What a valley does
A roof valley is the inside corner where two roof planes meet. Each plane sends its water toward the valley, and the valley channels it down to one point at the eave. Instead of rain falling evenly along the gutter, a large share of the water from two roof planes arrives at one spot, fast. On steep roofs, the water picks up speed in the valley and can shoot right over the front of the gutter.
That is why overflow at inside corners is one of the most common gutter complaints, even on new homes with clean gutters.
Signs of valley overflow
Look for water pouring over the gutter at one inside corner during heavy rain, a line of erosion or a hole in the mulch below that corner, stained siding and soffit at the corner, and in winter, a large ice formation at the same spot. Inside, a wet basement wall or crawl space at that corner often follows.
Fix 1: a valley diverter or splash guard
A splash guard (also called a valley splash diverter) is a short, angled piece of metal installed on the front lip of the gutter at the corner below the valley. It catches water that would shoot over the gutter and turns it back into the trough. It is inexpensive and often solves the problem alone. It should be fastened securely and sized to the gutter profile.
Fix 2: an outlet and downspout at the corner
If the nearest downspout is far from the valley, water has to travel along the gutter before it can leave, and the gutter fills up at the corner. Adding an outlet and downspout right at or near the corner gives that water an immediate exit. It is one of the most effective fixes on large roofs. See downspout size and placement.
Fix 3: larger gutters and downspouts
A 6-inch gutter holds considerably more water than a 5-inch one, and a 3x4-inch downspout drains about twice as much as a 2x3-inch downspout. On homes with several valleys and large roof areas, upgrading the gutters on the affected runs, or the whole house, removes the bottleneck. See 5-inch vs. 6-inch gutters.
Fix 4: check the valley itself
Debris collects in valleys, especially under trees, and forms dams that send water sideways under shingles or over the edge at odd angles. Valley flashing that ends short of the gutter can drip behind it. Keeping valleys clear and making sure the valley metal or shingles extend properly into the gutter area helps. A roofer can check valley details during a roof inspection.
Fix 5: guards that handle volume
Some gutter guards shed water off the front in very heavy flow, especially below valleys. If you have guards and overflow only at valley corners, ask whether the guard type can handle concentrated flow, or whether the corner section should have a different product or a splash guard. See are gutter guards worth it?
Upper roofs and dormers
Similar problems occur where an upper roof drains onto a lower roof, or where dormers create small valleys. Downspouts from upper gutters that empty onto lower roofs should spread the water with a diverter, and the lower gutter must be sized for the combined flow. See gutters overflowing in heavy rain.
Why it matters
Valley overflow drops a concentrated stream of water at a corner of the house in every heavy rain. Over years that erodes soil, saturates the foundation, rots fascia and soffit and stains siding. In cold climates, it builds ice at the corner. Fixing it is usually inexpensive compared with the damage it causes. See downspouts and your foundation.
Ice at valley corners
In cold climates, valley corners are also where ice builds first. Meltwater from two roof planes reaches the corner, refreezes in the gutter and builds an ice dam that can push water under shingles. Heat cables in the valley corner of the gutter and down the nearest downspout can keep a path open, and attic air sealing reduces the meltwater in the first place. See heat cables for gutters and ice dams and gutters.
Valleys over entries and walkways
When a valley drains toward a front door, garage door or walkway, overflow is not just a drainage problem but a safety one: water pours on people in rain, and ice forms where they walk in winter. These corners deserve the most robust fix: a splash guard, an outlet and downspout right at the corner, a larger gutter section and, in cold climates, heat cable. Routing the downspout away from the walkway completes the job. See gutters for porches, decks and patio covers.
Metal roofs and valleys
On metal roofs, valleys carry water even faster than on shingles, and snow slides down them in winter. Splash guards, larger gutters and snow guards above the gutter line help the gutter survive both. See gutters for metal roofs.
Testing a fix
After adding a splash guard or downspout, watch the corner in the next heavy rain. If water still shoots over, the gutter or downspout is still too small for the flow, or the guard is not positioned right. It is better to test and adjust than to assume the first fix worked.
New homes with complex roofs
Many newer homes have roofs with multiple gables, hips and valleys, but standard 5-inch gutters and a handful of 2x3-inch downspouts. Overflow at valley corners is one of the most common complaints on these homes. When planning gutters for a new home, or replacing builder-grade gutters, ask the installer to identify every valley and plan splash guards and outlets near them.