Decking · Cool & damp

How a composite deck ages in Seattle, WA

THE SHORT ANSWER

In Seattle, composite won’t rot, but the damp still matters: about 154 rainy days and 83% average humidity mean shaded boards grow surface mildew unless they are washed once or twice a year. Model estimate: 25–33 years of service.

Main threat
Moisture & decay
Climate profile
Cool & damp
Expected life
25–33 yrs
Wash
Spring & fall
Rainy days / yr
154
Days ≥ 90 °F
3
Simulated composite deck in Seattle, WA after 10 years
Year 10Slight early fade
MODEL SIMULATION
Seattle, WA · capped composite boards, south-facing, full sun
Color lossModerate
Surface wearMinimal
Mildew / rotModerate
Fails first hereShaded boards (surface mildew)

Storyboard

20 years in Seattle, washed yearly

Composite deck in Seattle, install: Install day
InstallInstall day
Composite deck in Seattle, year 1: Slight early fade
Year 1Slight early fade
Composite deck in Seattle, year 3: Slight early fade
Year 3Slight early fade
Composite deck in Seattle, year 5: Slight early fade
Year 5Slight early fade
Composite deck in Seattle, year 10: Slight early fade
Year 10Slight early fade
Composite deck in Seattle, year 15: Slight early fade
Year 15Slight early fade
Composite deck in Seattle, year 20: Slight early fade
Year 20Slight early fade

Seattle’s wear fingerprint

Each bar is scaled to the highest of the 25 cities we cover; the black line marks the median.

Rainy days / yr154.2 · High
Avg humidity83.2% · High
Decay index52.6 · Typical
UV index (avg)0.8 · Low
Freeze–thaw days29 · Typical
Days ≥ 90 °F3.1 · Low

NOAA station SEATTLE BOEING FLD (USW00024234, 7.3 km away), 1991–2020 normals · humidity & UV: NASA POWER.

What fails first here

  1. 1Surface mildewComposite does not rot, but pollen and dirt on shaded boards feed mildew. A wash in spring and fall prevents it.
  2. 2Early color fadeCapped boards lighten in the first 2–3 years, then level off. Stronger here than average: UV rank #25 of 25.
  3. 3Gaps & fastenersBoards expand and contract with temperature; check hidden clips and end gaps every few years.
  4. 4Heat underfootOnly about 3 days a year top 90 °F, so surface heat is rarely an issue.

Upkeep

Seattle maintenance calendar

Built from local normals: bar width is the share of days with rain; composite mainly needs washing.

Jan
Feb
MarWash & check for mildew
Apr
May
Jun
Jul
Aug
Sep
OctWash, clear gaps
Nov
Dec

Same city, other materials

How the alternatives age in Seattle

Cedar deck in Seattle at year 10 (simulation)
Cedar deck
Expected life here: 15–19 yrs · main threat: moisture & decay
See it age in Seattle →
Wood fence in Seattle at year 10 (simulation)
Wood fence
Expected life here: 14–18 yrs · main threat: moisture & decay
See it age in Seattle →

Pin-ready

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The 20-year storyboard as a tall pin, linking back to this page.

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Pinterest storyboard: composite deck in Seattle, WA, new to year 20

Composite decks in similar climates

Portland, ORNew York, NYBoston, MAChicago, ILBuffalo, NY

…and in very different ones

Phoenix, AZLas Vegas, NVAlbuquerque, NMAll 25 cities →

Questions Seattle owners ask

How long does a composite deck last in Seattle?

Our model puts a capped composite deck at 25–33 years in Seattle, close to typical warranty terms. The boards outlast the look: expect some early fading and surface mildew in shade.

How often should you clean a composite deck in Seattle?

Twice a year — in spring and fall. Dirt and pollen are the main thing to wash off.

Is composite worth it over cedar in Seattle?

In Seattle our model gives untreated cedar 15–19 years and composite 25–33. In a damp climate that gap is mostly rot you avoid.

Why does wood turn gray or black in Seattle?

Graying is UV plus weathering of the surface; black spots are usually mildew or algae feeding on that grayed layer in damp weather, not rot. A cleaner and a dry-season seal stop it.

Model simulation, not a guarantee: south-facing, full sun, no shade or ground-contact surprises. How it works.