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Hot/Cold Weather Adjustments

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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Weather decides more about a pour than almost anything else. Plan for heat, cold, wind and rain before the truck is ordered.

I am a:

Tap your level and the boxes below open to the advice for you. Remembered on this device.

What it isChanging the order, crew, protection and finishing to suit the heat, cold, wind and rain.
Your jobCheck the forecast, measure the concrete, and have protection on site before the truck.
The hard ruleBelow 5°C (41°F) air, keep the concrete at 10 to 25°C (50 to 77°F) while mixing and placing, and at 10°C or warmer for 72 hours (NBC 9.3.1.9).
The checkA concrete thermometer reading, taken and written down, not just the air temperature.

By the end you can

  • State the NBC cold-weather rule and what it protects.
  • Plan hot-weather placement to avoid fast drying and cracks.
  • Protect a pour from rain and wind.
  • Name the days you should not pour.
Stop and read

Fuel heaters in enclosed spaces make carbon monoxide, which can kill. Use vented or indirect heaters, keep fresh air coming in, and run a carbon monoxide alarm. Heat and cold both wear a crew down, so plan breaks, water and warm-up time. Wet concrete burns skin in any weather.

The big idea

Weather decides more about a pour than almost anything else. Heat speeds the set and dries the surface. Cold slows the set and can freeze the concrete before it has any strength. Wind and rain make both worse. Plan for the weather before the truck is ordered.

A temperature scale from zero to thirty degrees Celsius. Air below five degrees marks where cold-weather rules apply. A green band from ten to twenty-five degrees shows the allowed concrete temperature while mixing and placing. A slab under an insulated blanket shows keeping the concrete at ten degrees or warmer for seventy-two hours after placing.
Cold-weather limits. NBC Part 9 (9.3.1.9) for house-scale work: below 5°C (41°F) air, concrete at 10 to 25°C (50 to 77°F) while mixed and placed, and 10°C (50°F) or warmer for 72 hours after. Provinces adopt the code with changes, and engineered work follows the drawings.

Cold weather

Plastic concrete freezes at about -4°C (25°F), and an early freeze can cost it more than half its potential strength. A US guide (CIP 27) says to protect it from freezing until it reaches 3.5 MPa (500 psi), which for concrete kept at 10°C takes about two days. For concrete that will see freeze-thaw, it says protect until 24 MPa (3,500 psi).

  • The code rule. Below 5°C (41°F) air, keep the concrete between 10°C and 25°C (50 to 77°F) while mixed and placed, and at 10°C or warmer for 72 hours after placing. No frozen material or ice goes in the mix (NBC 9.3.1.9). An Ontario city information sheet cites the same Ontario Building Code article.
  • Order heated concrete. The plant may heat the water or aggregate. Use admixtures only if the supplier and engineer approve them, and no chloride or antifreeze product without approval.
  • Thaw the ground and forms. Remove snow, ice and frost, and do not place against ice or frost.
  • Cover right after finishing with insulated blankets, and use heated enclosures where needed. Corners and edges lose heat fastest.
  • Measure the concrete, not just the air.
  • Keep the slump low. Do not start finishing while it is still bleeding.
  • Do not strip forms or load the concrete until it has the strength the engineer requires. It gains strength slowly in the cold.

Example. Concrete placed at 2 p.m. on a Friday, with the air at 3°C (37°F), must stay at 10°C (50°F) or warmer until 2 p.m. on Monday. The date is made up.

Heaters and carbon monoxide

Do not use unvented gas or propane heaters in enclosed spaces. They make carbon monoxide, which can kill, and CIP 27 says fuel heaters in enclosures should be vented, partly to prevent carbonation that causes dusting. Use vented or indirect heaters, and never leave one running in a closed space without ventilation.

Confirm

Your province adopts its own edition of the code, and may add a written cold-weather plan or inspector approval. The By province section below says what I have and have not checked. For engineered work, the engineer sets the limits.

Hot weather

Heat speeds the set, dries the surface, and can cause plastic shrinkage cracks and cold joints. Part 9 gives no separate hot-weather limits. CSA A23.1 does, but I did not read it, so confirm the maximum concrete temperature with your supplier.

  • Watch evaporation. A concrete-admixture maker says plastic shrinkage cracking becomes a strong possibility above 1.0 kg/m²/h (0.2 lb/ft²/h), and that controls should be considered above about 0.5 kg/m²/h (0.1 lb/ft²/h). Lower humidity, hotter concrete or air, and more wind all raise it.
  • Pour early in the morning or in the evening, and shorten the delivery interval. Ask about retarders.
  • Shade and dampen the forms and base, without leaving standing water.
  • Have extra crew and be ready to finish fast.
  • Use an evaporation retarder or fogging above the surface, if it dries too fast. Do not trowel water into the surface.
  • Start curing at once, with compound, wet cover, or both.

Example. At 1.0 kg/m²/h, a 4 m by 6 m (13 ft 1 in by 19 ft 8 in) slab loses 24 m² × 1.0 = 24 kg, about 24 litres (6.3 US gal), of water from its surface every hour. The slab size is made up.

Rain and wind

  • Rain: cover the slab quickly and stop finishing. Do not let rain hit a fresh finish. Pump off standing water before you continue. Never add water to fix rain damage.
  • Wind: use windbreaks where you can. Bleed water disappears quickly, so work fast, and avoid early troweling that seals a dry crust. Use an evaporation retarder, and start curing at once.
  • Hot and windy: the worst finishing conditions. Expect fast drying, crusting and tearing, and add crew, shade and windbreaks.
  • Cold and windy: wind strips heat from the concrete and slows strength gain. Put blankets on sooner and leave them on longer.

When not to pour

  • Heavy rain or thunderstorms.
  • Freezing conditions with no protection plan.
  • Frozen ground, or forms covered in frost.
  • Very high wind with no windbreak.
  • A heat wave with no shade, no extra crew and no retarder.

Common mistakes

  • Finishing too early in hot or windy weather
  • Placing on frozen ground
  • Letting rain hit a fresh surface
  • No blankets in the cold
  • Ignoring fast evaporation
  • Wrong admixtures
Pro note

Weather decides the plan. Hot weather needs speed, cold weather needs protection, and rain and wind need cover.

Weather note

Check the temperature, wind, humidity and forecast before every pour. Weather changes quickly, so keep blankets, covers and a plan in the truck.

What the inspector looks at

Cold-weather placement may need the inspector's written approval, and they may check curing protection and temperature records.

✅ Check yourselfTap a question
What does NBC 9.3.1.9 require when the air temperature is below 5°C (41°F)?
Keep the concrete between 10°C and 25°C (50 to 77°F) while it is mixed and placed, and keep it at 10°C (50°F) or warmer for 72 hours after placing. No frozen material or ice may go in the mix.
You place concrete at 2 p.m. on Friday in a 3°C cold snap. When can the 72-hour protection end at the earliest?
2 p.m. on Monday, 72 hours later. The concrete itself must stay at 10°C or warmer the whole time, so measure the concrete, not just the air.
Why measure the temperature of the concrete, not just the air?
The code limits are on the concrete temperature. Heat from setting can keep a slab warm, but corners and edges lose heat fastest.
At the evaporation rate that makes plastic shrinkage cracking a strong possibility, how much water does a 4 m by 6 m (13 ft 1 in by 19 ft 8 in) slab lose in an hour?
The threshold is 1.0 kg/m²/h (0.2 lb/ft²/h). 24 m² × 1.0 = 24 kg, about 24 litres (6.3 US gal) of water per hour.
Rain starts on a fresh finish. What do you do?
Cover it quickly and stop finishing. Pump off standing water, and never add water to fix rain damage.
Why not use an unvented propane heater inside a closed enclosure over fresh concrete?
The fumes are dangerous to people, and a flueless fuel heater can damage the fresh surface. Use vented or indirect heat, and keep a carbon monoxide alarm running.
🧰 Gear for this stepMust-haveRent itNice to have
  • Must-haveMeasuring and monitoring (thermometers):
  • Nice to haveMeasuring and monitoring (meter):
  • Must-haveCold protection (blankets and sheeting):
  • Rent itCold protection (thaw blankets): ground-thaw blankets for frozen subgrade
  • Rent itHeat: vented or indirect heaters only, with a carbon monoxide alarm in any enclosed space
  • Must-haveHot and windy protection:
  • Must-haveCuring:
  • Must-haveFinishing support: extra screeds, floats and kneeboards, so the crew can finish fast when it is hot or windy
  • Must-havePPE:
👷 Who can do thisTap your level
Home DIY
A capable owner can pour a small slab in mild weather. Below 5°C (41°F), or in hot, windy conditions, the risks go up fast, so ask your supplier, and book a finisher or postpone.
Laborer or helper
Help move blankets, covers and windbreaks, and keep heaters and cords clear of fresh concrete. Never run a fuel heater in a closed space. Tell your lead if the ground looks frozen or rain is coming.
1st-year apprentice
You can place and protect concrete in moderate weather under supervision. Learn to read a concrete thermometer and to put blankets on right after finishing.
Apprentice, 2nd to 4th year
Place, finish and protect concrete under supervision. Take concrete temperature, not just air. Cover right after finishing, and tell your journeyperson if the ground is frozen, the forecast changes, or the surface is drying faster than you can finish.
Journeyperson
You own the weather plan: heated or retarded concrete, crew size, blankets, windbreaks and the record. Stop-work triggers: frozen ground or frost on forms, concrete outside the temperature limits, rain with no cover, or a heat or wind plan that has no extra crew.
Lead hand or foreman
Check the forecast the day before and again that morning, and stage blankets, covers and thermometers before the truck is called. Keep a temperature record. Enforce the triggers, and call the supplier early to change the order.
Contractor or business owner
Price heated concrete, blankets, heaters and extra crew for winter work, and write a weather clause into your contracts. Ask your lawyer or insurer about who carries the risk when weather damages a pour.
Starting a new business
Learn the cold and hot weather rules before you take winter or summer work. Budget blankets, thermometers and ventilation, and write down your temperature records. Confirm weather clauses and warranty terms with your lawyer and insurer.
By Province:

NBC 2020 Part 9, Article 9.3.1.9 is the cold-weather rule, and I read it as printed in the Vancouver Building Bylaw 2014. An Ontario city information sheet quotes the same wording as Ontario Building Code 9.3.1.9. I did not check the other provinces, which adopt the code with changes, so confirm the current edition for yours with the local authority. Engineered work follows the drawings and the engineer.

See Canada by Province for the full comparison.

Sources:

Last reviewed October 2026. General information, not engineering or legal advice.

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