Gutters are among the least considered components of a house and among the most consequential when they fail, because they fail continuously for months rather than in a single event.
Capacity is a real question here
How much water a gutter system needs to carry depends on the roof area draining into it, the roof pitch — steeper roofs deliver water faster — and local rainfall intensity.
Washington’s rainfall is persistent rather than intense, which sounds easier and is not, because it means the system works continuously through the wet months rather than occasionally.
Thurston County deserves specific mention. Olympia receives more annual rainfall than Seattle, on flatter ground with slower-draining soils. Capacity adequate in a drier location can be marginal there.
Downspouts matter as much as gutters
An underappreciated point. A large gutter feeding too few downspouts still overflows, because the constraint is the outlet rather than the channel.
More downspouts, well distributed, generally beats larger gutters with the same number of outlets. If your gutters overflow at particular points during heavy rain, the answer may be an additional downspout rather than a bigger gutter.
Materials
Aluminium. The most common. Seamless systems formed on site have fewer joints to leak, which matters in a climate where the system runs continuously.
Steel. Stronger and better able to handle the weight of debris or, east of the Cascades, ice. Heavier and subject to corrosion where coating fails.
Copper. Long-lasting and expensive, generally chosen for appearance on period properties.
Vinyl. Inexpensive and can become brittle with age and cold, which is more of a consideration east of the Cascades.
Coastal properties — Kitsap, coastal Pierce, the Skagit islands — should account for salt air, which affects fasteners and any exposed metal faster than inland.
Guards, honestly
Gutter guards reduce maintenance. They do not eliminate it, whatever the marketing says.
Their performance against conifer needles is considerably worse than against broad leaves, and Washington’s mature conifers produce exactly the debris guards handle least well. Needles are small enough to pass through many designs or to accumulate on top of them.
Where they earn their place is on a roof that is difficult or hazardous to access, where reducing the frequency of a dangerous task has real value. Treat never-clean-again claims with scepticism under Washington conifers.
Ice, east of the Cascades
Gutters in Spokane and Yakima carry ice as well as water. Ice adds substantial weight, and ice dams at the eave push water back under roofing.
Stronger hangers and more of them matter here. And as covered elsewhere, ice dams are caused by attic heat loss rather than by the gutter — the gutter is where the symptom appears.
Installation details worth asking about
Fall toward the downspouts, correct but frequently wrong after settling or poor installation. Hanger spacing, which determines whether the gutter sags under load. How the system is fixed to the fascia. Where downspouts discharge, which is the part homeowners most often overlook.
The discharge point
Worth repeating because it undoes everything else when wrong. A perfectly sized, perfectly installed gutter system that deposits water at the base of the wall has moved the problem rather than solved it.
Extensions, splash blocks or buried drainage carrying water well clear of the foundation is what completes the system. Watch during heavy rain to see where it actually goes.