5-Inch vs 6-Inch Gutters in Charlotte
We provide gutter cleaning, gutter guards, and seamless gutter installation across the Charlotte, North Carolina metro.
This is an arithmetic question, and every number in it is public. Here is the whole calculation, including the parts that make it less certain than it looks.
Why annual rainfall is the wrong number
Charlotte's annual rainfall total tells you almost nothing about what your gutters have to cope with. Roof drainage is sized on intensity - how hard it can rain in a short burst - because a gutter either keeps up with the flow arriving in that burst or it does not. Thirty inches spread evenly over a year would never trouble a downspout. Three inches in an hour will. That is why every serious sizing method starts with a short-duration intensity figure rather than a yearly total, and why a gutter that coped for a decade can be overwhelmed by one summer thunderstorm.
Where the intensity figure comes from
The reference source in the United States is NOAA's Atlas 14, which publishes precipitation frequency estimates for a specific point on the map. You give it a latitude and longitude and it returns a grid: rows for storm duration, columns for how often a storm of that severity is expected. For Charlotte, the partial-duration estimates run like this. In a 5-minute burst, a once-in-two-years storm delivers 5.68 inches per hour and a once-in-ten-years storm 7.27. Over 15 minutes those fall to 3.80 and 4.90. Over a full hour, the two-year figure is 1.65 inches per hour and the hundred-year figure is 3.19.
Intensity falls as duration lengthens, which is simply the observation that it cannot rain at cloudburst rates for an hour straight. The number the plumbing code's roof drainage tables are built around is the 100-year, 60-minute figure - for Charlotte, 3.19 inches per hour.
Turning rainfall into gallons per minute
The conversion is straightforward physics rather than a rule of thumb. One inch of rain falling on one square foot is one twelfth of a cubic foot of water. A cubic foot is about 7.48 gallons, so that inch delivers roughly 0.623 gallons in an hour, which is about 0.0104 gallons per minute. So flow equals 0.0104 multiplied by the roof area in square feet, multiplied by the rainfall intensity. At Charlotte's 3.19 inches per hour, every 1,000 square feet of roof plan area is delivering roughly 33 gallons per minute to whatever is beneath it. That figure is worth sitting with. A modest 2,000 square foot roof at design intensity is producing about 66 gallons a minute - a bath every ninety seconds, arriving continuously, and all of it has to leave through a few small openings.
What the code tables allow
The plumbing code publishes gutter capacity in gallons per minute. At a standard eighth-inch-per-foot slope, a 4-inch gutter carries 39 gallons per minute, a 5-inch carries 74, a 6-inch carries 110, and an 8-inch carries 247. Notice how uneven those steps are. Going from 5-inch to 6-inch is not a twenty per cent improvement to match the twenty per cent more metal - it is roughly a fifty per cent increase in capacity. Cross-sectional area grows faster than width, and that is why the upgrade is often better value than it sounds.
The calculation for a real roof
Divide capacity by flow per square foot and you get the maximum roof area each size can serve at Charlotte intensity. A 4-inch gutter covers roughly 1,180 square feet. A 5-inch covers about 2,230. A 6-inch covers about 3,320. Now apply that to actual houses. A single-storey ranch with 1,600 square feet split across two elevations is putting maybe 800 square feet onto each run - comfortably inside 5-inch. A two-storey house with a long unbroken rear elevation catching 2,600 square feet on one run is past it, and no amount of cleaning will change that.
This is the honest answer to a question we get constantly: why does a gutter that was cleaned last month still overflow? Sometimes because it is dirty again. Sometimes because it was never big enough, and the only thing cleaning bought was a quieter few weeks.
The trough is only half the system
Sizing the gutter while leaving the outlets alone moves the bottleneck rather than removing it. The same code publishes capacities for the vertical pipe: a 2-inch drain carries 34 gallons per minute, a 3-inch carries 87, and a 4-inch carries 180. Put that beside the gutter numbers and the arithmetic gets uncomfortable. A 6-inch gutter running at its rated 110 gallons per minute cannot empty through a single 2-inch outlet rated at 34. Either the outlets get bigger, or there are more of them, or the gutter overflows regardless of size. In practice this is why adding a downspout is sometimes a more effective fix than replacing a run - and considerably cheaper.
Slope matters more than people expect
The capacity figures above assume the gutter is actually falling toward the outlet at the standard slope. A run that has lost its fall - because hangers have loosened, or because it was hung level to look tidy against the fascia - does not achieve its rated capacity. This is one reason a sagging gutter accelerates its own decline. It holds water, the standing water adds weight, the weight loosens the hangers further, and each stage reduces the effective capacity that was calculated for it.
Four caveats, because the numbers are less exact than they look
The code figures are for semicircular gutters. The table's single dimensions are the diameters of a semicircle, and residential K-style gutters have a different cross-section. Treat these numbers as a conservative guide rather than an exact specification for K-style, and treat anyone quoting them to three significant figures with suspicion.
Rectangular downspouts are not in the pipe table. The 2x3 and 3x4 sizes actually fitted to houses are not the round pipe sizes the code tabulates. Converting between them is an inference, and we would rather flag that than quietly present an equivalence as fact.
The code edition matters and we have not confirmed which one applies locally. The figures here come from a recent International Plumbing Code edition. North Carolina adopts its own version of the plumbing code, and the tables can differ between editions. We are not going to cite 'the code' at you without having checked which one governs here.
This is design guidance, not a legal obligation. These tables govern how new drainage is designed. Nobody is coming to tell you the gutters on your existing house are illegal, and any company implying otherwise is selling with a misrepresentation.
Working out the area feeding one run
The number that matters is not the floor area of the house. It is the plan area of the roof draining to that particular gutter - the footprint of that roof plane viewed from above. Two things trip people up. First, pitch does not change plan area: a steeper roof has more surface, but the rain falls vertically, so the catchment is the same footprint. A steep roof does deliver that water faster, which matters, but it does not increase the total. Second, roofs rarely divide evenly. A house with a long rear elevation and a broken-up front may send two thirds of its water to a single rear run. That run is the one to size for, and it is the one that will be overflowing while everything else looks fine.
Pace out the footprint below the run that fails, multiply length by depth to the ridge, and you will have a good enough figure to compare against the 2,230 and 3,320 numbers above.
So what should you actually do
Work out roughly how much roof feeds your worst run - the one that overflows. If it is well under 2,000 square feet and it still spills, the problem is almost certainly debris, lost fall, or an undersized outlet, and replacing the gutter would be an expensive way to not fix it. If that run is catching more than about 2,200 square feet, the size itself is a genuine candidate, and going up is worth pricing against another decade of cleaning it more often. And if it is somewhere in between, the cheapest useful experiment is usually adding a downspout rather than replacing a run - it addresses the outlet capacity that the arithmetic above shows is often the real constraint.
Either way the useful diagnostic is free: stand outside in heavy rain and watch which run fails first, and whether it fails at the outlet or along its length.
Sources: NOAA Atlas 14 Volume 2 precipitation frequency estimates for Charlotte; International Plumbing Code storm drainage tables. Read 13 August 2026.