Decking · Cool & damp

How long does a pressure-treated deck last in Portland, OR?

THE SHORT ANSWER

In Portland, moisture — not sun — is the main enemy of a pressure-treated deck. The city sees about 157 days with rain a year, mostly from October to May, and its Scheffer decay index of 56.6 is moderate (#10 of 25 cities we cover). Unsealed boards gray within two years, then mildew and soft board ends become the real problem. Model estimate: 11–15 years unsealed, 15–20 years sealed on schedule.

Main threat

Moisture & decay

Climate profile

Cool & damp

Expected life (unsealed)

11–15 yrs

Fully gray by

Year 2

Best seal window

Jul – Aug

Sealed on schedule

15–20 yrs

Simulated pressure-treated deck in Portland, OR after 10 years

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

MODEL SIMULATION

Portland, OR · 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 Portland, unsealed

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

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

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

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

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

Portland’s wear fingerprint

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

Rainy days / yr157 · High

Avg humidity81.7% · High

Decay index56.6 · Typical

UV index (avg)0.9 · Low

Freeze–thaw days30.1 · Typical

Days ≥ 90 °F14.6 · Low

NOAA station PORTLAND INTL AP (USW00024229, 10.1 km away), 1991–2020 normals · humidity & UV: NASA POWER.

What fails first here

1Mildew & algaeA wet season from October to May and 82% average humidity keep shaded boards damp; dark spots start near walls, planters and gaps.
2Checks, splits and warpingMild here: limited UV and only 30 freeze–thaw days a year keep cracks shallow.
3GrayingCosmetic, 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.
4Rot at board ends & joistsThe real lifespan limiter in Portland: decay index 56.6. Treatment protects the wood, but cut ends and drilled holes lose it — brush end-cut preservative on during building.

Upkeep

Portland 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 Portland

Cedar deck

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

See it age in Portland →

Composite deck

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

See it age in Portland →

Wood fence

Expected life here: 13–17 yrs · main threat: moisture and decay

See it age in Portland →

Vinyl fence

Expected life here: 24–31 yrs · main threat: moisture and decay

See it age in Portland →

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 Portland, OR, 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 Portland owners ask

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

Our model puts it at 11–15 years unsealed and 15–20 years if it is sealed every 2–3 years. The national typical range is 10–15 yrs; Portland’s moisture and decay profile is why the local number differs.

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

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 Portland?

Our model gives an unsealed pressure-treated deck 11–15 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 Portland?

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.