5-Inch vs 6-Inch Gutters: Which Size Does Your Home Need?

For many homes, a 5-inch K-style gutter is enough. A 6-inch gutter makes sense for large or steep roof sections, for valleys that dump water onto a short run, and for long runs where you cannot add downspouts. The real answer comes from three numbers: how much roof drains to each gutter, how steep the roof is, and how hard it rains here.

K-style is the common gutter shape with a flat back and a decorative front that looks a bit like crown molding. The 5 or 6 is its width across the top, in inches.

How is gutter size worked out?

Gutter makers and sheet metal designers use the same basic method. EPA's moisture control guidance points to it too: size gutters and downspouts by roof area, pitch, and rainfall intensity. Berger, a gutter maker, lays it out in its gutter and downspout sizing guide. The Copper Development Association's design handbook uses the same steps.

1. Find the roof area that drains to each gutter run.
2. Adjust that area for the roof pitch.
3. Look up how hard it rains where you live.
4. Size the downspouts, then size the gutter to match.

You do not need to do the math to hire someone. But it helps to know what a good installer should be thinking about.

Step 1: How much roof drains to each gutter?

Measure each roof section that drains to one gutter run. Use the length of the eave times the flat distance from the eave to the ridge, as if you were looking down from above. That is the "plan area."

A typical ranch house splits into a front and a back section. A two-story house with dormers, porches, and valleys can have many sections, and some of them dump water onto others.

Step 2: Why does roof pitch matter?

Rain does not always fall straight down. Wind drives it into the roof face, so a steeper roof catches more of it. Water also runs off a steep roof faster and can shoot past the gutter edge.

Berger's guide adjusts the plan area with these factors:

Step 3: How hard does it rain in Cincinnati?

Gutters are sized for short, hard bursts, not daily totals. The standard is the heaviest 5 minutes of rain you would expect about once in 10 years, stated in inches per hour.

Berger's guide and the Copper Development Association handbook both list Cincinnati at 6.8 inches per hour for that storm. NOAA's Atlas 14, the federal rainfall frequency source, gives about 7.0 inches per hour for downtown Cincinnati. The numbers agree closely.

At that rate, both guides say each square inch of downspout opening can drain about 180 square feet of roof.

Step 4: How many downspouts, and what size?

The downspout is often the real limit. Berger puts it simply: the gutter can never be more effective than the outlet and downspout that drain it.

Using Berger's downspout sizes and the Cincinnati factor of 180 square feet per square inch:

Berger's guide also says downspouts under 7 square inches should serve only small areas such as porches. A 2x3 is 6 square inches. That is one more reason to ask about 3x4 downspouts on a large roof section.

Berger and the CDA both treat 50 feet of gutter as the most one downspout should serve. Berger ties that limit to how metal expands and contracts with temperature.

Two worked examples

A one-story back roof. The eave is 40 feet long. The flat distance to the ridge is 14 feet. That is 560 square feet. At a 5-in-12 pitch, multiply by 1.05 to get about 590 square feet. One 2x3 downspout can drain about 1,080 square feet. A 5-inch gutter with one downspout is a sound choice here.

A two-story back roof. The eave is 50 feet long. The flat distance to the ridge is 20 feet. That is 1,000 square feet. At a 9-in-12 pitch, multiply by 1.20 to get 1,200 square feet. That is more than one 2x3 downspout can drain. You have two good options:

• Keep a 5-inch gutter and add a second downspout.
• Go to a 6-inch gutter with a 3x4 downspout.

Sometimes the cheaper fix is one more downspout, not a bigger gutter. If there is no good spot for a second downspout, 6-inch with a 3x4 outlet is the better plan.

Berger's guide also lists a 100-year storm column. For Cincinnati it drops the drain area to 130 square feet per square inch of downspout. If you want more margin, size to that column.

How much more does a 6-inch gutter carry?

A wider gutter holds and moves more water. In Berger's table for half-round gutters, a 6-inch gutter carries about 50 percent more water than a 5-inch one. K-style gutters follow the same pattern, though the exact numbers differ by profile.

Pitch helps too. Berger calls for at least 1/16 inch of fall per foot. A gutter can be hung level for looks, but then it needs more downspouts.

When does a 6-inch gutter make sense?

Here is my checklist. If two or more of these fit a run, 6-inch is worth a look.

1. A large roof section drains to one run.
2. The roof is steep, 9-in-12 or more.
3. A valley pours water onto the run, often at an inside corner.
4. An upper roof drains onto a lower roof that feeds the run.
5. The run is long and there is no good spot for another downspout.
6. The gutter overflows in hard rain even when it is clean.

A 6-inch gutter is wider and heavier when full. It needs solid fascia and hangers driven into the framing. On a small porch or a short one-story eave, it can look bulky and add cost without much benefit.

How Shamrock can help

We install, replace, and repair seamless gutters in West Chester and across northern Cincinnati, and we size and pitch them to your roof. During a free estimate, we can look at each roof section and tell you where 5-inch is enough and where a larger gutter or another downspout makes sense.

FAQ

Can I mix 5-inch and 6-inch gutters on one house? Yes. Some homes need a larger gutter only on one or two runs, such as below a valley. The rest can stay 5-inch.

Will 6-inch gutters fix overflow? Only if the gutter is too small. Overflow is often caused by clogs, poor pitch, or too few downspouts. Rule those out first.

Do bigger gutters hold more ice in winter? A wider gutter can hold more ice and snow. That adds weight, so hanger spacing and solid fascia matter. Our ice dam guide covers winter loads.

Are 3x4 downspouts only for 6-inch gutters? Not always. It depends on whether the outlet fits the gutter. Berger notes that the downspout size must suit the bottom width of the gutter, so ask your installer what fits your profile.

Related guides

• Gutter installation in Cincinnati: sizes, materials, and guards
• Downspout drainage: keep roof water off your foundation
• Gutter guards compared: mesh, screens, and reverse curve