Technically yes, by $500 to $1,500 in tear-off and disposal costs on a typical Ontario home. But the practice is generally not recommended and may not comply with local building codes if a second layer already exists. Most Ontario municipalities cap roofs at two shingle layers under the Ontario Building Code.
The real problem is what happens underneath. Overlay roofing shortens the new roof’s lifespan because uneven surfaces stop proper adhesion and moisture gets trapped between the layers. Most manufacturer warranties require installation over a clean deck. Do an overlay and you’ve likely voided your coverage before the first winter is out. The upfront savings don’t survive contact with those two facts.
The same logic applies when asking whether it’s cheaper to roof over existing shingles more broadly. Short-term saving, long-term liability. For what a proper full replacement actually costs see how much it costs to replace a 2,000 sq ft roof in Ontario, and what the most expensive part of that project will be.