Here’s a specific scenario. It’s December, a real storm is coming through, and you’re watching water pour over the front edge of your gutter in a sheet. You had them cleaned six weeks ago. There’s nothing in them.
That’s not a debris problem. That’s a capacity problem, and it’s one of the most common things we get called out for in the valley.
The Number That Matters Isn’t the Annual Total
San Ramon gets somewhere around 18 to 20 inches of rain a year depending on where you are in the valley. Compared with almost anywhere east of the Rockies, that’s nothing, and it’s why gutters here get treated as an afterthought.
But annual rainfall is the wrong measurement entirely. What a gutter has to survive is peak flow: the most water that hits it in a single hour, not the total it handles across a year.
And our rain is unusually concentrated. Research from the Center for Western Weather and Water Extremes at Scripps found that in Northern California, roughly 60 to 120 hours of rainfall per year deliver more than half the annual total. Most of our water arrives in a small number of atmospheric river events, and one of those can drop an inch in an hour.
A gutter doesn’t experience 20 inches spread over twelve months. It experiences an inch an hour, several times a winter, with everything else being drizzle. Size the system for the average and it fails during the events that actually cause damage.
Why So Many Valley Homes Have a 5 Inch System
Five inch is the builder default nationwide. On a tract of several hundred homes it’s specified once for the whole development, priced by the foot, and installed the same way on every roofline regardless of what that particular roof does with water. That isn’t necessarily wrong, and five inch handles a great many homes here perfectly well. But “specified once for the subdivision” is different from “sized for your roof,” and the gap shows up in a hard December rain.
What actually determines whether a gutter overflows is how much roof area drains into a given run, how long that run is, how many downspouts it has, and whether water is being fed into it from above. A 40-foot run with one downspout collecting a large roof plane will struggle no matter how clean it is. The water has nowhere to go.
What the Sizes Actually Do
A 6 inch gutter is only an inch wider than a 5 inch, which is why the difference gets dismissed. But it carries roughly 40 percent more water, and the downspouts that pair with it are correspondingly larger. That’s not an incremental improvement. That’s the difference between coping with an atmospheric river and giving up at the front lip.
Here’s how we size it:
| Size | Best suited to |
| 5 inch | Runs under 40 feet with at least one downspout. Gutters that don’t currently overflow in normal rain. Runs that aren’t receiving drainage from an upper roof. |
| 6 inch | Runs over 40 feet that can’t accommodate a second downspout. Any run over 60 feet. Lower gutters catching water from upper gutters. Rooflines that funnel a large area into a short section. Inside valleys that overflow in heavy rain. Metal roofs. |
| 7 inch and larger | Available for unusual rooflines and high-volume situations. Less common on residential work, but it exists when the math calls for it. |
| Mixed sizes | You don’t have to pick one for the whole house. Running 6 inch on the two problem elevations and keeping 5 inch elsewhere is often the most cost-effective answer. |
That last row is worth sitting with, because most homeowners assume it’s an all-or-nothing replacement. It usually isn’t. If one side of your house handles storms fine and the north elevation floods every year, that’s where the money should go.
When 5 Inch is Genuinely Fine
Plainly: if your run is under 40 feet, it has a downspout, no upper roof is draining into it, and it doesn’t overflow during a real storm, your gutters are correctly sized. Leave them alone.
Upsizing a system that isn’t struggling is a waste of your money. We’d rather clean those gutters twice a year for a decade than sell you something you don’t need. If you call us out and that’s what we find, that’s what we’ll tell you.
The homes that need a second look are the ones with a symptom: water going over the edge in heavy rain, a section that erodes the same patch of landscaping every winter, an inside valley that fires a stream past the gutter entirely, or a persistent damp spot near the foundation on one elevation.
A Note for Dougherty Valley
If you’re in Windemere, your neighborhood was built between roughly 2002 and 2006, which puts those gutter systems at about twenty years old. Gale Ranch went up in phases from 2003 through the late 2010s, so it varies more by street.
Two things follow from that. The first is age: twenty-plus years of thermal cycling loosens hangers, opens seams, and flattens the pitch that moves water toward the downspout. Sometimes what looks like a capacity problem is really a repair problem, and a section that’s sagged half an inch will overflow at that low point long before the rest of the run is at capacity.
We check that first, because it’s the cheaper answer and it’s often the right one.
The second is the original spec. These are large homes with complex rooflines, multiple gables, and upper roofs draining onto lower ones, all fitted with the same builder-grade 5 inch profile. Some of those rooflines were always going to outrun the gutter attached to them.
How to Figure Out Which One You Have
You don’t need us to start. Next time a real storm comes through, walk the outside of the house and look.
Water spilling over the front lip along a long stretch of clean gutter is a capacity signal. Water overflowing at one specific low point while the rest of the run stays fine is usually pitch or a sagging hanger. Water shooting past the gutter entirely where a valley dumps in is a concentration problem, often best fixed with a larger gutter on that one elevation.
Note which elevation and roughly where. Five minutes of that is more useful than anything we can determine from the ground on a dry day.
We’ll Size It to Your Roof
Our free roofline assessment is exactly what it sounds like. A Solutionist walks your home, measures the runs, looks at where the roof planes drain and where the water concentrates, checks pitch and hanger condition on what’s already there, and tells you what size the system should be. Sometimes the answer is a full installation in 6 inch. Sometimes it’s one elevation. Sometimes it’s re-pitching a run you already have.
Book a free roofline assessment or call (925) 905-9555, and we’ll give you a straight answer either way.