For most construction work, no. Ontario only requires formal credentials in compulsory trades — Construction Electrician, Plumber, Sheet Metal Worker, Sprinkler and Fire Protection Installer, HVAC Mechanic, and a few residential classifications. In those trades you must be a registered apprentice or hold a valid Certificate of Qualification to work legally.

Everything else — general carpentry, framing, roofing, siding, drywall, concrete, bricklaying, welding, heavy equipment, general labour — is non-compulsory, meaning you can legally work without a certificate. Apprenticeship and certification are still available in most of those trades and usually move you up the wage range, but they aren’t a legal barrier to getting hired. Trades are registered through Skilled Trades Ontario.