Gutter sizing is usually decided by whatever the installer normally puts up. It is worth a few minutes of arithmetic instead, because the calculation is simple and the consequences of getting it wrong show up as fascia rot rather than as an obvious failure.
An inch of rain falling on 1,000 square feet of roof produces roughly 600 gallons of water. That is the number worth internalising, because it reframes what a gutter is doing.
Seattle's advantage is that our rain is not intense — we get roughly 37 inches spread across about 150 days, mostly as steady moderate rain rather than the violent cloudbursts that overwhelm systems in the Southeast. Peak intensity here is modest.
Our disadvantage is duration and debris. A gutter here is not asked to survive one extreme event; it is asked to keep working continuously for eight months under a canopy that fills it the entire time. That changes what "adequate capacity" means, and it is the reason the sizing conversation in Seattle is really a debris-tolerance conversation.
Five-inch K-style is the residential default and is genuinely adequate for most Seattle homes on rainfall grounds alone.
Six-inch carries substantially more water — the increase in cross-sectional area is larger than the one-inch difference suggests — and takes a larger downspout. It is worth the upgrade in four situations: large roof areas draining to few outlets, steep pitches that deliver water to the gutter fast and with momentum, long runs where water has far to travel, and homes under heavy conifer canopy.
That last case is the important one locally. The benefit of a six-inch gutter under trees is not raw capacity, it is that a partially blocked six-inch gutter still functions where a partially blocked five-inch gutter overflows. You are buying tolerance for the debris you know is coming.
Half-round profiles are common on historic homes for appearance reasons. They hold less water than K-style of the same nominal size, but they are considerably easier to clean because there is no flat bottom or inside corner for debris to pack into.
The most common under-specification in residential gutter systems is too few downspouts, and adding them improves real-world performance more than upsizing the trough.
A working guideline is one downspout for roughly every 30 to 40 feet of gutter run, with more where a large roof area drains to a single point — for instance where a valley discharges into one section of gutter, delivering the concentrated runoff of two roof planes to one place.
The reason more downspouts help so much is not total capacity, it is travel distance. Water entering a gutter has to reach an outlet. In a long run with one downspout at the end, water travels a long way through a channel that may be partly obstructed, and any blockage between the entry point and the outlet causes an overflow. Shorter distances to an outlet mean fewer opportunities for that.
Downspout size: the common 2x3 inch profile is a frequent bottleneck, particularly under conifers, because its narrow dimension clogs readily with needles. A 3x4 inch downspout carries considerably more and clogs far less. On a debris-heavy site it is one of the better small upgrades available.
Gutters need to fall toward the outlets, conventionally somewhere around a sixteenth of an inch per foot of run.
Too little and water stands in the trough, which accelerates corrosion, gives debris somewhere to settle and mat, and provides breeding habitat. Too much and the gutter reads visibly crooked against the fascia line, which looks wrong from the street.
The diagnostic is straightforward: after rain stops, look for standing water. A properly sloped gutter drains. A section holding water permanently has a slope or a sag problem, and both get worse over time as accumulated weight pulls it further out of line.
Hidden hangers screwed into solid fascia are the current standard, typically spaced around every 24 inches. Under heavy canopy, closer spacing is justified, because debris and water loading is greater and sustained.
Spike-and-ferrule attachment — a long nail driven through the front lip, through a spacer sleeve, into the fascia — is common on older homes and works loose over time as the wood cycles wet and dry. If your gutters are held by nails through the front edge, they are on a countdown, and the failure usually happens during a storm with a loaded gutter.
Seamless aluminium is the regional standard, formed on site so there are no joints along the run to leak — seams are where sectional gutters fail first.
Gauge matters more than people expect. Heavier aluminium resists denting from ladders, falling limbs, and the general abuse of a debris-heavy environment. Steel is stronger but corrodes at cut edges. Copper lasts generations, costs accordingly, and has the incidental benefit of shedding trace ions that suppress moss on surfaces below it.
For a typical Seattle house under trees: six-inch seamless aluminium in a heavier gauge, 3x4 inch downspouts, one downspout per 30 feet with additional outlets below valley discharges, hidden hangers at 24 inches or closer, correct slope to outlets, and discharge carried well away from the foundation.
That is a modest upgrade over a default installation and it materially changes how the system performs across eight wet months.
This post is part of our in-depth coverage of moss removal & gutter care in Seattle.
Gutters, Downspouts & Drainage → Moss Removal & Gutter Care →
Five-inch suits most homes on rainfall grounds since our rain intensity is modest. Six-inch is worth it under heavy conifer canopy, on large roof areas draining to few outlets, on steep pitches, and on long runs - because a partly blocked six-inch gutter still works where a five-inch one overflows.
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