No. Ontario building code and manufacturer guidelines both call for a full tear-off before installing a new roof. Layering shingles traps heat and moisture between surfaces and accelerates rot in the decking. The new roof starts its life already compromised.
There’s a narrow exception. If a roof has a single existing layer in good structural condition, no moisture damage, and it falls within code allowances, overlay is technically permitted. Even then most Ontario roofers push for tear-off. It’s the only opportunity in a 25 to 30 year window to inspect and repair the decking. Skipping it to save a few hundred dollars upfront is a bad trade.
How many years old is considered a new roof helps frame the decision. A roof under 10 years old in good condition is a completely different conversation than one pushing 20. Whether it’s cheaper to roof over existing shingles lands in the same place. Short-term saving, long-term liability. For full cost context on a proper replacement see how much it costs to replace a 2,000 sq ft roof in Ontario.