Decking · Cool & damp

How long does a pressure-treated deck last in Seattle, WA?

THE SHORT ANSWER

In Seattle, moisture — not sun — is the main enemy of a pressure-treated deck. The city sees about 154 days with rain a year, mostly from October to April, and its Scheffer decay index of 52.6 is moderate (#14 of 25 cities we cover). Unsealed boards gray within two years, then mildew and soft board ends become the real problem. Model estimate: 12–15 years unsealed, 16–21 years sealed on schedule.

Main threat

Moisture & decay

Climate profile

Cool & damp

Expected life (unsealed)

12–15 yrs

Fully gray by

Year 2

Best seal window

Jul – Aug

Sealed on schedule

16–21 yrs

Simulated pressure-treated deck in Seattle, WA after 10 years

Year 10Decay starting at board ends, heavy mildew, deep checks

MODEL SIMULATION

Seattle, WA · treated southern pine boards, south-facing, full sun, no sealant

Drag through time 0 — 20 years

Color lossSevere

Surface checkingSevere

Mildew / rotHigh

Fails first hereCut ends that lost their treatment, and joists

Storyboard

20 years in Seattle, unsealed

Pressure-treated deck in Seattle, install: Fresh, green-tinted boards — **Install**Fresh, green-tinted boards

Pressure-treated deck in Seattle, year 1: Silvering begins — **Year 1**Silvering begins

Pressure-treated deck in Seattle, year 3: Fully gray — **Year 3**Fully gray

Pressure-treated deck in Seattle, year 5: Mildew in shade, checks and some warping — **Year 5**Mildew in shade, checks and some warping

Pressure-treated deck in Seattle, year 10: Decay starting at board ends, heavy mildew, deep checks — **Year 10**Decay starting at board ends, heavy mildew, deep checks

Pressure-treated deck in Seattle, year 15: Soft spots — probe for rot, heavy mildew, deep checks — **Year 15**Soft spots — probe for rot, heavy mildew, deep checks

Pressure-treated deck in Seattle, year 20: Soft spots — probe for rot, heavy mildew, deep checks — **Year 20**Soft spots — probe for rot, heavy mildew, deep checks

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

1Mildew & algaeA wet season from October to April and 83% average humidity keep shaded boards damp; dark spots start near walls, planters and gaps.
2Checks, splits and warpingMild here: limited UV and only 29 freeze–thaw days a year keep cracks shallow.
3Rot at board ends & joistsThe real lifespan limiter in Seattle: decay index 52.6. Treatment protects the wood, but cut ends and drilled holes lose it — brush end-cut preservative on during building.
4GrayingCosmetic, and fast: our model has the surface fully silver by year 2. Not damage on its own, but it means the protective surface is gone.

Upkeep

Seattle maintenance calendar

Built from local normals: bar width is the share of days with rain; sealant needs 2–3 dry days between 50 and 88 °F.

Jan

Feb

MarInspect ends & fasteners

Apr

May

JunWash off mildew

JulSeal / stain

AugSeal / stain

Sep

OctClear leaves & gaps

Nov

Dec

Same city, other materials

How the alternatives age in Seattle

Cedar deck

Expected life here: 15–19 yrs · main threat: moisture and decay

See it age in Seattle →

Composite deck

Expected life here: 25–33 yrs · main threat: moisture and decay

See it age in Seattle →

Wood fence

Expected life here: 14–18 yrs · main threat: moisture and decay

See it age in Seattle →

Vinyl fence

Expected life here: 25–32 yrs · main threat: moisture and decay

See it age in Seattle →

Pin-ready

Save it for when you build.

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

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Pinterest storyboard: pressure-treated deck in Seattle, WA, new to year 20

Pressure-treated decks in similar climates

…and in very different ones

Phoenix, AZ

Las Vegas, NV

Albuquerque, NM All 25 cities →

Questions Seattle owners ask

How long does a pressure-treated deck last in Seattle?

Our model puts it at 12–15 years unsealed and 16–21 years if it is sealed every 2–3 years. The national typical range is 10–15 yrs; Seattle’s moisture and decay profile is why the local number differs.

When is the best time to seal a pressure-treated deck in Seattle?

July and August: the months with the fewest rainy days while average temperatures sit between 50 and 88 °F. Most sealants need 2–3 dry days to cure.

Cedar or pressure-treated: which lasts longer in Seattle?

Our model gives an unsealed pressure-treated deck 12–15 years here and untreated cedar 15–19 years. Where rot is slow, cedar’s stability wins: treated pine checks and warps faster in sun and frost.

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.