner and the downspout. Leaves, oak tassels and shingle granules collect where flow slows down. Flush the downspout too; a partly blocked downspout makes the whole run back up toward the corner.
- Add a valley splash guard. A short metal deflector fastened to the front lip of the gutter where the valley water lands. It catches water that would overshoot and turns it down into the gutter. It is inexpensive and usually solves sheet-over overflow at a single valley.
- Correct the slope. The gutter should fall steadily toward its downspout. If the corner sits at a low point with no outlet, water pools there and spills over. Re-hanging the run fixes this.
- Add a downspout at or near the corner. Giving the valley water its own exit close to where it lands is often the most effective fix for volume problems. Plan where that downspout will discharge before adding it.
- Upsize that run. A larger gutter profile, larger downspouts, or both, on the sections that take valley water. This is the fix when the corner simply cannot carry the water even with a clear outlet.
Our gutter installation and replacement service can size and place gutters and downspouts for exactly these trouble spots and check the roof edge feeding them.
Gutter Guards and Corners
Gutter guards keep leaves out, but at a valley they can make overflow worse. Fast, concentrated water can skim across the surface of some guard types and off the edge instead of falling through. If you have guards and the overflow started after they went in, the corner is the first place to check.
If your home is in the City of Austin's Wildland-Urban Interface area, keep the wildfire code in mind when choosing parts. The City's WUI code page says "gutters and downspouts shall be constructed of non-combustible materials," and its zone tables call for leaf guards.
What Overflow Does Below
Water spilling over a corner lands in the same spot every storm. That can mean:
- Soil washed out and a trench forming in the bed below the corner.
- Water soaking the ground right next to the foundation.
- Splash-back staining and wetting the bottom of the siding.
- Water running behind the gutter onto the fascia if it overflows over the back edge.
A related problem is an upper roof whose downspout empties onto a lower roof, which concentrates water in a similar way. See our guide to a downspout draining onto a lower roof.
Sources: SMACNA Downspout and Gutter Sizing Calculator (rainfall intensity basis, ASMM Table 1-2); City of Austin Fire Department, Wildland-Urban Interface Code page.
Frequently Asked Questions
Why does my gutter overflow at the corner even when it is clean?
Inside corners usually sit below a roof valley, which collects water from two roof planes. In heavy rain, that section gets more water, and faster water, than the rest of the gutter, so it can overflow even when there is no clog.
Do valley splash guards work?
For water that shoots over the front of the gutter at a valley, a splash guard is often enough. It will not fix a gutter that is overflowing because it is full; that needs a better outlet or a larger gutter.
Should I add a downspout at the corner?
If the corner overflows from volume rather than overshoot, a downspout close to where the valley water lands is often the most effective fix. Plan where it will discharge so the water ends up away from the house.
Can gutter guards cause overflow at corners?
They can. Fast, concentrated valley water can skim across some guard surfaces instead of falling into the gutter. If overflow started after guards were installed, check the corners first.
Is a dripping gutter corner the same problem?
No. Water dripping from the seam is a leaking corner joint, which is a sealing or replacement issue. Water spilling over the edge is an overflow problem.
