WorkWindow

Concrete Pouring Weather in Longmont, CO: 10-Day Windows & Best Months

Longmont gives you roughly 83 workable concrete pouring 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

Typical bagged-mix requirements for small DIY pours, scored against Longmont's forecast above. The 48-hour freeze rule dominates every other row; structural work follows engineer/ACI specs instead of this table.

Typical label thresholds for concrete pouring — the ruleset behind every Longmont verdict above.
CheckThresholdWhy it matters
Air temperature 40–90°F — ideal 50–85°F DIY pours work from 40–90°F; 50–85°F is the sweet spot.
Overnight low ≥40°F during the first 48 h A low under 40°F inside the first 48 hours puts you in cold-weather concreting — not a DIY window.
Dry before no soaking (≥1.0") in the prior 24 h What fell before you start matters as much as what falls after — surfaces hold water invisibly.
Dry after <0.1" rain for 6 h after (12 h light rain after finishing still risks surface marks) The make-or-break window: rain here undoes the work, not just delays it.
Wind ≤20 mph (rapid surface drying up to 28 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 concrete pouring 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 concrete pouring 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.

Sealing the same driveway later? Sealcoating in Longmont wants warmer nights (50°F+) than the pour did.

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. Clear the 48-hour rule first: two nights over 40°F. May opens that door in Longmont; January (14°F average lows) slams it.
  2. Stage everything before mixing: forms braced, base compacted and damp, tools rinsed, help booked.
  3. Cut plastic sheeting and weight it at the pour's edge — Longmont sees rain on 23% of August days, and the 6-hour rule doesn't negotiate.
  4. Resist the watery mix: it finishes easier today and dusts forever after. Low slump wins.
  5. Work the sequence: screed on the sheen, float as it dulls, and leave bleed water alone.
  6. Edge and joint with an edger + float set — control joints every 2–3 slab-thicknesses in feet.
  7. Hold the moisture in (sheeting or misting); when June nights dip toward 50°F, a curing blanket is the difference.
  8. Traffic schedule: feet at 24–48 hours, tires near day 7. Anything structural runs on engineer/ACI specs, not this checklist.

Gear that saves a window

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FAQ

What temperature is too cold to pour concrete?

The line is a 40°F low inside the first 48 hours; an actual freeze (32°F) physically damages young concrete. Longmont's January nights average 14°F — firmly out — while August nights hold near 54°F. Small pours only; structural work follows engineer/ACI specs.

Can you pour concrete before rain?

The engine wants 6 protected hours; light rain later actually feeds the cure. With 31% rain-day odds in May versus 10% in December, Longmont's dry-season pours barely think about this rule and wet-season pours live by the radar. For small DIY pours; structural schedules follow ACI.

How long does concrete need to be protected from freezing?

Keep it above freezing for at least the first 48 hours (the engine calls any sub-40°F low in that window a NO). In Longmont that rules out roughly January-adjacent months entirely and makes shoulder-season pours a two-night forecast decision. DIY scope; anything structural follows engineer/ACI cold-weather practice.

Is it OK to pour concrete in hot weather?

To a 90°F high, yes — with shade, cool mix water, a damp subgrade, and a dawn start. Above 90°F the surface sets while the core is plastic and shrinkage cracks map the slab. Longmont averages 27 such days in July, which is why summer pours here move to first light.

How long before you can drive on new concrete?

A week before tires, 24–48 hours before feet — at August-typical Longmont temperatures (88°F highs). When nights slide toward 14°F, add days: cure speed is temperature. Early loads print permanent marks; the bag's cure table wins every argument.

Best season for concrete work in Longmont?

May through june — the months with 40°F+ nights, sub-90°F days, and manageable rain. August leads at 22 workable days; January bottoms out near 0.

Other projects in Longmont

Concrete Pouring 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.