Fencing · Cool & damp

How a wood fence ages in Portland, OR

THE SHORT ANSWER

In Portland, moisture — not sun — is the main enemy of a wood fence. 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). Untreated boards gray within two years, then mildew and soft post bases become the real problem. Model estimate: 13–17 years untreated, 17–23 years sealed on schedule.

Main threat

Moisture & decay

Climate profile

Cool & damp

Expected life (untreated)

13–17 yrs

Fully gray by

Year 2

Best seal window

Jul – Aug

Sealed on schedule

17–23 yrs

Simulated wood fence in Portland, OR after 10 years

Year 10Decay starting where boards meet the ground, heavy mildew, surface checks

MODEL SIMULATION

Portland, OR · cedar pickets on wood posts, no stain

Drag through time 0 — 20 years

Color lossSevere

Surface checkingHigh

Mildew / rotHigh

Fails first herePost bases and picket bottoms

Storyboard

20 years in Portland, untreated

Wood fence in Portland, install: Fresh boards — **Install**Fresh boards

Wood fence in Portland, year 1: Silvering begins — **Year 1**Silvering begins

Wood fence in Portland, year 3: Fully gray — **Year 3**Fully gray

Wood fence in Portland, year 5: Mildew in shade, surface checks — **Year 5**Mildew in shade, surface checks

Wood fence in Portland, year 10: Decay starting where boards meet the ground, heavy mildew, surface checks — **Year 10**Decay starting where boards meet the ground, heavy mildew, surface checks

Wood fence in Portland, year 15: Rot at the base — posts may lean, heavy mildew, deep checks — **Year 15**Rot at the base — posts may lean, heavy mildew, deep checks

Wood fence in Portland, year 20: Rot at the base — posts may lean, heavy mildew, deep checks — **Year 20**Rot at the base — posts may lean, 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.
2GrayingCosmetic, 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.
3Checks, splits and cuppingMild here: limited UV and only 30 freeze–thaw days a year keep cracks shallow.
4Rot at post basesThe real lifespan limiter in Portland: decay index 56.6. Posts rot at the soil line first.

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

JulStain / seal

AugStain / seal

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 & decay

See it age in Portland →

Composite deck

Expected life here: 25–32 yrs · main threat: moisture & 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.

Save to Pinterest Download pin

Pinterest storyboard: wood fence in Portland, OR, new to year 20

Wood fences 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 wood fence last in Portland?

Our model puts it at 13–17 years untreated and 17–23 years if it is sealed every 2–3 years. The national typical range is 15–20 yrs; Portland’s moisture & decay profile is why the local number differs.

When is the best time to seal a wood fence 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.

How long do fence posts last in Portland?

Posts are usually the first part of a wood fence to go, because they sit in damp soil. With a decay index of 56.6 here, plan on inspecting post bases from about year 8.

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.