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Deck Staining Weather in Longmont, CO: 10-Day Windows & Best Months

Longmont gives you roughly 83 workable deck staining days a year, concentrated May through June. The single best month is August, averaging 22 days that clear every check — highs of 88°F, lows near 54°F, and a 23% daily rain chance. Below: the live 10-day check and Longmont's full month-by-month table.

GOOD — a clean label day MARGINAL — one borderline check NO — hard fail or stacked flags

The rules this check uses

Every Longmont verdict above traces to this table — typical stain-label requirements across major manufacturers. Water-based and oil-based formulas differ mainly in the dry-after row.

Typical label thresholds for deck staining — the ruleset behind every Longmont verdict above.
CheckThresholdWhy it matters
Air temperature 50–90°F Air temperature while applying and for the first hours of dry time.
Overnight low ≥40°F during the first 24 h Overnight low during the cure window.
Dry before ≤0.05" rain in the prior 24 h; watch back to 48 h What fell before you start matters as much as what falls after — surfaces hold water invisibly.
Dry after <0.05" rain for 24 h after (48 h oil-based formulas want 48 h dry) The make-or-break window: rain here undoes the work, not just delays it.
Evening dew-point spread ≥5°F from 6–11 pm Temperature minus dew point from 6 pm to 11 pm. A small spread means dew will settle on fresh stain.
Daytime humidity ≤85% Humid air slows evaporation, stretching dry times into the risky evening hours.
Wind ≤15 mph (brush or pad only up to 20 mph) Scored on the windiest working hour; the marginal band changes the method, not the day.

Always follow your product label — formulas vary. Treat the table as the consensus range across brands — the label in your hand is the final word.

Best months for deck staining in Longmont

How Longmont months rank: temperature-rule days minus NOAA rain odds, from 1991–2020 normals. An estimate for planning, not a forecast.
MonthAvg highAvg lowRain odds/dayWorkable days 
January 44°F 14°F 12% 0
February 47°F 17°F 16% 0
March 57°F 26°F 17% 0
April 63°F 32°F 24% 0
May 73°F 41°F 31% 14
June 84°F 50°F 28% 22
July 91°F 55°F 22% 3
August 88°F 54°F 23% 22
September 80°F 46°F 21% 22
October 66°F 33°F 18% 0
November 54°F 24°F 15% 0
December 45°F 14°F 10% 0

Longmont compresses the whole deck staining year into May through June. Miss those 83 workable days and the next real window is months out: by July, average lows hit 55°F against a 40°F floor. Plan the prep work in advance and treat every GOOD chip as spendable. Neighboring towns shift by a month or more — the Colorado comparison shows where Longmont sits.

July here fails on heat, not rain: the average high of 91°F sits over the 90°F label ceiling, and 27 of 31 days typically top 90°F. Midsummer work moves to dawn or waits for August.

The rain odds swing hard across the year — 10% of days in December up to 31% in May. Season the plan accordingly: prep in the wet months, apply in the dry ones.

Same-weekend planning note: the dew and overnight rules here track exterior painting in Longmont almost rule for rule — a clean staining day usually paints too.

Climatology here is measured at Longmont 2 Ese, Co Us (2.0 km away). Treat the monthly numbers as the neighborhood average; a shaded north-side deck runs colder and damper than any of them. The exact formula lives in the methodology.

Longmont by the numbers

Prep checklist

  1. Pick the window first: you need roughly 2 dry days (24 h cure plus buffer), and Longmont averages rain on 31% of May days versus 10% in December — the strip above finds the pair.
  2. Wash the deck, then give Longmont's air 48 hours to pull the water back out — a pressure washer shortens the scrub, not the dry time.
  3. Check moisture before opening the can — under 15% on a wood moisture meter; after a May soak, end grain lags the surface by a day.
  4. Quick pass with sandpaper and a nail set, then sweep the gaps; stain drips find every crack.
  5. Mask where deck meets siding (painter's tape) and drop cloth under the rails.
  6. Morning start, shaded side first — full sun puts a board 20–30°F above air temperature, past the 90°F ceiling on a 88°F day.
  7. Thin coats, wiped edges: pads or a pump sprayer below 15 mph wind; brush-only from 15 to 20 mph.
  8. Quit about 2 hours before sunset — Longmont's August nights average 54°F, and the dew-point spread closes fastest right after dark.

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FAQ

What temperature is too cold to stain a deck?

Below 50°F air temperature, or any night under 40°F inside the 24-hour cure. Cold is what actually frames Longmont's season: average lows sit at 32°F in April and 33°F in October, so shoulder-season afternoons can pass while their nights fail.

How long does deck stain need to dry before rain?

About 24 hours for water-based stain, up to 48 for oil-based — rain of 0.05" or more inside that window can spot or streak the film. In Longmont, May brings measurable rain on 31% of days, so finding two clean days is the real scheduling job; December (10%) makes it easy.

Can I stain a deck in direct sunlight?

Direct sun is a surface-temperature problem: add 20–30°F to the forecast for a board in full sun. With Longmont July highs averaging 91°F, sunlit boards regularly pass the 90°F limit even when air temperature reads fine. Chase the shade and finish 2 hours before sunset.

How dry should wood be before staining?

Under about 15% moisture content, with no 0.05"+ rain in the previous 24 hours (and ideally 48). After a soak, Longmont wood needs a full day or two of drying — longer in May, when rain returns on 31% of days. The sprinkle test works: if water beads instead of soaking in, wait.

Water-based vs oil-based stain in a wet climate?

Water-based needs a shorter dry window (24 h vs 48) — decisive where rain is frequent. Longmont's wettest month sees rain 31% of days, so the shorter cure roughly doubles your usable windows; the engine marks oil's 24–48 h tail as MARGINAL when rain lands there.

What months are best for staining in CO?

For Longmont specifically: August, September and June, led by August with 22 workable days (average high 88°F, rain on 23% of days). The season shuts by June when nights fall through the 40°F floor.

Other projects in Longmont

Deck Staining nearby

Guides

Climatology: NOAA 1991–2020 normals via LONGMONT 2 ESE, CO US (2.0 km from Longmont center, elevation 4950 ft); live outlook by Open-Meteo.