Decking · Mixed

How a cedar deck ages in Chicago, IL

THE SHORT ANSWER

In Chicago, moisture — not sun — is the main enemy of a cedar deck. The city sees about 121 days with rain a year, mostly from April to June, and its Scheffer decay index of 55.3 is moderate (#11 of 25 cities we cover). Untreated boards gray within two years, then mildew and soft board ends become the real problem. Model estimate: 14–18 years untreated, 19–24 years sealed on schedule.

Main threat

Moisture & decay

Climate profile

Mixed

Expected life (untreated)

14–18 yrs

Fully gray by

Year 2

Best seal window

Jul – Sep

Sealed on schedule

19–24 yrs

Simulated cedar deck in Chicago, IL after 10 years

Year 10Heavy mildew, deep checks

MODEL SIMULATION

Chicago, IL · south-facing deck, full sun, no sealant

Drag through time 0 — 20 years

Color lossSevere

Surface checkingHigh

Mildew / rotHigh

Fails first hereBoard ends, joists and shaded boards

Storyboard

20 years in Chicago, untreated

Cedar deck in Chicago, install: Fresh boards — **Install**Fresh boards

Cedar deck in Chicago, year 1: Silvering begins — **Year 1**Silvering begins

Cedar deck in Chicago, year 3: Fully gray — **Year 3**Fully gray

Cedar deck in Chicago, year 5: Mildew in shade, surface checks — **Year 5**Mildew in shade, surface checks

Cedar deck in Chicago, year 10: Heavy mildew, deep checks — **Year 10**Heavy mildew, deep checks

Cedar deck in Chicago, year 15: Soft spots — probe for rot, heavy mildew, deep checks — **Year 15**Soft spots — probe for rot, heavy mildew, deep checks

Cedar deck in Chicago, year 20: Soft spots — probe for rot, heavy mildew, deep checks — **Year 20**Soft spots — probe for rot, heavy mildew, deep checks

Chicago’s wear fingerprint

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

Rainy days / yr121.3 · Typical

Avg humidity81% · High

Decay index55.3 · Typical

UV index (avg)0.9 · Low

Freeze–thaw days67.1 · High

Days ≥ 90 °F19.8 · Low

NOAA station CHICAGO MIDWAY AP (USW00014819, 14.9 km away), 1991–2020 normals · humidity & UV: NASA POWER.

What fails first here

1Mildew & algaeA wet season from April to June and 81% average humidity keep shaded boards damp; dark spots start near walls, planters and gaps.
2Surface checkingAbout 67 freeze–thaw days a year work water into cracks and pry them open.
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 Chicago: decay index 55.3. Cut ends wick water — probe them with a screwdriver each spring.

Upkeep

Chicago 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

Mar

AprInspect ends & fasteners

May

JunWash off mildew

JulSeal / stain

Aug

SepSeal / stain

OctClear leaves & gaps

Nov

Dec

Same city, other materials

How the alternatives age in Chicago

Composite deck

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

See it age in Chicago →

Wood fence

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

See it age in Chicago →

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

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Pinterest storyboard: cedar deck in Chicago, IL, new to year 20

Cedar decks in similar climates

…and in very different ones

Phoenix, AZ

Las Vegas, NV

Albuquerque, NM All 25 cities →

Questions Chicago owners ask

How long does a cedar deck last in Chicago?

Our model puts it at 14–18 years untreated and 19–24 years if it is sealed every 2–3 years. The national typical range is 15–20 yrs; Chicago’s moisture & decay profile is why the local number differs.

When is the best time to seal a cedar deck in Chicago?

July and September: 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.

Is composite better than cedar in Chicago?

Probably, if low upkeep matters: composite removes rot, the main local failure mode. Our model gives composite 25–32 years here versus 14–18 for untreated cedar.

Why does wood turn gray or black in Chicago?

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.