Cold-weather masonry in Ontario: the 5 °C rule and how builders keep going

Updated October 2026 · By GTA Tarping INC, Newmarket

What the OBC requires

• Mortar and masonry must be maintained at a temperature of at least 5 °C during installation.
• That temperature must be maintained for at least 48 hours after the masonry is laid.
• Frozen materials must not be used in mortar.

The City of Markham publishes a builder tip on masonry construction in cold weather that summarises these points for site supers; it's worth sharing with your crews.

Tarp-and-heat set-ups that work

• Full enclosure of the working elevations: tarps on the scaffold from the base to above the working height, closed at the corners.
• Sealed base: tarps closed at grade so heat doesn't escape under the scaffold.
• Insulated tarps where heat must be held: especially overnight and through long cold spells.
• Heater clearance and ventilation: heaters placed with clearance from tarps and materials, and the enclosure ventilated, in line with O. Reg. 213/91 s. 49.
• Material storage: brick, block and sand kept covered and warm so nothing frozen goes into the mortar.

Protecting unfinished walls overnight

Cover the top of unfinished walls to keep out snow and rain, keep the enclosure heated through the 48-hour window, and check the enclosure after wind. A tarp that has blown loose overnight can undo a day's work.

Checklist for site supers

• Check the forecast daily and plan heat before temperatures fall below 5 °C.
• Confirm the enclosure is closed and secure before masons start.
• Confirm who supplies and monitors heaters.
• Keep materials covered and free of frost.
• Protect newly laid masonry for at least 48 hours.
• Record temperatures if your quality programme requires it.

FAQ

Can masons lay brick below 5 °C in Ontario?

Only if the mortar and masonry are kept at 5 °C or warmer during installation and for 48 hours after, which usually means a tarped, heated enclosure.

Where does the 5 °C rule come from?

Ontario Building Code Article 9.20.14.1.

Who is responsible for heating the enclosure?

It varies by job. Usually the masonry contractor or builder supplies heat; confirm it in the contract.

Need an enclosure? See winter scaffolding tarping or read What winter tarping costs. Get a quote