Every fall we get calls that open the same way. "The gutters overflow in a hard rain, I think I need bigger gutters."
Sometimes that's right. Usually it isn't. In our experience the trough is rarely the bottleneck. The downspouts are. Swapping 5-inch gutters for 6-inch on a house with two downspouts serving a hundred feet of eave gets you a bigger bathtub with the same undersized drain.
The plumbing of it
A gutter is a horizontal reservoir. Its job is holding water for a few seconds while it moves toward an exit, and the exit is the downspout.
A 5-inch K-style gutter holds roughly 1.2 gallons per running foot. A 6-inch holds about 2.0, which works out to something like a 40 percent bump in cross section. That's a real difference and on big steep roofs it matters.
But the standard 2x3 downspout most homes have drains only about 12 gallons a minute under gravity. Going to a 3x4 roughly doubles it. So on a house with one small spout per long run, the gutter fills faster than the spout empties it, water backs up, and it pours over the front edge. Which looks exactly like a gutter that's too small, and isn't.
The rule we work from is one downspout per 30 to 35 feet of run, and more than that wherever a large roof area funnels into a single trough. Valleys are the usual offender. Two big slopes meeting in a valley dump an enormous volume into about eighteen inches of gutter, and no amount of trough size fixes a valley with its nearest downspout forty feet away.
Diagnosing it yourself, in the rain
Go outside during a real downpour. Not a drizzle. The kind of storm we get in July.
If water sheets over the front edge along the entire run, the system is undersized or clogged, and the next thing to notice is whether it's overflowing everywhere or in one place.
If the overflow concentrates at one end near the low point, the gutter is doing its job and the downspout can't keep up. That's capacity, and adding a second spout at the far end usually ends the problem outright.
If water shoots over the gutter right below a valley, the valley is overshooting. A valley diverter or splash guard plus a downspout near that discharge point fixes it. A whole new gutter system doesn't.
If water drips behind the gutter and runs down the fascia, that's a drip edge problem rather than a gutter problem. Water is wicking under the shingle edge and behind the trough. We see it on older homes around here that were built before drip edge was standard.
And if nothing comes out of the downspout while the gutter is full, you have a blockage, almost always at the elbow where the spout turns to run down the wall. Shingle granules, roof grit and a couple of maple seeds compact into something remarkably solid at that bend.
What we recommend around here
We install both 5-inch and 6-inch seamless gutter, and we size it to the roof rather than to a preference.
Five-inch with adequate downspouts handles most homes in Holmes and Tuscarawas Counties without complaint. It looks proportionate on a typical farmhouse or ranch fascia and it costs less.
Six-inch earns its keep on steeply pitched roofs, on large uninterrupted expanses, and on houses where the architecture forces long runs with nowhere good to put a spout. Steeper pitch means water arrives at the eave carrying more speed and less time to settle into the trough, and a 12/12 roof will throw water clean past a 5-inch gutter that a 4/12 would have caught.
The 3x4 downspouts are worth the small upcharge nearly everywhere and close to mandatory on lots with heavy tree cover, because the wider spout clogs far less readily. This region is not short on trees.
Mostly, though, placement beats size. We'd rather add a third downspout than upsize a whole house.
Two things people get wrong about guards
Gutter guards keep leaves out. They don't add capacity, and most designs take a little away, because water now has to get past a screen or around a reverse curve before it reaches the trough. On a system that already overflows, guards make it worse.
Fix the capacity first, then add guards if the trees warrant it. Doing it the other way around is how people end up convinced guards don't work.
Slope and hangers
Two details that never show up in a homeowner's diagnosis and cause a surprising number of problems.
Gutters need pitch, a slight fall toward the downspout on the order of a quarter inch per ten feet. Too little and water stands in the trough, which is where the sediment line and the mosquitoes come from. Too much and you can see it from the street, which nobody wants.
Hanger spacing matters more in our winters than it does in most climates. Ice load is what pulls gutters off a house here. Hangers set too far apart let a loaded gutter sag into a permanent low spot that then never drains right again, so we space them tighter than the minimum.
When replacement makes sense
Repair handles isolated problems: a sagging section, a leaking seam, a missing downspout, bad drip edge. Replacement makes sense when the system is genuinely undersized for the roof, when seams have started leaking in several places at once, or when the fascia behind it has begun to rot, since at that point you're rebuilding the mounting surface anyway.
If your gutters overflow every hard rain and you've had enough of it, we'll come look at the whole system, tell you which of those five problems you have, and quote what fixes it. More about our gutter services here.
