Last Updated: October 23, 2026
Short answer
Most houses are well served by 5-inch K-style gutters. A 6-inch gutter makes sense where a large or steep roof area drains to one run, or a valley dumps water at one corner, and it usually pairs with 3x4-inch downspouts. Size each run from its own roof area, pitch, valleys and outlets, not from the old gutter.
Key takeaways
- Size is decided per run: the roof area draining to that eave, how steep it is, and whether a valley feeds it.
- A 6-inch gutter holds noticeably more than a 5-inch and usually pairs with larger 3x4-inch downspouts.
- Outlet count and downspout size often limit a system before the gutter width does.
- NOAA's Atlas 14 design-storm estimates are not published for Kent, so sizing leans on the roof itself and Kent's long, steady wet season.
The short answer: 5-inch for most runs, 6-inch where the roof demands it
The right gutter size is the one that carries the water from the roof above it to an outlet without spilling, and that is worked out one run at a time. Gutters of Kent installs seamless gutters in both 5-inch and 6-inch K-style across Kent and South King County, with the work done by licensed and insured contractors, and the size is set at the walk-around for each roof edge rather than copied from what came off the house.
A 5-inch K-style suits a typical roof plane draining to a typical eave. A 6-inch K-style holds noticeably more water and earns its place in three situations: a large roof area draining to a single run, a steep roof that catches and sheds more wind-driven rain, and a valley that concentrates water from two roof planes onto one short stretch of gutter. The rest of this guide explains each input and the places where going bigger solves nothing.
What decides the size of a gutter run
Roof drainage area, measured from above
The drainage area for a run is the part of the roof that sheds into it, measured in plan, as if you were looking straight down. That is different from the square footage of shingles. A wide rear slope that runs from the ridge to one eave sends all its water to one gutter, while a hip roof splits its water among four eaves. Two houses with the same floor area can need different gutters because their roofs divide the water differently.
Roof pitch
A steep roof presents more surface to rain blown at an angle, and water leaves it faster. Sizing methods account for this by increasing the effective drainage area as pitch rises, so a steep slope is treated as larger than its plan footprint. Pitch also affects how far water shoots past a gutter’s front lip in a downpour, which matters as much as volume on very steep roofs.
Valleys
A valley is the inside angle where two roof planes meet. It gathers water from both and delivers it to a short section of gutter, usually near an inside corner. That spot can overflow while the rest of the run copes easily. The fix depends on how much water arrives: a 6-inch gutter on that run, an outlet placed close to where the valley lands, or a diverter that spreads the flow.
Outlets and downspouts
A gutter can only empty as fast as its outlets let water leave. A 2-by-3-inch downspout has a cross-section of 6 square inches; a 3-by-4-inch downspout has 12, double the area, and is far less likely to clog with needles. A long run pitched to one outlet at the far end makes the whole run carry the whole roof’s water past every point where it could have left. Splitting it to outlets at both ends halves the load on each. The guide to how many downspouts you need covers spacing, and downspout sizes compares 2x3 and 3x4.
5-inch vs 6-inch gutters compared
| 5-inch K-style | 6-inch K-style | |
|---|---|---|
| Capacity | Suits a typical roof plane per run | Holds noticeably more water per foot of run |
| Usual downspouts | 2x3 or 3x4 inch | 3x4 inch |
| Fits best | Average roof areas, hip roofs, one-story eaves | Large or steep areas to one run, valley corners, long uninterrupted eaves |
| Debris | Narrower trough and smaller outlets clog sooner | Wider trough and larger outlets pass needles and leaves more readily |
| Look | Standard proportion on most houses | About an inch wider and deeper; more visible on a low eave |
| Cost | Base per-foot price | Usually a few dollars more per foot than 5-inch |
For budgeting, 5-inch seamless aluminum, installed, runs a typical Seattle-area range of about $8 to $13 per linear foot. These are budgeting figures, not quotes, and the Kent gutter installation cost guide explains what moves a house within the range.
Why there is no design-storm number for Kent
Formal gutter sizing tables pair a drainage area with a design rainfall intensity: the heaviest short burst of rain expected over a set return period. In much of the country that figure comes from NOAA’s Atlas 14 precipitation-frequency estimates. Atlas 14 is not published for Kent, so this site does not quote a design storm intensity, and any gutter calculation that claims a precise Kent figure is borrowing one from somewhere else.
What NOAA does publish is the Kent station’s 1991 to 2020 climate normals: 40.09 inches of precipitation a year, with November averaging 6.23 inches, December 5.75 and January 5.77, against 0.67 inches in July. The station averages 96.7 days a year with at least a tenth of an inch of rain and 6.6 days with an inch or more. That is a long, steady wet season rather than a climate of frequent cloudbursts. For sizing, it shifts weight toward outlets that stay clear through months of rain and debris, and toward enough downspout area that a partly clogged drop does not stop the run.
Oversized gutters: when bigger helps and when it does not
“Oversized” usually means stepping a residential run up to 6-inch, sometimes with larger 3x4 or wider outlets, and on commercial buildings it can mean larger custom profiles; commercial gutters covers those. Bigger capacity is the right call when the roof truly sends more water than a 5-inch run with clear, well-placed outlets can carry.
It is the wrong call when the overflow has another cause, and a larger gutter will spill just as readily if:
- the outlet or bottom elbow is clogged, because a full trough of any size still has nowhere to drain;
- the run is pitched away from its outlet, so water collects at the low end with no drop;
- a valley lands far from any outlet, and the corner overwhelms before the water reaches one;
- the drip edge stops behind the gutter, so water runs down the fascia rather than into the trough.
The guide to overflowing gutters separates those causes, and water behind gutters covers the last one. A bigger gutter is also heavier when full, which puts more load on each hanger and on the fascia it screws into. That is one more reason the walk-around checks the board before anything is hung, and why gutter hangers are spaced to the load.
How the size is chosen at the house
At the walk-around each roof edge is measured with its corners, and the roof above it is read for area, pitch and valleys. Outlet positions come next: where the roof sends its water, and where a downspout can run down the wall and let go somewhere it is allowed to go. Kent City Code 7.04.180 keeps roof runoff out of the sanitary sewer, and under the King County manual Kent adopts, a splash block can disperse water from no more than 700 square feet of roof. A large roof area draining to one outlet can exceed that, which is a reason to split the run regardless of gutter size. Where downspouts should drain explains the options.
Then the size: 5-inch where the area and outlets comfortably handle it, 6-inch where a run still carries more than that. On a replacement, the old system is evidence, not a template. Stains on the fascia below one corner or a trench in the soil under one stretch show where the old gutter was losing, and the new layout answers that spot with an outlet, a size change or both. Gutter replacement covers the rest of that scope.
Before you decide
Watch your current gutters in a steady November rain and note exactly where water escapes: over the front, at a corner, behind the gutter or at the bottom of a downspout. That one observation separates a sizing problem from an outlet, pitch or drip edge problem. Pair it with the seamless vs sectional comparison if you are replacing the whole system, then Get an Estimate to have each run sized against its roof.
Questions about your own house? Call (253) 352-5321 or get an estimate.