Two Roof Slopes Into One Gutter: Size the Run for Both

If your roof has two slopes that drain toward the same stretch of gutter, the trough at the bottom of that junction does not see the water from one roof plane. It sees the water from both, arriving at the same spot in the same storm. Most gutter quotes are written as if each run collects from a single slope, and that works for a plain gable. It does not work where a valley, a hip, or a cross-gable pours into one trough. This page is for the homeowner comparing quotes who wants to know whether the run was sized for what actually lands in it.

Where the extra water comes from

A valley is the line where two roof planes meet at an angle and form a channel. Rain that falls on either plane runs down toward that channel, and at the bottom of the valley it leaves the roof in one concentrated stream. If the valley ends at the eave above a gutter, the gutter receives the flow from both planes at a single point. A hip roof does the same at its corners, and a cross-gable shows the pattern where a shorter wing ends against a longer one. In each case the water arriving at one stretch of trough is larger than what either plane would send alone.

The load depends on the square footage each slope drains, the steepness of the pitch, the rainfall intensity your installer uses, and whether the water arrives as a sheet or a stream. Sheet flow spreads across the whole trough. Valley flow arrives as a concentrated stream, and it can pour over the front lip of the gutter at the point where it lands if the trough is undersized there. That overshoot is the failure homeowners notice first, usually as a stain on the fascia or a splash line on the siding below the junction.

What the trough has to carry

Gutter capacity is set mostly by cross-section and slope. A five inch K-style trough and a six inch trough differ in how much water they hold before the flow backs up. Trough size is only part of the picture. The gutter also has to be pitched toward the outlet so the water moves rather than sitting. A run with a flat section at the valley end will hold water there, and the overflow will start at that low spot even if the trough is large.

When two slopes share one run, the trough at the junction should be checked against the combined area, not against either slope on its own. Ask the installer to state which slopes feed each section of the run. A quote that says "gutter, 60 feet, 5 inch" without showing where the valley lands does not tell you whether that section was sized for the valley.

The downspout at the bottom of the valley

The downspout is where the trough's capacity either holds or fails. A 2 by 3 inch downspout drains less than a 3 by 4 inch downspout, and a single downspout at the bottom of a valley may be asked to pass water that a 2 by 3 cannot clear quickly enough in a heavy storm. Installers often respond with a larger downspout at the valley outlet, a second outlet placed at the low point, or an extra downspout that splits the flow. Each choice changes the layout, and each should appear on the written scope.

Outlet placement matters as much as outlet size. A downspout placed several feet away from the valley's lowest point forces the water to travel along the trough, and the trough has to carry it there. Placing the outlet directly under the valley shortens that travel. Ask where the outlet sits relative to the valley, not only how many outlets the run has.

Hanger spacing near the valley deserves the same attention. The hangers nearest the outlet carry the most water weight during a storm and the most ice weight in winter. If the quote does not state the spacing for that section, ask for it in writing, because a trough that sags a half inch at the valley will hold water even after the flow has passed.

Downspout straps and elbows at the valley outlet take the same load as any other downspout, but they take it more often. A loose strap at the bottom of a valley drain can let the elbow swing, and that movement starts a joint leak that you may not see until the wall below is wet. Check the straps after the first heavy storm following the install.

Freeze and thaw at the valley outlet

St. Louis winters cycle the metal and the sealant at the outlet many times between November and March. Water that sits in an undersized section can freeze, and ice in a trough adds weight to the hangers nearest the valley. Where a valley drains into a trough with a flat spot, the ice tends to form at that spot first. If the spot is also where the downspout connects, the frozen mass can crack the connection or push the elbow out of line.

Heat cable is sometimes proposed as a fix for ice at the valley outlet. A heat cable can keep a path open, but it does not change the slope or the capacity of the trough. Treat it as a separate line on the quote, not as a substitute for sizing the run correctly.

Where a gutter guard fits

A gutter guard over a valley run changes the flow as well. A guard that sheds water well at a sheet-flow section may hold water at a valley section if the guard surface is dense enough to slow entry. The valley is where leaves and shingle grit concentrate, so the guard there gets the most debris. If you are adding guards to a run that includes a valley, ask the installer which guard type covers the valley section and how the entry rate was checked at that point. Do not accept a general statement that the guard works across the whole run.

Guards also do not change the trough's capacity. A guard sitting over an undersized trough only makes the overflow harder to see until it happens. If the trough was undersized before the guard went on, the guard does not fix that.

What to ask in writing

1. Which slopes drain into each section of the run, and where the valley or hip outlet sits.
2. The trough size and the pitch across the valley section.
3. The downspout size at the valley outlet, and whether a second outlet or a larger downspout is included.
4. The hanger spacing in the valley section and how the straps and elbow at the outlet are fastened.
5. Whether any heat cable or ice management is priced as a separate line.
6. Whether the guard, if one is included, is specified for the valley section.

Keys Roofing puts a written scope with photos and measurements in the homeowner's hands. If you are comparing gutter and guard quotes for a home with a valley or hip junction, the office at (314) 220-2333 can walk through the run with you before you choose.